Rebar Calculator
Bars each way = (side − 2 × edge clearance) ÷ spacing, rounded down, plus one; each runs the side less the clearance, plus laps. A 20 × 20 ft slab at 18 in on centre, 3 in clear, takes 14 bars each way: 546 lineal ft, or 29 sticks of 20 ft with 5% waste. Pour: Concrete Calculator.
How it's calculated: Bars one way = ⌊(span − 2 × edge clear) ÷ spacing⌋ + 1; run = span − 2 × edge clear + laps; lineal feet = bars × run, both ways; sticks = ⌈lineal feet × (1 + waste allowance) ÷ stock length⌉.
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 × (1 + waste) ÷ 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.
- How much rebar do I need for a 20×20 slab?
- At 18 in on centre with 3 in edge clearance: 14 bars each way, 546 lineal ft, or 574 ft with 5% waste — 29 bars of 20 ft stock. Closer spacing or a second mat adds proportionally more.
- How many rebar ties do I need?
- One per grid intersection if you tie every crossing — 196 for 14 bars each way — or about half that if you tie every other one. Allow roughly 1.5 in of tie wire per tie.
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.