Tile Mortar Calculator
How much thinset mortar do you actually need? This free mortar and thinset calculator gives DIY tile setters and contractors three things in one screen: the right ANSI mortar class for your substrate and tile size, the right trowel notch, and an accurate bag count built from blended TDS data. No more buying the wrong product because the box label said "for tile" — and no more running short on a 100-sq-ft floor because the calculator used optimistic coverage.
Three independent variables drive every mortar take-off. First, the ANSI class — dictated by substrate (Schluter Ditra forces unmodified A118.1; plywood forces A118.11; pools or exterior force A118.15) and tile size (anything ≥ 15" requires the H suffix per ANSI A108.02 §4.1.4.3.1). Second, the trowel notch — published by each manufacturer per square foot, and notably the NTCA 2024/2025 Reference Manual now warns against the 1/2"×1/2" square notch because the wide spacing prevents proper ridge collapse. Third, the back-butter multiplier — 1.00 for small tile, 1.15 for 95% specs, 1.25 for LFT, 1.30 for gauged porcelain panels.
Built on ANSI A108.5 / A108.02 / A108.19, ANSI A118.1 through .15 plus A118.3 and A136.1, the NTCA 2024/2025 Reference Manual, and the TCNA Handbook for Ceramic, Glass and Stone Tile Installation. Coverage values cross-checked against published TDS data from Laticrete (254 Platinum, 4-XLT, MULTIMAX, Tri-Lite), Mapei (Ultraflex 1/2/LFT, Granirapid, Adesilex P10), Custom Building Products (VersaBond, MegaLite, ProLite), ARDEX (X 5, X 77, X 90 OUTDOOR), and Bostik (Single-Flex, Reflex, Glass-Mate). Free, no signup.
Mortar / Thinset Calculator
Auto-pick the right ANSI mortar class for your substrate and tile size, choose a trowel, and get a bag count built on industry-blended TDS coverage values from Laticrete, Mapei, Custom, ARDEX, and Bostik.
Project area
Net tiled area — subtract cabinets, fixtures, openings before entering.
For irregular footprints, enter the total square footage as length × 1 (e.g. 73 sq ft = 73 × 1).
Substrate and tile
The two inputs that determine which ANSI class is acceptable.
A118.4 modified. Use T suffix on walls. · Back-butter recommended over 12"×12".
Which ANSI mortar — modified, unmodified, epoxy or mastic? See the selection guide
Trowel and bag size
Trowel auto-picks by tile category if left on auto.
Lightweight 25 lb (Titanium / MULTIMAX) and 30 lb (MegaLite / ProLite / Tri-Lite) bags yield the same coverage as conventional 50-lb bags per Laticrete and Custom Building Products TDS.
How does the notch size change coverage per bag? See the trowel diagram
Application flags
These add ANSI suffixes (H/T/E/F) and adjust the coverage requirement.
What is the 80% / 95% coverage rule, and why back-butter? See the coverage diagram
Getting thinset right: trowel, coverage & class
Three things decide whether tile stays down: the trowel notch (it sets the mortar bed and the coverage), the mortar contact under the tile (ANSI measures contact, not how much you spread), and the ANSI class for your substrate and exposure. These engineering-style diagrams cover each one — use the “see the diagram” links beside the inputs above to jump to the figure you need.
The trowel-coverage diagram is why the calculator ties mortar coverage to notch size. A bigger notch lays a deeper bed for larger tile, so it uses more mortar and covers fewer square feet per bag. Matching the notch to the tile is what makes the bag count come out right — the notch, not just the area, sets the yield.
The coverage-rule diagram explains why the calculator assumes proper contact under the tile, not just combed ridges. Standing ridges leave voids, so the tile has to be set to collapse them and back-buttered to reach the contact the standard requires for wet and large tile. Under-buttering leaves hollow spots the mortar estimate does not fix.
The mortar-class diagram is why substrate and exposure pick the product, not just the tile. The classic trap is a sheet membrane, which needs unmodified mortar because modified cannot cure sealed between two impervious layers, while mastic is for interior dry wall tile only. The calculator flags the class because the wrong one never fully cures.
Calculation Formulas
A = net tiled area, B = back-butter / coverage multiplier, M = job-site mortar waste (10% default), C = manufacturer-published coverage at the chosen trowel notch. Always rounded up — bags are sold whole.
Example:
100 sq ft × 1.15 (back-butter for 12×24) × 1.10 (10% waste) ÷ 55 sq ft/bag (1/4"×3/8" notch, conservative) = 2.30 → 3 bags.
Tile under 8" with 80% interior-dry coverage uses no multiplier. Tile 8"–15" or any 95%-spec install (wet, exterior, natural stone) needs 1.15. LFT (≥15") needs 1.25. Gauged porcelain panels and ungauged stone need 1.30.
Example:
24×48 LFT over CBU: B = 1.25, requires H suffix and a 1/2" or 3/4" notch.
Conservative defaults are blended low-end values from the 50-lb-equivalent TDS sheets of Laticrete, Mapei, Custom, ARDEX, and Bostik. The 1/2" square coverage is published only for legacy compatibility — NTCA 2024/2025 advises against the 1/2"×1/2" trowel.
Example:
1/4"×3/8" notch over CBU with 8–12" tile: C = 55 sq ft per 50-lb bag at the conservative band, 70 sq ft at the manufacturer midpoint.
Substrate sets the floor: A118.1 over Schluter on concrete, A118.4 over CBU, A118.11 over plywood. Tile size raises the bar: ≥15" requires LFT-rated A118.4-H (or A118.15-H). Wet, exterior, and submerged force A118.15. Glass and panel force a white modified product.
Example:
18×18 porcelain over CBU on a residential floor: A118.4HTE (modified, large/heavy, sag-resistant, extended open time).
ANSI A108.5 §2.2.3 mandates 80% mortar contact with no void over 2 sq in. The TCNA stone bulletin and ANSI for exterior / wet raise that to 95%. ANSI A108.19 sets ≥ 85% per sq ft for gauged porcelain panels with no voids within 2" of corners on natural stone.
Example:
Standard porcelain on a CBU bathroom floor: 80% spec. Same tile on a shower floor or natural stone anywhere: 95% spec.
Per Schluter Systems FAQ quoting the TCNA Handbook, modified mortar trapped between two impervious layers can take 14–60 days to dry. Unmodified Portland-cement above Ditra/Kerdi cures by hydration and is the only category Schluter approves (along with Schluter SET / ALL-SET / FAST-SET).
Example:
Plywood subfloor + Ditra: bag #1 A118.11 (1/4"×1/4" trowel) to bond Ditra. Bag #2 A118.1 unmodified (1/4"×3/8" or larger by tile) to set tile.
Notch volume assumes 50% fill of the plane after ridge collapse. Larger notches deposit more mortar per square foot — and lower the bag coverage proportionally. Match notch size to the back-butter requirement, not just to the tile face.
Example:
1/4"×3/8" notch lays ~22 ci/sq ft → 22 × 144 ÷ 1728 ≈ 1.83 cubic feet per 100 sq ft.
Type 1 passes 7-day water-immersion shear ≥50 psi but is rated only for intermittent wet (e.g. residential tub-surround above splash). Type 2 is dry only. Mastic cures by water evaporation and fails between two impervious layers.
Example:
Subway tile on a powder-room wall: mastic OK. Same tile on a shower wall or kitchen floor: mortar required.
ANSI E suffix means ≥30-min open time at 75°F / 50% RH. Real-world open time in a hot, dry attic at 95°F drops to 10–15 min. Shade the area, pre-dampen porous substrates, and mix smaller batches.
Example:
A118.4E mortar at 75°F has 30 min open time; the same product at 95°F open is closer to 12–15 min — recompute waste accordingly.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| ANSI Coverage Floor | 80% interior dry / 95% wet / 85% gauged porcelain panel | ANSI A108.5 §2.2.3 + A108.19 + TCNA stone bulletin. No void may exceed 2 sq in (golf-ball diameter). Minimum 3/32" bed thickness after embedment. |
| LFT Threshold | Any tile with at least one edge ≥ 15" (380 mm) | Per ANSI A108.02 §4.1.4.3.1. Large-format tile requires substrate flatness ≤ 1/8" in 10 ft and ≤ 1/16" in 2 ft, plus an A118.4-H or A118.15-H mortar. |
| Lippage Allowance | 1/32" + warpage (1/16"–<1/4" joints) · 1/16" + warpage (≥1/4" joints) | ANSI A108.02 §4.3.7. Tighter joints have less room to absorb tile warpage and substrate variance. |
| Back-Butter Multipliers | 1.00 / 1.15 / 1.25 / 1.30 | Tile <8" interior dry / 8–15" or 95%-spec / LFT ≥15" or ungauged stone / gauged panel or large glass. Direct multiplier on bag count via the calculator formula. |
| Standard Bag Sizes | 50 lb (default) · 30 lb lightweight · 25 lb micro / sand-free · 11 lb retail | Lightweight 25 lb (Laticrete Titanium / MULTIMAX) and 30 lb (Custom MegaLite / ProLite, Laticrete Tri-Lite) bags yield the same coverage as conventional 50 lb. Epoxy ships in 1.8-gal kits or 10-lb units. |
| Open Time / Pot Life (A118.4E) | 30 min open · 2–3 hr pot life · 15–20 min adjustability | All values at 75°F, 50% RH. Fast-set (F suffix) drops pot life to 45–90 min and adjustability to 5–10 min. |
| Conservative Coverage Defaults (sq ft per 50-lb bag) | 80 (1/4") / 55 (1/4"×3/8") / 38 (1/2") / 30 (3/4" U) | Calculator default is the manufacturer low-end × 0.85 to account for substrate texture, trowel wear, and back-butter material loss. Toggle to "standard" for manufacturer midpoint. |
| Tile-on-Tile Bond Coat | Modified A118.4 per TCNA F250 | Existing glazed tile must be sanded / abraded for mechanical bond. Confirm the existing installation is sound (no hollow tiles) before bonding to it. |
| Submersion Cure | 21–28 days (A118.15) / 72 hr (Granirapid + Ultracolor Plus FA per Mapei TDS) / 7–14 days (A118.3 epoxy) | Pool plaster / submerged installations must cure before water fill. Mapei's published 72-hour figure is product-system-specific; verify against your selected product's current TDS. |
| Freeze-Thaw Cycles | 20 cycles (ANSI A118.15 minimum) · 150+ cycles (ARDEX X 90 OUTDOOR) | Exterior installations require freeze-thaw certification. ARDEX Americas product page lists 150+ cycles for X 90 OUTDOOR — 7× the ANSI minimum. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ANSI A108.5 — Installation of Ceramic Tile with Dry-Set Cement Mortar or Latex-Portland Cement Mortar(ANSI A108.5)
View StandardThe North American installation standard for thinset-applied ceramic, porcelain, and stone tile. §2.2 governs mortar application: keying, combing direction, embedment, minimum bed thickness after embedment, and the 80% / 95% coverage requirement.
Key Requirements:
- •§2.2.2 Minimum 3/32" bed thickness after embedment; no skinned-over mortar
- •§2.2.3 ≥ 80% mortar contact for interior dry installations
- •§2.2.3 ≥ 95% mortar contact for exterior, interior-wet, and natural stone (per TCNA stone bulletin)
- •Maximum void area not to exceed 2 sq in anywhere in the field
- •Comb in one direction with the flat side keyed first; embed perpendicular to ridges to collapse them
ANSI A108.02 — General Requirements: Materials, Environmental, and Workmanship(ANSI A108.02)
View StandardCompanion standard defining substrate tolerances, large-format tile (LFT) handling requirements, and lippage allowances. §4.1.4.3.1 is the LFT clause that drives the calculator's H-suffix logic.
Key Requirements:
- •§4.1.4.3.1 LFT defined as any tile with at least one edge ≥ 15" (380 mm)
- •LFT substrate flatness: ≤ 1/8" in 10 ft (3 mm in 3 m) and ≤ 1/16" in 2 ft (1.6 mm in 0.6 m)
- •Standard tile (<15") substrate flatness: ≤ 1/4" in 10 ft and ≤ 1/16" in 1 ft
- •§4.3.7 Lippage allowance: 1/32" + tile warpage for joints 1/16" to <1/4"; 1/16" + warpage for joints ≥1/4"
- •§4.3.8.2 Offset patterns >33% on tiles with side >15" require owner-approved mock-up
ANSI A118.1 / .4 / .11 / .15 — Cementitious Tile-Setting Mortars(ANSI A118 series)
View StandardThe shear-bond hierarchy. A118.1 is unmodified dry-set Portland-cement; A118.4 is modified; A118.11 adds plywood-bond performance; A118.15 is the improved-modified high-bond class. Suffixes H (heavy/large), T (sag-resistant), E (extended open time), F (fast-setting), P (plywood-bonded) qualify any class.
Key Requirements:
- •A118.1 28-day shear ≥ 150 psi porcelain, ≥ 200 psi glazed wall (unmodified, hydration cure)
- •A118.4 28-day shear ≥ 200 psi porcelain, ≥ 300 psi glazed wall (modified, polymer-film cure)
- •A118.15 28-day shear ≥ 400 psi porcelain, ≥ 450 psi glazed wall (improved modified)
- •A118.11 plywood-bond performance overlay — required for direct-to-plywood installations
- •Suffix H = no slump under tile weight (LFT mandatory). T = ≤ 0.02" slip in 20 min for vertical. E = ≥ 75 psi at 30-min reskin. F = final set < 4 hr, 50 psi at 4 hr.
ANSI A118.3 — Chemical-Resistant, Water-Cleanable Tile-Setting Epoxy Mortar(ANSI A118.3)
View StandardTwo-part epoxy mortar standard. Required for swimming pools, chemical-exposure floors, green marble (water-sensitive), and resin-backed mosaic. Tile-to-tile shear ≥ 1,000 psi.
Key Requirements:
- •Tile-to-tile shear strength ≥ 1,000 psi at 28 days
- •Pot life 45–60 min at 70°F (drops sharply with temperature)
- •Submersion ready 7–14 days after installation
- •Strict resin / hardener mix ratio per manufacturer — no field adjustment
- •Required for green marble / serpentine and resin-backed natural stone (water warps these stones)
ANSI A136.1 — Organic Adhesives for Installation of Ceramic Tile(ANSI A136.1)
View StandardThe mastic standard. Type 1 passes 7-day water immersion shear ≥ 50 psi (intermittent wet rated). Type 2 is dry only. Cures by water evaporation — fails between two impervious layers.
Key Requirements:
- •Type 1: ≥ 50 psi shear after 7-day water immersion (residential tub-surround above splash OK)
- •Type 2: dry-only rating
- •Maximum tile size 12"×12" wall, 8"×8" floor
- •Never permitted in showers, steam rooms, exterior, swimming pools, or floors > 8"×8"
- •Substrate and tile must permit drying (water evaporation cure)
ANSI A108.19 — Installation of Gauged Porcelain Tile and Gauged Porcelain Tile Panels / Slabs(ANSI A108.19)
View StandardSpecialty standard for the 3 ft+ gauged porcelain panels and slabs (e.g. Laminam, MaxFine, Neolith). Coverage spec is per square foot, flat-back trowel both sides, two-person handling minimum.
Key Requirements:
- •≥ 85% mortar contact per square foot — measured locally, not averaged
- •Voids ≤ 2 sq in maximum
- •No voids within 2 in of corners on natural stone or panel edges
- •Flat-back trowel applied to BOTH the substrate AND the back of the panel
- •Comb at the panel only after both sides are flat-coated; do not double-comb
NTCA 2024/2025 Reference Manual — Mortar Coverage and Trowel Selection(NTCA Reference Manual 2024/2025)
View StandardThe North American Tile Contractors Association field-installation manual. The 2024/2025 edition contains the precautionary statement against the 1/2"×1/2"×1/2" square-notch trowel, the relaunched Trowel & Error video method, and updated trowel-size recommendations matched to tile-size brackets.
Key Requirements:
- •1/2"×1/2"×1/2" square-notch precautionary statement: wide notch spacing prevents proper ridge collapse, especially with 1/16"–1/8" grout joints
- •Substitute the 3/4"×9/16"×3/8" U-notch held at 30° for equivalent volume with cleaner collapse
- •Trowel & Error verification: pull a tile after embedment to verify ≥ 80% / 95% coverage
- •Comb in one direction; embed perpendicular to the ridges with downward pressure
- •Re-comb if the bed has skinned over — never wet-back with water
TCNA Handbook for Ceramic, Glass, and Stone Tile Installation(TCNA Handbook)
View StandardAnnual reference handbook from the Tile Council of North America. Each installation method is keyed (F-series for floors, B-series for showers / wet, W-series for walls). Methods specify the required mortar class, membrane, substrate, and grout for each assembly.
Key Requirements:
- •F101 / F102 mortar-set pavers on cured concrete slab using ASTM C270 Type S mortar
- •F250 tile-on-tile bond coat using A118.4 modified per TCNA
- •B414 / B415 / B420 traditional shower with clamping-ring drain — pre-slope required
- •B421 / B422 topical bonded waterproofing (Schluter Kerdi, Laticrete Hydroban, USG Durock) — no pre-slope
- •Schluter Ditra / Kerdi: unmodified A118.1 above the membrane to set tile (or Schluter SET / ALL-SET / FAST-SET)
ANSI A137.2 — Specifications for Glass Tile (under revision)(ANSI A137.2)
View StandardGlass tile classification, dimensional, and physical-property standard. Drives the white-mortar requirement (gray bleeds through translucent glass) and the 3/16" V-notch / minimum-mortar-transfer rule for mosaic glass. The 2022 publication is currently under revision per TCNA's official ANSI Standards page.
Key Requirements:
- •White A118.4ET modified mortar mandatory for translucent glass tile
- •Mosaic V-notch (3/16"×5/32") to minimize show-through on smooth-back glass
- •Back-butter all glass tile > 2" with the flat side of the trowel
- •Matched products: Laticrete 254 Platinum White, Mapei Adesilex P10 + Keraply, Bostik Glass-Mate, Custom Glass Tile Premium
- •Confirm current ANSI A137.2 edition before specifying — 2022 version is under active revision
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Climate, Open Time, and Pot Life
Hot dry climates compress every mortar timing window — order more mortar and mix smaller batches
ANSI E-suffix open-time minimums are tested at 75°F / 50% RH. Real-world hot dry conditions drop open time to roughly half published values. Pot life shrinks the same way. Cold conditions stall hydration and prevent polymer-film coalescence, which causes modified mortars to fail outright below 50°F.
Regional Examples:
Substrate Tolerance and the LFT Flatness Floor
Tile size dictates substrate prep, not the other way around
ANSI A108.02 §4.1.4.3.1 sets two flatness floors: 1/4" in 10 ft for tile under 15" and 1/8" in 10 ft for LFT (≥15"). Going from a 12×12 to a 12×24 tile on the same floor often forces a self-leveling underlayment pour. The calculator's H-suffix flag is a red flag for substrate prep, not just mortar choice.
Regional Examples:
Schluter and the Modified-Mortar-Trap Problem
Two impervious layers + polymer = drying period of 14 to 60 days
Schluter Systems' published rule (per their FAQ quoting the TCNA Handbook) is: standard polymer-modified mortars between Ditra/Kerdi and tile cannot dry. Drying time depends on geography, humidity, and whether the install is interior or exterior — anywhere from 14 days to over 60. Solution: unmodified A118.1 above the membrane, modified A118.11 below to bond to plywood.
Regional Examples:
Glass, Stone, and the White-Mortar Rule
Translucent and water-sensitive materials mandate specific mortar formulations
Glass tile shows whatever color is behind it — gray Portland cement bleeds through and ruins the install. Light-colored marble and limestone behave the same way at the joints. Green marble, serpentine, and white moisture-sensitive marble warp under water-based mortars and require A118.3 epoxy.
Regional Examples:
Wet, Submerged, and Exterior Freeze-Thaw
Three escalations beyond standard A118.4 — submersion, exterior freeze-thaw, and pool chemistry
Standard A118.4 modified is fine for residential interior. Anything wet, exterior, or submerged escalates to A118.15 or A118.3 epoxy. Cure timing matters: ANSI A118.15 must reach full cure (21–28 days at 70°F) before water fill, with limited fast-cure exceptions per specific product TDS.
Regional Examples:
Mastic Misuse and the Ban-Worthy Edge Cases
Where mastic survives in 2026 — and where it absolutely doesn't belong
ANSI A136.1 Type 1 mastic is still legal for interior dry walls with tile ≤ 12"×12" wall and ≤ 8"×8" floor. Real-world misuse — showers, kitchen backsplashes behind ranges, large-format wall tile, exterior siding tile — is the most common DIY thinset failure. The calculator blocks mastic on any wet substrate or any tile > 12".
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Share this calculator with your customers
Run a contractor site, supply store, or blog? Drop this free tile mortar calculator straight onto your own page so your visitors can estimate materials without leaving. No signup, works on any website — just copy and paste.
Want to Learn More?
Pick the right ANSI mortar (A118.1/.4/.11/.15/.3 or A136.1 mastic) for substrate and tile, choose a trowel notch, and calculate bag count.
Read the Mortar & Thinset Guide
Heavy material — watch the weight limit
Concrete, brick, and masonry hit tonnage caps fast. Most dumpsters cap heavy material at 10 tons, and overage fees stack quickly. See the disposal guide before you load.
Read the heavy-debris guide →
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How to Use This Calculator
- Enter the project area: length × width in feet (subtract cabinets, fixtures, and openings before entering).
- Pick the substrate: drywall, cement backer board, plywood, concrete slab, Schluter Ditra over concrete, Schluter Ditra over plywood (two-mortar plan), liquid waterproof membrane, SLU, heated floor, pool / submerged, or exterior / freeze-thaw.
- Pick the tile size category: glass mosaic, subway, 8–12", 12–15", LFT 15–24", LFT 24"+, gauged porcelain panel, glass ≥ 6", or natural stone. The category drives required ANSI suffixes (H for LFT) and back-butter multiplier.
- Choose a trowel notch (or leave on auto). The calculator auto-recommends a notch by tile size — V-notch for mosaics, 1/4"×1/4" for subway, 1/4"×3/8" for 8–12", 3/4" U-notch for LFT, 1/2" Euro for panels.
- Set application flags: vertical / wall (adds T suffix), extended open time (E suffix), fast-set (F suffix), epoxy (force A118.3), mastic (force A136.1 — only valid on dry walls with small tile), force ≥ 95% coverage (auto-on for stone / panel / wet / exterior).
- Pick a bag size: 50 lb standard, 30 lb lightweight (MegaLite / ProLite / Tri-Lite), 25 lb micro (Titanium / MULTIMAX), 11 lb retail, or epoxy 1.8-gal kit / 10-lb unit.
- Click Calculate: see the recommended ANSI class with suffixes, the coverage spec (80% or 95%), the back-butter multiplier, the chosen trowel notch with conservative + standard coverage bands, and bag count. Schluter-over-plywood automatically returns a two-mortar plan: A118.11 below the membrane plus A118.1 above to set tile.
Why the bag count uses conservative coverage by default
Manufacturer TDS coverage values are nominal — published as ranges (e.g. "80–95 sq ft per 50-lb bag at 1/4"×1/4""). Real-world yield runs 10–20% below the midpoint when ≥ 80% coverage is verified by the NTCA Trowel & Error method. The calculator defaults to manufacturer low-end × 0.85 to absorb substrate texture, trowel wear, ambient temperature, back-butter material loss, and the gap between published and field-verified coverage. A "Standard coverage" toggle switches to manufacturer midpoint if you want the optimistic number for budgeting. Either way, every line item shows the recommended trowel, the conservative-and-standard coverage band, the back-butter multiplier applied, and the adjusted square footage so you can see exactly how the bag count was derived.
Frequently Asked Questions
How much thinset mortar do I need for tile?
Bags = ⌈ (Area × back-butter multiplier × (1 + mortar waste)) ÷ coverage per bag ⌉. For 100 sq ft of 12×24 porcelain over CBU at a 1/4"×3/8" notch, conservative coverage: 100 × 1.15 × 1.10 ÷ 55 = 2.30 → 3 bags of A118.4HTE. Use the conservative coverage band by default — TDS midpoints are optimistic in real conditions. Bags are sold whole, so always round up. The calculator handles all of this automatically and shows the back-butter multiplier so you can see how the count was derived.
What ANSI mortar class do I need for my tile and substrate?
Substrate sets the floor and tile size raises the bar. A118.1 unmodified above Schluter Ditra/Kerdi (always). A118.11 modified for direct-to-plywood. A118.4 modified is the workhorse for cement backer board and concrete with everyday tile. A118.4 with H/T/E suffixes (large/heavy, sag-resistant, extended open time) for tile ≥ 8" or LFT. A118.15 improved-modified for tile ≥ 24", gauged porcelain panels, exterior, submerged, glass ≥ 6", or commercial 450-psi shear specs. A118.3 epoxy for pools, chemical exposure, green marble, or resin-backed mosaic. A136.1 mastic ONLY for interior dry walls with tile ≤ 12"×12" wall and ≤ 8"×8" floor — never showers or floors larger than 8"×8". The mortar class diagram matches the ANSI class to your substrate.
What trowel size should I use for my tile?
Tile size drives notch size — match longest tile edge to notch volume. Glass mosaic ≤ 2": 3/16"×5/32" V-notch. Subway and tile up to 6"–8": 1/4"×1/4" square. 8"–12" tile: 1/4"×3/8" square. 12×24 to 18×18: 1/2"×1/2" or 3/4"×9/16"×3/8" U-notch. 24×24 and 24×48 (LFT): 3/4" U-notch held at 30°. Gauged porcelain panels: 1/2" Euro / slant notch with flat-back trowel both sides. The NTCA 2024/2025 Reference Manual now warns against the 1/2"×1/2" square notch — wide notch spacing prevents proper ridge collapse, especially with 1/16"–1/8" grout joints. Custom Building Products explicitly recommends substituting the 3/4" U-notch for equivalent volume with cleaner collapse. The trowel coverage diagram pairs notch size to tile size.
Can I use modified thinset over Schluter Ditra?
No — not standard polymer-modified mortars. Schluter Systems explicitly prohibits A118.4 modified above Ditra and Kerdi. The reason, quoting the TCNA Handbook: modified mortars cure partly by drying (latex-film coalescence) and trap moisture between two impervious layers (the membrane and porcelain tile), with drying periods that range from 14 to 60 days depending on geography and climate. Use unmodified A118.1 above Ditra/Kerdi to set tile, OR Schluter's own SET, ALL-SET, or FAST-SET. Over plywood, you need a TWO-MORTAR plan: A118.11 modified BELOW the membrane to bond to plywood (plywood needs polymer to bond), then A118.1 unmodified ABOVE the membrane to set tile. The calculator returns both bag counts when you pick the Schluter-over-plywood substrate.
What is the 80% / 95% coverage rule and how do I verify it?
ANSI A108.5 §2.2.3 mandates ≥ 80% mortar contact for interior dry installations and ≥ 95% for exterior, interior-wet, and natural stone (per the TCNA stone bulletin). No void may exceed 2 sq in (about a golf-ball diameter) and the bed must be ≥ 3/32" thick after embedment. Gauged porcelain panels per ANSI A108.19 require ≥ 85% per square foot. Verification is the NTCA Trowel & Error method: pull a tile after embedding and look at the back. If you see uncollapsed ridges, voids over 2 sq in, or bare spots near corners, the trowel is undersized, the bed has skinned over, the tile wasn't back-buttered, or you didn't embed perpendicular to the ridges. Re-comb the bed and reset. Comb in ONE direction; embed perpendicular to ridges with downward pressure to collapse them. The coverage rule diagram shows collapsing the combed ridges.
What does "back-butter" mean and when do I have to do it?
Back-buttering is troweling a thin skim of mortar onto the back of the tile with the FLAT side of the trowel before setting it into the combed mortar bed. It fills back-of-tile texture (lugs, ridges, voids) and is the single biggest factor in achieving spec coverage on tile larger than 8". Required: any tile ≥ 8", any tile in a 95%-coverage installation (wet, exterior, natural stone), any LFT (≥ 15"), all gauged porcelain panels, all glass tile ≥ 2", and all ungauged stone. Calculator multipliers: 1.00 for tile under 8" interior dry, 1.15 for 8–15" or 95% spec, 1.25 for LFT, 1.30 for gauged porcelain panels and large glass — directly applied to your bag count.
Can I use mastic in a shower or on a kitchen floor?
No — and this is the most common DIY thinset failure. ANSI A136.1 mastic (Type 1 or Type 2) is permitted ONLY on interior dry walls with tile ≤ 12"×12" wall and ≤ 8"×8" floor. Type 1 passes 7-day water immersion shear ≥ 50 psi, which qualifies it for residential tub-surround above the splash line, but explicitly excludes showers, steam rooms, exterior, swimming pools, and floors larger than 8"×8". Mastic cures by water evaporation and fails between two impervious layers — within 12–18 months in a shower, it re-emulsifies and tiles slide off the wall in sheets. Use modified A118.4 thinset and a topical waterproofing membrane in any wet area. The calculator blocks mastic on any wet substrate or any tile category larger than 12".
What's the difference between A118.4 and A118.15 modified mortars?
A118.4 is the standard modified dry-set thinset class — 28-day shear strength ≥ 200 psi porcelain / ≥ 300 psi glazed wall. A118.15 is the improved-modified high-bond class — 28-day shear ≥ 400 psi porcelain / ≥ 450 psi glazed wall, more than double the bond. Use A118.4 (with H/T/E suffixes as needed) for residential floors and walls with everyday tile. Step up to A118.15 for tile ≥ 24", gauged porcelain panels, exterior with freeze-thaw, submerged or pool, glass tile ≥ 6", or any commercial spec calling for 450-psi shear. A118.15 mortars almost always meet A118.11 (plywood-bond performance) but not the reverse — read the actual TDS rather than assuming. Premium A118.15 examples: Laticrete 254 Platinum, MULTIMAX Lite, Mapei Granirapid, ARDEX X 77, Custom MegaLite / ProLite.