Rebar Calculator
Plan the rebar grid for a concrete slab, patio or footing pad. Enter the slab size, bar spacing and edge clearance to get the number of bars to buy and the total linear feet.
- Total linear feet
- 858 ft
- Bars lengthwise
- 20 × 19.5 ft
- Bars widthwise
- 20 × 19.5 ft
- Intersections to tie
- 400
How this calculator works
Bars stop a few inches short of the slab edges (the edge clearance), so the grid is slightly smaller than the slab.
The number of bars in each direction is the grid width divided by the spacing, plus one. Each bar is as long as the grid in its direction.
Total linear feet include extra for lap splices where bars overlap. That total is divided by the bar length to get the number of bars to buy.
Example: a 20 × 20 ft slab with a 12 in grid
- Grid size with 3 in clearance on each side: 20 − 0.5 = 19.5 ft each way.
- Bars each way: 19.5 × 12 ÷ 12 = 19.5, rounded down to 19, plus 1 = 20.
- Linear feet: (20 × 19.5) × 2 = 780 ft, plus 10% for laps = 858 ft.
- Bars to buy: 858 ÷ 20 = 42.9, so 43 bars of 20 ft. Intersections to tie: 400.
Quick reference
| Slab size | 12 in grid | 18 in grid |
|---|---|---|
| 10 × 10 ft | 11 | 8 |
| 12 × 12 ft | 16 | 11 |
| 20 × 20 ft | 43 | 31 |
Tips before you buy
- Support rebar on chairs or dobies so it sits in the middle third of the slab, not on the ground.
- Lap splices are typically 40 bar diameters — about 20 inches for #4 (½ in) rebar. Check your local requirements.
- Many patios and sidewalks use welded wire mesh or fiber instead of rebar; driveways and structural slabs usually need rebar.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
With a 12 inch grid and 3 inch edge clearance, a 20 × 20 ft slab needs about 858 linear feet including lap splices — 43 bars of 20 ft.
What spacing should rebar be in a slab?
For residential slabs and driveways, #3 or #4 bars at 12 to 18 inches on center in both directions are common. Follow your engineer or local code.
Related calculators
Last updated October 25, 2026