Rebar Calculator
How much rebar do you actually need? This free rebar calculator gives DIY homeowners and concrete pros the exact reinforcing-steel take-off for a slab, driveway, footing, or round pad: bar count in each direction, total linear feet and meters, weight in pounds and tons, the number of 20, 40, or 60 ft sticks to buy, and the lap-splice length — all from one form.
It runs on verified ASTM A615 nominal bar weights and the canonical grid formula bars = floor((dimension − 2 × cover) ÷ spacing) + 1, applied in both directions with proper edge cover. Project presets auto-fill safe non-structural spacing and cover for sidewalks, patios, driveways, garage slabs, and footings, and the spacing check flags anything over the 18-inch ACI maximum.
This is a material estimator, not a structural design tool. Bar size, spacing, and layout for any load-bearing element must come from stamped drawings, local code, or a licensed engineer. No pricing, no signup — just the quantities you take to the supplier.
Rebar Calculator
Estimate reinforcing steel for slabs, driveways, footings, and round pads — exact bar count by direction, total linear feet, weight in pounds and tons, the number of 20-ft sticks to buy, lap-splice length, plus tie-wire and chair estimates. Built on verified ASTM A615 bar weights and the ACI 318-19 spacing-and-cover formula. Free, no signup.
Layout & bar
See what the bar number and weight mean
ACI 318-19 §20.6 cover: 3" cast against earth (footings), 2" formed & exposed to weather, ¾" for interior slabs not exposed.
Lap, waste & stock length
Reading a rebar take-off
Three things trip up a rebar order: what the bar number and weight actually mean, how to count the bars without dropping the one at the far edge, and the placement details that keep the steel working — cover and lap splices. 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 size chart is why the calculator can turn a bar number into a weight. The number is the diameter in eighths of an inch, and weight per foot is fixed by size regardless of grade, since grade is yield strength, not thickness. That is why you pick a bar number for spacing and a grade for strength independently.
The grid-count diagram explains the plus-one rule behind the bar count. You subtract cover from both edges, divide the clear span by the spacing, and add one for the far-edge bar, in each direction, keeping spacing within the maximum. That plus-one is why a slab needs one more bar per direction than a simple span-over-spacing guess.
The cover-and-lap detail is why the estimate includes chairs and extra bar for laps. Bar is held up on chairs to keep its concrete cover, and where a run is spliced the bars lap a set multiple of their diameter, staggered and tied. That lap length is real bar the calculator adds — steel that never spans new ground but is required at every splice.
Calculation Formulas
The clear span is the dimension minus cover on BOTH edges. Dividing by spacing and adding 1 counts the bar sitting at the far edge. Omitting the '+1' under-counts; ignoring edge cover gives the wrong bar length. This is the single most common error in competitor tools.
Example:
20 ft slab, 12" spacing, 2" cover: clear span = 240 − 4 = 236"; bars = floor(236 ÷ 12) + 1 = 19 + 1 = 20 bars.
Each bar spanning the length is cut to the clear length; each bar spanning the width to the clear width. Bars spanning the length are spaced across the width, and vice-versa. Multiply by the number of mats (2 for top + bottom face).
Example:
20 × 20 ft slab, #4 @ 12", 2" cover: 20 bars × 19.67 ft + 20 bars × 19.67 ft ≈ 787 LF for one mat.
Use the published nominal weight per foot — not a live density derivation — because rib deformations and mill tolerance make the table the ordering standard (accurate to ≈ ±2%). Weight is identical across Grade 40/60/80/100 for a given bar size.
Example:
787 LF of #4 (0.668 lb/ft) ≈ 526 lb ≈ 0.26 tons.
Derived from steel density (490 lb/ft³). Shown for understanding only — the calculator always serves the verified lookup table for ordering.
Example:
#4 (0.5"): 2.67 × 0.5² = 0.668 lb/ft, matching the table.
Field rule of thumb for Grade 60 in ~30 MPa concrete. ACI 318-19 Class B tension lap = 1.3 × development length; Class A = 1.0 × ℓd. Bars larger than #11 are generally not lap spliced. Stagger splices and tie every lap.
Example:
#5 bar (0.625"): lap = 40 × 0.625 = 25". Common field values: #4 ≈ 20–24", #5 ≈ 25–30", #6 ≈ 30–36".
Laps are only needed when a single continuous run exceeds one stock length. When every bar fits within a stick, the full stock length is usable. Rebar is sold in whole 20, 40, and 60 ft lengths — always round up.
Example:
526 lb of #4 over 787 LF, +10% waste = 866 LF; bars are 19.67 ft so they fit a 20 ft stick — ⌈866 ÷ 20⌉ = 44 sticks.
A continuous footing or grade beam splices each longitudinal bar wherever the run exceeds a stock length. Multiply by the number of continuous bars (commonly 2–4).
Example:
60 ft run, #5, 20 ft sticks, 25" lap: usable = 17.9 ft → 4 pieces, 3 laps; each bar = 60 + 3 × 2.08 ≈ 66.25 ft.
Bars are spaced across the diameter; each bar's length is the chord at that offset by the Pythagorean theorem. The center bar is the full 2R. Two-way layouts double the total for a perpendicular band.
Example:
10 ft round pad, #4 @ 12", 3" cover: R = 57"; center 114" + chords 111.4 + 103.4 + 88.3 + 61.5 (×2) ≈ 70.3 ft one way, ≈ 140.6 ft two-way.
One tie per intersection is the field default (some specs every other, or four per intersection for seismic). Chairs / dobies support the mat off the dirt at roughly 4 ft on-center both ways. 16-gauge tie wire, ~0.75 ft per tie.
Example:
20 × 20 bar grid = 400 intersections ≈ 400 ties; chairs at 4 ft = 6 × 6 = 36.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| #3 Bar — 3/8" | 0.376 lb/ft · 0.11 in² | Smallest common size. Sidewalks, light patios, and as ties/stirrups. 11.3 mm soft-metric label is #10S, not a new bar. |
| #4 Bar — 1/2" | 0.668 lb/ft · 0.20 in² | The residential workhorse — driveways, garage slabs, footings, foundation walls. The default for most flatwork. |
| #5 Bar — 5/8" | 1.043 lb/ft · 0.31 in² | Heavier footings, grade beams, retaining walls, and slabs under vehicle or equipment load. ≈ 15M (CSA). |
| #6 Bar — 3/4" | 1.502 lb/ft · 0.44 in² | Structural footings, columns, and walls. Note a #6 ≈ 20M but differs ~6% in area — never silently equate them. |
| Concrete Cover — Cast Against Earth | 3 in (75 mm) | ACI 318-19 §20.6 and IRC R403 minimum for footings poured directly against soil. Protects steel from corrosion. |
| Concrete Cover — Formed, Exposed | 2 in (#6+) / 1.5 in (#5 & smaller) | Formed surfaces exposed to weather or earth. Interior slabs/walls not exposed drop to 3/4" (#11 & smaller). |
| Max Spacing — Temp/Shrinkage | ≤ 3 × thickness or 18 in | ACI 318-19 §24.4.3.2 crack-control limit for slabs. The calculator flags spacing over 18". Minimum steel ratio ρ = 0.0018 (Grade 60). |
| Lap Splice (field rule) | 40 × bar diameter (min 12 in) | Conservative Grade 60 planning value. ACI Class B = 1.3 × development length. Bars over #11 are generally mechanically coupled, not lapped. |
| Stock Lengths | 20 / 40 / 60 ft | 20 ft is the common retail length; 40 and 60 ft are supply-house / commercial. A 20-ft #18 weighs ~272 lb and is crane-handled. |
| Standard 90° Hook Extension | 12 × bar diameter | ACI 318-19 Table 25.3.1 tension hook. 180° hook extension = greater of 4 d_b or 2.5". Minimum inside bend diameter 6 d_b for #3–#8. |
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.
ASTM A615 / A615M — Deformed Carbon-Steel Reinforcing Bar(ASTM A615)
View StandardThe general-construction material spec covering deformed carbon-steel bar sizes #3–#18. Defines nominal dimensions, deformation requirements, and grade (yield) classes. The nominal weights this calculator uses for ordering come straight from A615.
Key Requirements:
- •Bar # = diameter in eighths of an inch for #3–#8 (#4 = 4/8" = 0.500")
- •Grades 40 [280], 60 [420], 80 [550], 100 [690] — Grade 75 was removed; substitute Grade 80
- •Grade 60 (60 ksi / 420 MPa) is the US structural default
- •Nominal weights: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb/ft (≈ ±2% real tolerance)
- •#2, #12, and #13 are NOT standard A615 sizes
ASTM A706 / A706M — Low-Alloy Weldable / Seismic Bar(ASTM A706)
View StandardLow-alloy reinforcing bar with controlled chemistry for welding and seismic ductility. Required for welded splices (ACI 318 §25.5.7) and in seismic design categories D–F. Identical nominal weights to A615.
Key Requirements:
- •Carbon equivalent (CE) ≤ 0.55% (A706 §6.4) for weldability
- •Covers Grade 60 and Grade 80
- •Required where bars are welded or for seismic detailing (SDC D–F)
- •Standard A615 Grade 60 should NOT be welded — use tie wire or specify A706
- •Dual-grade bars marked 'W' and 'S' meet both A706 and A615
ACI 318-19 — Building Code Requirements for Structural Concrete(ACI 318-19)
View StandardThe governing US standard for structural concrete, adopted by reference in the IBC. Sets concrete cover, minimum temperature/shrinkage steel and spacing, lap splice classes, hook geometry, and column minimums. The calculator's spacing/cover defaults and the 18" crack-control flag come from here.
Key Requirements:
- •§20.6 cover: 3" cast against earth, 2"/1.5" formed exposed, 3/4" interior not exposed
- •§24.4.3.2 temperature/shrinkage spacing ≤ lesser of 3× thickness or 18"; ρ ≥ 0.0018 (Grade 60)
- •§25.5 lap splices: Class B tension lap = 1.3 × ℓd, min 12"
- •§25.3 hooks: 90° extension = 12 d_b; 180° = greater of 4 d_b or 2.5"
- •§10.7.3 columns: min 4 bars (rectangular ties) / 6 bars (spiral)
IRC R403 — Residential Footings & Foundations(IRC R403)
View StandardThe residential code section for footing and stem-wall reinforcement minimums. Plain footings sized per Table R403.1 may need no rebar absent local amendments; reinforced footings follow the one-bar-top / one-bar-bottom convention this calculator uses for its footing preset.
Key Requirements:
- •One #4 bar within 12" of the top of the footing and one near the bottom (common minimum)
- •Seismic SDC D0–D2: add vertical #4 dowels at ≤ 4 ft o.c.
- •Frost depth is local-code driven, not computed — verify with the building department
- •Concrete cover for footings cast against earth = 3"
- •Local amendments frequently override — always confirm jurisdiction-specific rules
CRSI Manual of Standard Practice(CRSI MSP)
View StandardThe Concrete Reinforcing Steel Institute's fabrication and placing reference — bar identification/marking, standard hook and bend tables, and detailing conventions used by fabricators. Notes where CRSI fabrication is more conservative than the ACI code minimum.
Key Requirements:
- •Bar markings: mill ID → size → type letter (S = A615, W = A706) → grade marks
- •Standard hook and bend-diameter tables for fabrication
- •Stirrup/tie bend diameter: CRSI fabrication uses 5 d_b for #3–#5 vs ACI's 4 d_b code minimum
- •Tag and bundle conventions for shop tickets and field placement
- •Lap and development length tables by f'c and grade
CSA G30.18 / EN 10080 / AS-NZS 4671 — Regional Bar Systems(CSA G30.18 · EN 10080 · AS/NZS 4671)
View StandardMetric and regional reinforcing standards. The calculator uses US #-bars, but ordering abroad means a different designation system — and a US # never silently equals a metric mm bar, because the area differs.
Key Requirements:
- •Canada (CSA G30.18): 10M–55M soft-metric area basis; Grades 400W/500W (weldable)
- •Europe/UK (EN 10080 / BS 4449): 8–40 mm diameter, grades B500B/B500C, T/H prefix
- •Australia/NZ (AS/NZS 4671): N-bars (deformed, 500 MPa, D500N) N12–N40; R-bars plain round
- •10M ≈ #3/#4, 15M ≈ #5, 20M ≈ #6 (but a #6 vs 20M differs ~6% in area)
- •Soft-metric US labels (#13 = #4, #16 = #5) are the SAME physical bar, not a new size
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.
Frost Depth Drives Footing Geometry
Footing depth is local-code driven, not computed — and it changes your bar quantities
Footings must bear below the local frost line, so the same building needs far deeper (and more heavily reinforced) footings in a cold climate than a warm one. The calculator estimates steel for a footing run you define; the required depth and bar schedule come from your jurisdiction.
Regional Examples:
Seismic Design Category Changes the Detailing
SDC D–F requires A706 ductile bar, tighter ties, and more laps
In moderate-to-high seismic regions, reinforcement is not just sized for gravity — it is detailed for ductility. That means weldable A706 bar, closer tie/stirrup spacing, hooks at specific angles, and staggered laps the calculator does not auto-generate.
Regional Examples:
Bar System & Grade by Region
US #-bars vs metric — never auto-translate the designation
Outside the US, reinforcement is specified in soft-metric (Canada) or hard-metric (Europe, Australia) systems with different grades and stock lengths. A #5 ≈ 15M is close, but a #6 vs 20M differs ~6% in cross-section, so substitutions must be checked by area, not number.
Regional Examples:
When Rebar Isn't the Right Reinforcement
Fiber, welded wire mesh, and post-tension are different products
Not every slab gets a rebar grid. Light flatwork is often welded wire mesh; some slabs use fiber for crack control; and engineered slabs use stressed post-tension tendons. Treating these as interchangeable over- or under-builds the slab.
Regional Examples:
Cover, Coatings & Corrosion Exposure
Marine, de-icing, and below-grade exposure call for more cover or coated bar
Concrete cover is the first line of corrosion defense, and harsh exposures push it up or call for epoxy-coated, galvanized, or stainless bar. Coatings also add weight the calculator's bare-steel nominal values do not include.
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
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Want to Learn More?
How much rebar to order — bar sizes, spacing and cover defaults, the +1 grid formula, lap splices, and weight per foot from ASTM A615.
Read the How Much Rebar Do I Need? Size, Spacing & Weight
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
- Pick a project type — driveway, patio, garage slab, footing, or round pad — to auto-fill a safe starting bar size, spacing, and cover.
- Choose the mode: two-way slab grid, continuous-bar footing, or circular slab.
- Enter your dimensions (length and width, or footing run, or diameter) and confirm the bar size, spacing, and concrete cover.
- Adjust the waste factor (default 10%), stock stick length (20/40/60 ft), and lap length if you need to override the 40 × diameter default.
- Read your results: bar count by direction, total linear feet, weight in lb and tons, sticks to buy, lap length, and tie-wire/chair estimates. Copy or print the take-off.
Understanding the Bar-Count Formula
Across any covered span the number of parallel bars is floor((dimension − 2 × cover) ÷ spacing) + 1. The "+1" counts the bar at the far edge, and cover is subtracted from both edges before dividing — the two places most tools slip. Bar length equals the opposite clear span, and the same logic runs in both directions for a slab. Weights are ASTM A615 nominal values (accurate to about ±2%) and are identical across Grade 40/60/80/100, because grade sets yield strength, not cross-section.
Frequently Asked Questions
How much rebar do I need for a slab?
Run the bar count in each direction: bars = floor((dimension − 2 × cover) ÷ spacing) + 1. For a 20 × 20 ft driveway with #4 bar at 12 inches on-center and 2 inches of cover, the clear span is 240 − 4 = 236 inches, so floor(236 ÷ 12) + 1 = 20 bars each way. Each bar is about 19.7 ft long, giving roughly 787 linear feet for a single mat — about 526 lb of #4, or 44 twenty-foot sticks with 10% waste. Enter your size and spacing above and the calculator does both directions, the weight, and the stick count for you.
What is the rebar bar-count formula?
The number of parallel bars across a covered span is floor((dimension − 2 × cover) ÷ spacing) + 1. The clear span is the slab dimension minus the concrete cover on both edges. Dividing by the on-center spacing and adding 1 counts the bar sitting at the far edge. The two most common mistakes — and the reason competitor tools disagree — are dropping the '+1' (which under-counts by one bar per direction) and forgetting to subtract edge cover (which gives the wrong bar length). This calculator applies one convention consistently in both directions. The grid-count diagram shows the +1 grid formula.
How much does #4 rebar weigh per foot?
A #4 bar (1/2 inch) weighs 0.668 lb per foot — so a 20-ft stick is about 13.4 lb. These are ASTM A615 nominal weights, the industry ordering standard, accurate to roughly ±2% because of rib deformations and mill tolerance. Other common sizes: #3 = 0.376, #5 = 1.043, #6 = 1.502, #7 = 2.044, and #8 = 2.670 lb/ft. The number stamped on the bar is its diameter in eighths of an inch (#4 = 4/8 = 1/2 inch). Weight is identical across Grade 40, 60, 80, and 100 — grade is yield strength, not cross-section. The rebar size chart shows the bar number as eighths of an inch.
What size rebar and spacing do I use for a driveway or garage slab?
For a residential driveway or garage slab carrying car and light-truck loads, #4 bar at 12 to 16 inches on-center is the durable standard, with 2 inches of cover; tighten to #4 at 12 inches for heavy trucks or RVs. Sidewalks and small patios often need only #3 at 18 inches — or welded wire mesh — if the base is good and control joints are cut. The presets above fill these in automatically. For any slab that carries a structure (footings, foundation walls, equipment pads), get the size and spacing from stamped plans or a licensed engineer rather than a rule of thumb.
How much overlap (lap splice) do I need for rebar?
The field rule of thumb is 40 times the bar diameter — so a #4 bar laps about 20 inches, a #5 about 25 inches, and a #6 about 30 inches, with a 12-inch minimum. ACI 318-19 defines a Class B tension lap as 1.3 times the development length; use the larger value when uncertain, stagger splices so they don't line up, and tie every lap. Bars larger than #11 are generally mechanically coupled rather than lapped. The calculator adds laps automatically when a continuous run exceeds one stock length, and you can override the lap length in the advanced options. The cover and lap diagram shows the 40× lap and required cover.
What is the standard spacing and cover for rebar in a slab?
For temperature and shrinkage steel, ACI 318-19 §24.4.3.2 limits spacing to the lesser of 3 times the slab thickness or 18 inches — the calculator flags anything over 18 inches. Concrete cover per §20.6 is 3 inches when cast directly against earth (footings), 2 inches for formed surfaces exposed to weather (1.5 inches for #5 and smaller), and 3/4 inch for interior slabs and walls not exposed to weather. Cover is the corrosion barrier, so keep the steel up on chairs and off the dirt — never lay bar directly on the subgrade.
Does this calculator include rebar prices or cost?
No. Like every calculator on the site, it is pricing-free — it gives you quantities (bar count, linear feet, weight, sticks, lap length, tie wire, and chairs), not dollar figures. Rebar prices move constantly by region, mill, and grade, so take your weight or linear-foot total to a supplier for a current quote. Pairing the weight with your own per-pound or per-ton price gives a quick budget; pair the slab dimensions with the Concrete Calculator to size the pour itself.
Can I use this for a footing or a round pad?
Yes. Switch the mode to 'Footing / grade beam' to estimate continuous longitudinal bars along a trench or perimeter — enter the total run and how many bars run in it, and the calculator splices each bar where it exceeds a stock length. Switch to 'Circular slab / pad' to lay out a round slab using the chord method: bars are spaced across the diameter and each is cut to the chord at that offset, with an optional perpendicular band for a two-way grid. Footing reinforcement minimums follow IRC R403, but confirm depth, frost line, and bar schedule with your local building department.