Calculators/Rebar mat

Rebar calculator for slabs

Bar count in both directions, total linear feet and total weight for a flat mat — at your spacing, your cover and your bar size.

The slab

Laps are not included. Required splice length depends on bar size, concrete strength, coating and the governing code — take it from your drawings.

The mat

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Check the result against the job. This does the arithmetic from the values you enter. Building codes, span tables and material specifications vary by jurisdiction and by product, and the governing document is your approved drawings and your local code. Nothing you type here leaves your browser: the calculation runs on your device and we never receive it.

How this is checked. Every formula on this site runs from one shared calculation engine, and every one has a test fixture whose expected answer was worked out from the geometry or taken from a figure the trade already publishes — never from what the code returns. Some land exactly, which is the point: a 6/12 hip over a 144″ run is exactly 216″, a 45° crown in a square corner bevels at exactly 30°, and a 30° conduit bend has a multiplier of exactly 2. Where a number is a convention rather than arithmetic — a code limit, a waste allowance, bundles per square — it is an input with a stated default rather than a hidden constant. Found something wrong? Tell us at [email protected] and we will fix it and say so.

The plus one, and the cover

Two things trip up a hand count. The first is that n spaces need n + 1 bars — the last bar closes the far side and is easy to leave out. The second is cover: the mat does not run to the edge of the slab, so the usable width is the slab less cover on both sides, and every bar is shorter than the slab dimension for the same reason.

Bar numbers are eighths

The bar number is its diameter in eighths of an inch. A #4 is four eighths, which is half an inch. A #8 is eight eighths, an inch. That is the whole system, and it is why the jump from #4 to #5 is a quarter-inch step in diameter but a 56% step in weight.

Weight matters as soon as anyone has to carry it. A 20 × 20 slab at 12 inches each way in #4 is a little over 500 pounds of steel — the difference between a two-person lift and a delivery to the pour.

Quantity is not design. Bar size, spacing, cover and lap all come from a structural drawing or a code-prescribed detail. This counts what a specified layout needs; it does not decide what the layout should be.

Common questions

How do you calculate rebar for a slab?

Take the slab dimensions, subtract the edge cover from each side, then divide the usable width by the spacing and add one for the bar that closes the far side. Do the same the other way for the bars running across, then multiply total linear feet by the weight per foot of the bar size.

How much does rebar weigh per foot?

A number 3 bar is 0.376 pounds per foot, a number 4 is 0.668, a number 5 is 1.043 and a number 8 is 2.67. The bar number is its diameter in eighths of an inch, so a number 4 is four eighths, which is half an inch.

Does this include lap splices?

No, deliberately. Required lap length depends on the bar size, the concrete strength, whether the bar is epoxy coated, and the governing code. A number invented here would be wrong more often than right, so add laps from your own drawings or specification.

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