Rebar calculator
This lays out a grid and totals the steel — counts and lengths only. It does not choose the bar size, the spacing or the cover for any load; those are set by an engineer or your local requirements, and a wrong guess there is not a wasted stick of bar but a failed slab. Order to a design you trust.
Enter the slab size, the spacing you were told to use and the edge clearance, and the tool lays out a two-way grid: how many bars run each way, the total length of bar to order, and how many intersections you will tie.
Worked example
A 20 ft × 12 ft slab, bars at 12 inches on centre, 3 inches clear of each edge. The usable field is 19.5 ft × 11.5 ft. Across the 11.5 ft width at 1 ft spacing that is 11 spaces, so 12 bars running the length at 19.5 ft each. Along the 19.5 ft length, 19 spaces means 20 bars running the width at 11.5 ft each. That is 234 + 230 = 464 ft of bar; add 10% for laps and you order about 510 ft, with 240 intersections to tie.
The formula
bars = floor(usable ÷ spacing) + 1 · length = Σ (bars × run) × (1 + waste)
Trim the clearance off each side to get the usable field, divide by the spacing and add one for the edge bar. Each set's total length is its bar count times how far it runs; the two sets add together, and the lap factor tops it up. Intersections are simply one count times the other.
What this tool assumes
A single rectangular slab and one spacing both ways. Spacing, clearance and the lap factor are all yours to set — nothing about the bar is fixed, because everything that matters structurally is a decision you bring to the page, not one it makes.
Questions
- How do I work out how much rebar a slab needs?
- Count the bars each way and add up their lengths. Along the width, a bar every so-many inches gives you a set running the length; along the length, another set runs the width. This tool takes your slab size, your spacing and your edge clearance and returns both bar counts, the total linear footage and the number of grid intersections to tie.
- Does this tell me what size rebar to use?
- No, and that is deliberate. Bar size and spacing for a structural slab come from an engineered design or your local requirements, never from a quantity tool. This page assumes you already know the spacing you were told to use and simply lays out the grid and totals the steel to order.
- Why the +1 in the bar count?
- Because a grid has a bar at both edges as well as in the gaps. If a run fits five spaces it holds six bars — the fencepost rule. The tool divides the usable width by the spacing, drops any part-space, and adds one, so the outer bars at each edge are always counted.
- What is the edge clearance for?
- Steel is kept back from the edge of the pour so concrete fully surrounds it — the cover. The clearance you enter is trimmed off each side before the grid is laid out, so the bars stop short of the forms and the counts reflect real, buildable positions rather than bars running out to the very edge.
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