A 12 × 20 ft slab reinforced with #4 rebar at 18 inches each way, kept 3 inches from the edges, needs 8 bars running the length and 14 running the width: 317 feet of bar. Cut from 20 ft stock that is 22 bars, 25 with a 10% allowance, weighing 334 lb, plus 112 chairs to hold the mat off the ground. That works out to 0.88 lb of steel placed per square foot.
How rebar is calculated
In each direction, the bars fill the slab minus the cover on both sides. Divide that clear width by the spacing, round down, and add one for the first bar. Each bar is as long as the slab minus the cover at both ends.
Bars one way = (width − 2 × cover) ÷ spacing, rounded down, + 1; bar length = length − 2 × cover
For the example:
- Bars along the 20 ft length: (12 ft − 6 in) ÷ 18 in = 7.67, rounded down to 7, + 1 = 8 bars of 19 ft 6 in
- Bars across the 12 ft width: (20 ft − 6 in) ÷ 18 in = 13, + 1 = 14 bars of 11 ft 6 in
- Net length: 8 × 19.5 + 14 × 11.5 = 317 ft
Every run fits in a 20 ft bar, so there are no splices. The 11 ft 6 in bars cannot share a stock bar (two would need 23 ft), so each takes its own: 22 bars. With 10% extra, 25 bars at 13.36 lb each is 334 lb to buy; the 317 ft placed weighs 212 lb, and the offcuts make up the difference.
Rebar weight by bar size
Nominal weights from ASTM A615, the standard US reinforcing bars are made to. The bar number is its diameter in eighths of an inch.
| Bar | Metric | Diameter | lb per ft | 20 ft bar |
|---|---|---|---|---|
| #3 | 10M | ⅜ in | 0.376 | 7.52 lb |
| #4 | 13M | ½ in | 0.668 | 13.36 lb |
| #5 | 16M | ⅝ in | 1.043 | 20.86 lb |
| #6 | 19M | ¾ in | 1.502 | 30.04 lb |
To find the weight of any rebar order, multiply the total feet by the lb per foot. The calculator reports both the weight you buy (whole stock bars) and the weight you place.
Lap splices
Where a run is longer than a stock bar, two bars overlap and are tied. The International Residential Code's Table R608.5.4(1) sets the lap for Grade 60 bars at 30 inches for #4, 38 inches for #5 and 45 inches for #6. The table has no #3 row; its Grade 60 values are 60 bar diameters, so the calculator starts #3 at 22½ inches and lets you change it. A 40 ft footing run in #4 needs two splices with 20 ft bars (one splice only reaches 37 ft 6 in), so it uses 45 ft of bar: three stock bars.
Rebar calculator for a slab, driveway or circular slab
Slab and driveway modes lay a rectangular grid; enter the slab by length and width or by square feet (laid out as a square of the same area). The spacing can differ each way. Circular slab mode clips a square grid to the circle, bar by bar, so the chords near the edge are short. The drawing shows the mat to scale.
Chairs follow the rule Hercules Rebar Chair gives from CRSI guidance: with bars 12 inches apart or closer, a chair at every other crossing; farther apart, a chair at every crossing; never more than 4 feet between supports. Buy chairs as tall as the cover your plans call for.
Rebar for footings, grade beams and walls
Footing mode runs continuous bars along the footing, with optional cross bars. The IRC's provisions for high seismic areas (R403.1.3) call for one No. 4 bar 3 to 4 inches from the bottom of the footing and 3 inches of cover where concrete is cast against earth. Wall mode draws vertical and horizontal bars at their spacings; formed faces exposed to weather need 1½ inches of cover for #5 and smaller bars (R608.5.4.1).
The bar size and spacing for a structural footing, wall or beam come from your plans or building department, not from this calculator. It counts what you specify.
Rebar per square foot
For budgeting, the figure under the result divides the placed weight by the slab area. At #4 on an 18 inch grid it is 0.88 lb per square foot; on a 12 inch grid it rises to about 1.3.
Next steps
Get the concrete for the same slab in the concrete slab calculator or the concrete footing calculator, price the job including a rebar grid in the concrete cost calculator, or work out every other shape in the concrete calculator.