How rebar is calculated
Bars run both ways in a grid. Keep them back from the edges (the "cover"), count how many fit at your spacing, and add a lap wherever one bar has to join another.
Bars one way = (width − 2 × cover) ÷ spacing, rounded down, + 1
Each bar's length = length − 2 × cover
Long runs: extra bars overlap by the lap length
Each bar's length = length − 2 × cover
Long runs: extra bars overlap by the lap length
Worked example
A 20 × 20 ft slab with 3 in cover leaves 19.5 ft each way. At 12 in spacing that's 19.5 × 12 ÷ 12 = 19.5, rounded down to 19, + 1 = 20 bars each way, each 19.5 ft. Each fits in one 20 ft bar, so buy 40 bars. That's 780 ft of #4 bar at 0.668 lb/ft, about 521 lb.
Rebar sizes
| Bar | Diameter | Weight per foot |
|---|---|---|
| #3 | 0.375 in | 0.376 lb |
| #4 | 0.500 in | 0.668 lb |
| #5 | 0.625 in | 1.043 lb |
| #6 | 0.750 in | 1.502 lb |
The bar number is its diameter in eighths of an inch: #4 is 4/8 = ½ inch.
Limitations
- Design comes first: bar size, spacing, cover and lap length should come from your plans or local code. This page only does the counting.
- Cut-offs: leftover pieces from long runs are counted as waste; a careful cutting plan can save a little.
- Chairs and ties: you'll also need supports to hold the bars in the middle of the slab, and tie wire for the intersections.