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Rebar in a slab is laid as a grid. In each direction the Rebar Calculator takes the slab dimension in inches, subtracts the edge cover at both sides, divides by the bar spacing, rounds down and adds one bar, and makes each bar the other dimension less the cover at both ends. The default cover is 3 in, the minimum in ACI 318-19 Table 20.5.1.3.1 for concrete cast against and permanently in contact with earth. Short bars are cut two or more to a stick; runs longer than a stick are lapped 40 bar diameters, a common field rule — structural work follows the drawings or ACI 318. Lineal feet are multiplied by the bar weight in ASTM A615 Table 1: 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5 and 1.502 for #6. A 20 × 20 ft slab with #4 bars at 12 in each way takes 20 bars each way, each 19.5 ft — 780 lineal feet, 40 sticks of 20 ft, 521 lb of steel and 400 intersections to tie.
Rebar for a slab is a grid: in each direction, bars = (dimension in inches − 2 × edge cover) ÷ spacing, rounded down, plus one, and each bar is the other dimension less the cover at both ends. ACI 318-19 Table 20.5.1.3.1 requires 3 in of cover for concrete cast against and permanently in contact with earth. Weight is lineal feet × the ASTM A615 bar weight. Runs longer than a stick are lapped; 40 bar diameters is a common field rule, but the drawings or ACI 318 govern structural work.
| Bar | Diameter | lb per ft | 20 ft stick | 40 d lap |
|---|---|---|---|---|
| #3 | 0.375 in | 0.376 | 7.5 lb | 15 in |
| #4 | 0.5 in | 0.668 | 13.4 lb | 20 in |
| #5 | 0.625 in | 1.043 | 20.9 lb | 25 in |
| #6 | 0.75 in | 1.502 | 30.0 lb | 30 in |
| #7 | 0.875 in | 2.044 | 40.9 lb | 35 in |
| #8 | 1 in | 2.67 | 53.4 lb | 40 in |
Lay out a grid: bars each way = (slab dimension in inches − 2 × edge cover) ÷ spacing, rounded down, plus one. Each bar is the other dimension less the cover at both ends. A 20 × 20 ft slab with #4 bars at 12 in and 3 in cover needs 20 bars each way, each 19.5 ft — 780 lineal feet, 40 twenty-foot sticks and about 521 lb of steel.
From ASTM A615 Table 1: #3 0.376 lb/ft, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.670 lb/ft. A 20 ft stick of #4 weighs about 13.4 lb; #5 about 20.9 lb.
Residential slabs and driveways commonly use #3 or #4 bars at 12 to 18 in each way, but slab-on-ground reinforcement is not set by a single code table — follow the plans or your engineer. The bar number is its diameter in eighths of an inch: #4 is 1/2 in.
ACI 318-19 Table 20.5.1.3.1 requires 3 in of concrete cover where concrete is cast against and permanently in contact with earth. Keep bars 3 in in from the forms at the edges, and support them on chairs or dobies so they end up in the slab, not on the ground.
A common field rule is 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5. It is a rule of thumb, not a code value — required lap lengths depend on bar size, steel grade and concrete strength, so structural slabs, walls and footings follow the drawings or ACI 318.
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