Rebar Calculator
Rebar quantity, spacing and lineal feet for a slab or footing grid.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Bars one way = ⌊ (span − 2 × edge clear) ÷ spacing ⌋ + 1
Run length per bar = slab dimension − 2 × edge clear, + lap length per splice
Total lineal feet = Σ (bars × run length); sticks = lineal feet ÷ stock length
Run the calculation to see each step with your own numbers.
20 × 20 ft slab, bars 18 in o.c. each way, 3 in edge clear, 2 ft laps, 20 ft stock, 5% waste.
Usable = 19.5 ft each way → bars = ⌊19.5 ft ÷ 18 in⌋ + 1 = 14 each way
Run per bar = 19.5 ft (fits one 20-ft stick, no lap)
Lineal = 14 × 19.5 + 14 × 19.5 = 546 ft → order 574 ft → 29 sticks; 196 intersections
- A single flat mat of straight bars each way. Follow the structural drawings for bar size, spacing, cover, lap length and whether a second (top) mat is required — this calculator counts one mat.
- Lap length defaults to ~2 ft (≈ 40 bar diameters for #4/#5); confirm against your specification and the bar size used.
- Bars are supported on chairs/dobies at the correct height — those are counted separately.
- Edge dowels, corner bars, and reinforcement around openings are extra.
- How far apart should rebar be in a slab?
- For a residential slab-on-grade, 16–18 in on centre each way with #3 or #4 bar is common, but spacing, bar size and cover must come from the engineered drawings for anything structural.
- How long is a lap splice?
- Roughly 40 bar diameters where two bars overlap end-to-end — about 20 in for #4 bar and 25 in for #5 — but the real figure is on the drawings and depends on the concrete strength and bar coating. Enter your specified lap.
- Does this include chair supports or a top mat?
- No. It counts one flat layer of steel. Chairs or dobies to hold the bar at the right height, edge dowels, corner bars and any second mat are separate.
Grid layout
Standard reinforcing-mat layout (CRSI Manual of Standard Practice): bars on-centre plus one, lap splices ≈ 40 db. Counting is standard geometry.
About the Rebar Calculator
The Rebar Calculator lays out a single flat mat of reinforcing bar for a slab or footing — the number of bars each way, the total lineal feet including lap splices, the number of stock lengths to buy, and the grid-intersection count for tie wire. Bar size, spacing, cover and lap length must come from the structural drawings for anything load-bearing; the calculator counts one mat, not a top-and-bottom cage.
What you enter
- Slab length
- Slab width
- Bar spacing (on-centre, both ways) — 12–18 in for slabs-on-grade; follow the structural drawings.
- Edge clearance (cover from each edge) — Concrete cover / clear distance from the slab edge to the first bar.
- Lap splice length — Where bars are spliced end-to-end. ≈ 40 bar diameters — ~20 in for #4, ~25 in for #5.
- Bar stock length — Rebar is commonly sold in 20 ft lengths.
- Waste allowance — Default 5%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Bars running lengthwise — the exact result of the calculation.
- Bars running widthwise — the exact result of the calculation.
- Total rebar (lineal, incl. laps) — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Stock lengths to buy — the exact result of the calculation.
- Grid intersections (tie every one or every other) — ≈ 1.5 in of tie wire per tie. A 3.3-lb roll of 16-ga ties ≈ 300+ intersections.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Concrete Calculator — How much concrete a rectangular slab or footing needs, in cubic yards or cubic metres.
- Concrete Footing Calculator — Concrete volume for strip and pad footings.
- Concrete Column Calculator — Concrete volume for round or square columns.
- Cubic Yard Calculator — Convert a length, width and depth into cubic yards, cubic feet or cubic metres.
Part of the Steel & Reinforcement calculators.