Mortar / Thinset 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
For irregular footprints, enter the total square footage as length × 1 (e.g. 73 sq ft = 73 × 1).
Substrate and tile
A118.4 modified. Use T suffix on walls. · Back-butter recommended over 12"×12".
Trowel and bag size
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.
Application flags
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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".
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.
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.
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.