Rebar calculator
How much rebar a concrete slab grid needs, in linear feet and 10 or 20 ft bars.
How to use it
Enter the slab's length and width in feet, the grid spacing, and how far the bars stop short of the slab edges. The calculator lays out a grid of bars both ways and shows the total linear feet and how many stock bars to buy.
How it's calculated
- Inner size = slab size − 2 × edge clearance, so the bars stay inside the concrete.
- Bars running the long way = inner width ÷ spacing, rounded down, + 1. Each is the inner length.
- Bars running the short way = inner length ÷ spacing, rounded down, + 1. Each is the inner width.
- Linear feet = the two sets added together.
- Stock bars = linear feet × 1.1 ÷ bar length, rounded up. The 10% covers lap splices, where two bars overlap to act as one, and cutting waste.
| Slab | Grid | Linear feet | 20 ft bars |
|---|---|---|---|
| 10 × 10 ft | 18 in | 133 | 8 |
| 20 × 12 ft | 18 in | 317 | 18 |
| 24 × 24 ft | 12 in | 1,128 | 63 |
(With 3 in edge clearance.)
Tips
- Bar size and spacing come from your plans or local code. Driveways and patios commonly use #3 or #4 bar at 12 to 18 in on center, but that isn't a substitute for a design.
- Keep the bars in the middle of the slab, on chairs or concrete blocks, not lying on the ground. Rebar on the ground does almost nothing.
- Tie the bars where they cross with wire ties, so the grid doesn't shift during the pour.
- Lap splices are typically 30 to 40 bar diameters long; check your plans.