How Much Rebar Do I Need for a Slab? Chart by Size and Spacing
Count the bars each way, multiply by their length, and add overlaps where bars join. A 20 × 20 ft slab with #4 rebar at 16 inches on center needs about 31 sticks of 20 ft rebar (585 ft, about 391 lb). The rebar calculator works it out for any slab.
How to work it out
- Bars running the length: (slab width − 6 in) ÷ spacing, rounded down, plus 1.
- Bar length: slab length − 6 in, so the steel stays 3 inches from each edge.
- Repeat the other way for the bars running across.
- Overlaps: where a run is longer than one stick, bars overlap by 40 bar diameters, which is 20 inches for #4.
- Sticks: total length plus 5% for cutting, divided by 20 ft.
For the 20 × 20 example: 15 bars each way, each 19.5 ft long with one overlap, for 585 ft in total.
20 ft sticks of #4 rebar by slab size and spacing
| Slab | 12 in | 16 in | 18 in | 24 in | Weight at 16 in (lb) |
|---|---|---|---|---|---|
| 8 × 8 ft | 7 | 5 | 5 | 4 | 60 |
| 10 × 10 ft | 10 | 8 | 7 | 5 | 102 |
| 10 × 12 ft | 13 | 10 | 9 | 7 | 119 |
| 12 × 12 ft | 15 | 11 | 10 | 8 | 138 |
| 12 × 20 ft | 25 | 19 | 17 | 13 | 232 |
| 16 × 16 ft | 27 | 20 | 18 | 14 | 248 |
| 20 × 20 ft | 41 | 31 | 29 | 21 | 391 |
| 24 × 24 ft | 64 | 48 | 43 | 32 | 605 |
| 24 × 30 ft | 79 | 60 | 53 | 40 | 761 |
| 30 × 40 ft | 133 | 101 | 90 | 67 | 1,283 |
Rebar sizes
| Size | Diameter | Lb per ft | Typical use |
|---|---|---|---|
| #3 | 0.375 in | 0.376 | Patios, walkways, light slabs |
| #4 | 0.5 in | 0.668 | Driveways, garage floors, footings |
| #5 | 0.625 in | 1.043 | Foundations, walls, heavy loads |
Rebar spacing by project
| Project | Typical bar | Spacing |
|---|---|---|
| Patio or walkway, 4 in thick | #3 | 18 to 24 in each way |
| Driveway or garage floor, 4 to 6 in | #4 | 12 to 18 in each way |
| Steps and landings | #3 or #4 | 12 to 18 in each way |
| Footings | #4 or #5 | 2 or 3 bars running the length, as plans or code require |
| Walls and foundations | #4 or #5 | Set by plans or an engineer |
Tighter spacing means more steel and more cost, but less chance of cracks opening up. Keep the bars at least 3 inches from the edges and the ground, so concrete covers them and they don't rust.
How much does rebar cost?
Rebar usually costs about $0.75 to $1.40 per linear foot, so a 20 ft stick of #4 runs roughly $15 to $28. For the 20 × 20 ft slab above, 31 sticks come to about $470 to $870, plus chairs and tie wire. Steel prices move a lot, so check your local yard. Many stores sell #3 and #4 in 10 ft lengths too, and will cut 20 ft sticks to fit your vehicle.
Rebar or wire mesh?
Welded wire mesh is cheaper and quicker for thin patios and walkways, but it's hard to keep in the middle of the slab. Rebar on chairs stays where you put it and is the better choice for driveways and anything carrying vehicles. Either way, the steel should sit near the middle of the slab's thickness, not on the ground. Plan on a chair about every 3 ft each way.
Codes and engineers set the steel for structural slabs, footings and foundations. Use these numbers for estimating and follow your plans where you have them.