By Zencalcy Editorial TeamLast reviewed About us
A slab mat is two grids crossing each other. Count the bars in each direction from the spacing, add a lap allowance for the overlap at splices, and convert linear feet to weight with the bar's published weight per foot. This calculator does all three so you can order by the stick or by the ton.
Rebar mat quantity, linear feet and weight for a slab.
Bars = (L ÷ s + 1) + (W ÷ s + 1) Linear ft = bars × span × (1 + lap)
The +1 accounts for the bar at the far edge. A 10% lap allowance is typical for a slab with splices; add 5% waste. Bar weights are #3 0.376, #4 0.668, #5 1.043, #6 1.502 and #8 2.670 lb/ft.
| Symbol | Meaning |
|---|---|
| L, W | Slab dimensions. |
| s | Bar spacing, centre to centre. |
| N | Total number of bars. |
Each direction needs 16 bars (15 spaces plus one), so 32 bars of 20 ft = 640 linear feet. With 10% laps that is 704 ft, or 470 lb of #4 bar.
Continue with the Gravel Calculator, Sand Calculator, Rock Calculator, or the River Rock Calculator — or browse more materials & quantities and all construction calculators.
#3 or #4 at 16–24 inches on centre is typical for a residential slab on grade. Use #4 at 12 inches for a driveway.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026