Rebar calculator
How much rebar do I need?
Estimate the total length of rebar and number of 40-ft sticks for concrete slabs based on slab dimensions and spacing.
Rebar calculator
Rebar calculator result
Rebar placement guidelines
- Slab on grade: Use #3 or #4 rebar at 12-18 inches spacing
- Driveways: #4 rebar at 12-18 inches
- Footings: #4 or #5 rebar, 2-4 continuous bars
- Retaining walls: #4 or #5 rebar at 12-24 inches
Rebar should be placed in the middle third of the slab thickness. Use wire chairs or rebar supports to hold it at the correct height during the pour.
Worked example
For example, for a 10×8 ft slab, #4 rebar at 12-in spacing:
- Horizontal bars: (8 × 12 ÷ 12) + 1 = 9 bars at 10 ft = 90 ft
- Vertical bars: (10 × 12 ÷ 12) + 1 = 11 bars at 8 ft = 88 ft
- Total length = 90 + 88 = 178 ft
- 40-ft sticks needed = 178 ÷ 40 = 4.45 → 5 sticks
- Add 10% for overlaps and bends: 5 × 1.10 = 6 sticks
Common mistakes
| Mistake | Why it matters |
|---|---|
| Using bars that are too far apart | Maximum spacing is typically 18 in — 12 in is better for most residential slabs |
| Not supporting bars off the ground | Use rebar chairs or dobies to keep rebar in the middle of the slab |
| Forgetting to overlap at splices | Overlap bars by at least 40 bar diameters (20 in for #4) at joints |