Key takeaways
- This counts the steel for a grid you've chosen. It doesn't decide whether a slab needs that grid.
- Halving the spacing about doubles the steel.
- The 2021 IRC says reinforcement in a slab-on-ground must stay between the center and upper third during the pour.
Enter your slab's size and the bar size, spacing and edge distance from your design. This tool counts materials. It doesn't tell you what your slab needs, which depends on loads, soil and local rules.
How the calculator works
- Bars each way: the distance across, less the edges, ÷ spacing, rounded down, plus one
- Total feet: bars × their length, plus any laps where one stock length isn't long enough
- Stock lengths: total feet ÷ stock length, rounded up
- Weight: total feet × the bar's weight per foot
This is a minimum, as it doesn't allow for cutting waste. Lay out your cuts so that offcuts get used.
Worked example
A 12 × 10 foot slab with #4 bars at 18 inches, held 3 inches back from the edges:
- 7 bars of 11.5 feet, and 8 bars of 9.5 feet
- Total: 80.5 + 76 = 156.5 feet
- At least 8 stock lengths of 20 feet
- About 105 lb
How spacing changes the count
| Spacing | Total on this slab |
|---|---|
| 12 in | 229 ft |
| 16 in | 177.5 ft |
| 18 in | 156.5 ft |
| 24 in | 114.5 ft |
Bar sizes
| Bar | 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 |
We read these weights on a calculator site rather than the standard itself, so treat them as planning figures. It says rebar is typically sold in 20, 30, 40 and 60-foot lengths, but stores often carry shorter ones, so ask what's available and what fits your vehicle.
Placing the steel
2021 IRC Section R506.2.4 says that where reinforcement is provided in a slab-on-ground, it must be supported to stay between the center and the upper one-third of the slab during the pour. So plan for supports, as steel left at the bottom doesn't meet that figure.
We haven't sourced lap lengths or edge distances, so we leave them as your entries. Your design or building department should give you those.
Frequently asked questions
Does a patio slab need rebar?
It depends on your soil, loads and local rules, so ask your building department or a concrete contractor. We don't make that call.
How far apart should rebar be?
That comes from the slab's design. The calculator shows the steel for whatever spacing you enter.
How much rebar do I need for a 10 × 12 slab?
With #4 bars at 18 inches and 3 inches from the edges, about 156.5 feet, before cutting waste.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 5, Section R506.2.4
- Inch Calculator: Rebar weight and size (secondary source for weights)