How to use the rebar calculator
- Enter the slab's length and width in feet.
- Choose the grid spacing. 16 or 18 inches on center is common for patios and garage floors; 12 inches for driveways and heavier loads. Follow your plans or local code if you have them.
- Pick the bar size. #4 (1/2 inch) is the usual choice for slabs; #3 is common for light patios and walkways.
- Read off the sticks to buy, the weight (useful for delivery) and the number of chairs.
Rebar formulas
bars one way = (slab width − 6 in) ÷ spacing + 1
bar length = slab length − 6 in
overlap where bars join = 40 × bar diameter (20 in for #4)
sticks = total length × 1.05 ÷ stick length
The calculator keeps bars 3 inches in from each edge so they stay covered by concrete, adds the overlap for any run longer than one stick, and adds 5% for cutting. Chairs (or dobies) hold the grid in the middle of the slab: plan on one about every 3 ft each way.
Rebar for common slab sizes
#4 bar in 20 ft sticks.
| Slab | Spacing | Total length (ft) | 20 ft sticks | Weight (lb) |
|---|---|---|---|---|
| 10 × 10 ft | 16 in | 152 | 8 | 102 |
| 12 × 12 ft | 16 in | 207 | 11 | 138 |
| 12 × 20 ft | 18 in | 317 | 17 | 212 |
| 20 × 20 ft | 16 in | 585 | 31 | 391 |
| 20 × 24 ft | 18 in | 664 | 35 | 444 |
| 24 × 24 ft | 12 in | 1208 | 64 | 807 |
| 30 × 30 ft | 16 in | 1434 | 76 | 958 |
Rebar sizes and weights
| Size | Diameter | Weight per ft | Weight per 20 ft stick | Overlap (40 × diameter) |
|---|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.5 lb | 15 in |
| #4 | 0.5 in | 0.668 lb | 13.4 lb | 20 in |
| #5 | 0.625 in | 1.043 lb | 20.9 lb | 25 in |
Building codes and engineers set rebar for structural work like footings, foundations and retaining walls. Use this calculator for estimating, and follow your plans where you have them.
Frequently asked questions
How much rebar do I need for a 12x12 slab?
With #4 bar at 16 inches on center, a 12 × 12 ft slab takes 9 bars each way, each 11.5 ft long, about 207 ft in total. That is 11 sticks of 20 ft with 5% for cutting, weighing about 138 lb.
What spacing should rebar be in a slab?
For patios and garage floors, 16 to 18 inches on center each way is common. Driveways and slabs carrying heavy loads often use 12 inches. Follow local code or your plans.
How much should rebar overlap?
A common rule is 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5. Tie the overlapping bars together with wire.
How far from the edge should rebar be?
Keep rebar about 3 inches in from the edges and in the middle of the slab’s thickness, held up on chairs, so it is fully covered by concrete.