Rebar Calculator
Enter your slab and choose the bar spacing in the Reinforcement panel. You get total feet of #4 rebar, the number of 20 ft bars and the weight.
Last updated November 13, 2026
How to calculate rebar for a slab
- Divide the slab width by the spacing and add 1. That is the number of bars running the long way.
- Multiply by the slab length to get their total length.
- Do the same in the other direction.
- Add both totals, then add about 10% for lap splices.
Example: a 10 ft × 10 ft slab with bars 12 inches apart has 11 bars each way. 11 × 10 + 11 × 10 = 220 ft, which is 11 bars of 20 ft.
Rebar for common slab sizes
Grid in both directions, before lap splices. Bars are counted in 20 ft lengths.
| Slab size (ft) | 12 in spacing | 18 in spacing | 24 in spacing |
|---|---|---|---|
| 10 × 10 | 220 ft (11 bars) | 140 ft (7 bars) | 120 ft (6 bars) |
| 12 × 12 | 312 ft (16 bars) | 216 ft (11 bars) | 168 ft (9 bars) |
| 16 × 16 | 544 ft (28 bars) | 352 ft (18 bars) | 288 ft (15 bars) |
| 20 × 20 | 840 ft (42 bars) | 560 ft (28 bars) | 440 ft (22 bars) |
| 24 × 24 | 1,200 ft (60 bars) | 816 ft (41 bars) | 624 ft (32 bars) |
Rebar sizes and weights
| Bar size | Diameter | Weight per foot |
|---|---|---|
| #3 | 3/8 in | 0.376 lb |
| #4 | 1/2 in | 0.668 lb |
| #5 | 5/8 in | 1.043 lb |
| #6 | 3/4 in | 1.502 lb |
| #7 | 7/8 in | 2.044 lb |
| #8 | 1 in | 2.670 lb |
The bar number is the diameter in eighths of an inch. The calculator estimates #4 bar, the most common size in residential slabs. For another size, multiply the total feet by the weight in the table.
Wire mesh instead
Switch the Reinforcement panel to wire mesh and the calculator counts 5 ft × 10 ft sheets with 10% added for overlap.
Limits
This is a quantity estimate for slabs. It does not tell you whether a slab needs steel or what size and spacing your code requires. Footings, walls and structural slabs need a design.
Frequently asked questions
How much rebar do I need for a 20×20 slab?
With #4 bar at 12 inches each way, 840 ft, which is 42 bars of 20 ft before lap splices. At 18 inches it is 560 ft, or 28 bars.
How do I calculate rebar for a slab?
Divide the slab width by the spacing and add one to get the number of bars running lengthwise. Multiply by the slab length. Repeat in the other direction and add the two totals.
How much does #4 rebar weigh?
#4 rebar weighs 0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
Do I need rebar or wire mesh?
That depends on what the slab carries and on your local code. Mesh is common in patios and walks. Rebar is common in driveways and slabs that carry loads.
How much should rebar overlap?
Where bars meet end to end they must overlap. The required lap length depends on bar size and code, so add about 10% to the total length as an allowance and confirm the lap with your inspector or engineer.