How to calculate rebar for a slab
Keep the steel 3 inches in from every edge — that's the concrete cover code requires for concrete cast against the ground. Then count bars each way: the width inside the cover divided by the spacing, rounded down, plus one. Do the same for the other direction.
Example: a 12' x 10' patio with #4 bars at 18". Inside the 3" cover it's 138" x 114". That's 7 bars running the length and 8 running the width — 157 linear feet, or 9 twenty-foot sticks with a little extra for cutting.
Lap splices
When a run is longer than one stick, overlap the bars and tie them. A common rule is 40 bar diameters: 15" for #3, 20" for #4 and 25" for #5. The calculator adds the laps before counting sticks.
Bar sizes and weights
| Bar | Diameter | Weight per foot |
|---|---|---|
| #3 | 3/8" | 0.376 lb |
| #4 | 1/2" | 0.668 lb |
| #5 | 5/8" | 1.043 lb |
Chairs and ties
Rebar only works in the middle of the slab, not lying on the dirt. Set it on chairs about every 3 feet each way, and tie the bars where they cross. Many builders tie every other crossing on light patio grids; the calculator counts every one so you won't run short.
Need the concrete itself? Use the concrete calculator.