The formula
Grid size = slab size − 2 × edge distance Bars one way = grid width ÷ spacing, rounded down, + 1 (each as long as the grid length) Bars to buy = bars ÷ how many fit in one stock bar, or more bars plus overlap if a run is longer Weight = total length × pounds per foot Ties = one at every crossing
Example: a 20 × 20 ft slab with #4 bars 12 inches apart each way, 3 inches from the edges, has a 19.5 ft grid: 20 bars each way, each 19.5 ft long, so 40 bars of 20 ft. That is 780 ft of bar weighing about 521 lb, with 400 crossings to tie and about 49 chairs.
Quick answers for common slabs
| Slab | 20 ft #4 bars | Linear feet | Weight | Ties |
|---|---|---|---|---|
| 10 × 10 ft | 10 | 190 ft | 127 lb | 100 |
| 12 × 12 ft | 24 | 276 ft | 184 lb | 144 |
| 10 × 20 ft | 20 | 385 ft | 257 lb | 200 |
| 20 × 20 ft | 40 | 780 ft | 521 lb | 400 |
| 24 × 24 ft | 96 | 1,208 ft | 807 lb | 576 |
| 30 × 30 ft | 120 | 1,870 ft | 1,249 lb | 900 |
Bar sizes and weights
| Bar | Diameter | Weight per foot | 20 ft bar |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb |
| #4 | 1/2 in | 0.668 lb | 13.4 lb |
| #5 | 5/8 in | 1.043 lb | 20.9 lb |
The number of a bar is its diameter in eighths of an inch. Where two bars join, they overlap; a common rule is 40 bar diameters, which is 20 inches for #4, but the overlap and spacing on your plans always come first.
Placing the bars
- Keep bars about 3 inches from the edges and the ground. Concrete cast against soil needs 3 inches of cover so the steel doesn’t rust.
- Set the grid on chairs or concrete blocks every 3 to 4 ft so it sits in the middle of the slab, not on the ground.
- Tie the crossings with wire. Many crews tie every other crossing in the middle and every crossing at the edges.
Sources and assumptions
Bar weights are the standard nominal weights for deformed steel reinforcing bars (ASTM A615). The 3 inch cover for concrete cast against earth is from the ACI 318 building code for structural concrete. Bars shorter than the stock length are cut from full bars, and leftover pieces are not reused.
Questions
How much rebar do I need for a 20 × 20 slab?
With #4 bars 12 inches apart each way and 3 inches from the edges: 40 bars of 20 ft, about 780 ft or 521 lb.
How far apart should rebar be in a slab?
Slabs on the ground commonly use #3 or #4 bars 12 to 18 inches apart each way, but driveways, garages and footings have their own requirements, so follow your plans or local code.
How much does a 20 ft piece of #4 rebar weigh?
About 13.4 lb, at 0.668 lb per foot.
How far should rebar be from the edge of the slab?
About 3 inches from the edges and the ground, so the concrete covers and protects the steel.
References and review
Units convert with exact definitions: 1 ft = 12 in = 0.3048 m, 1 yd³ = 27 ft³. Results round up to whole bags, sheets and pieces so you don’t come up short. Last reviewed October 20, 2026. If your bag, pallet or supplier lists a different figure, use theirs: change the matching field above.