Rebar calculator for slab: the grid
A slab is reinforced with a grid of bars running both ways. The calculator works out each direction separately:
- Cover — bars stop 3 in short of every edge, the cover ACI 318 requires for concrete cast against the ground. That keeps the steel from rusting at the slab's edge.
- Bars — the remaining width ÷ spacing, rounded down, plus one.
- Length — the other side less 6 in of cover.
- Splices — rebar comes in 20 ft lengths, so a 30 ft run needs a second bar overlapped by the lap length.
For a 20 × 20 ft patio at 18 in spacing that's 19.5 ft of usable width, 13 spaces and 14 bars each way: 28 bars, each 19.5 ft long.
| Slab | Spacing | #3 bars (20 ft) | #4 bars (20 ft) | #4 weight |
|---|---|---|---|---|
| 10 × 10 ft | 18 in | 7 | 7 | 93 lb |
| 12 × 20 ft | 18 in | 17 | 17 | 222 lb |
| 20 × 20 ft | 18 in | 29 | 29 | 383 lb |
| 20 × 20 ft | 12 in | 41 | 41 | 547 lb |
| 24 × 24 ft | 16 in | 47 | 48 | 635 lb |
Counts include 5% for cuts and the lap splices a 24 ft run needs; #3 bars weigh 44% less than #4 for the same count.
Rebar weight calculator
| Bar size | 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 |
| #6 | 3/4 in | 1.502 lb | 30.0 lb |
The bar number is its diameter in eighths of an inch. Weight matters when the steel is priced by the pound or the hundredweight, and when you're deciding whether a load fits in a pickup — 30 bars of #4 is about 400 lb.
Lap splices, chairs and ties
Laps. Where one bar ends and the next begins, overlap them by about 40 bar diameters and tie them together: 15 in for #3, 20 in for #4, 25 in for #5. Stagger laps so they don't all fall on one line.
Chairs. Rebar only helps if it's inside the concrete, not lying in the dirt underneath it. Set it on plastic chairs or concrete dobies so it sits near the middle of a 4 in slab — the calculator allows one about every 3 ft each way. Pulling the bars up with a rake during the pour doesn't work reliably.
Tie wire. Every crossing can be tied; many crews tie every other one in the field and every crossing at the edges. A 3.5 lb coil of 16-gauge tie wire goes a long way.
Our take: for a patio or walk on well-compacted base, #3 at 18 in or welded wire mesh is plenty to control cracks. Driveways and slabs carrying vehicles benefit from #4 at 12–18 in. Footings and structural slabs follow their drawings. Size the concrete itself with the slab calculator, or the footing calculator for continuous footings.
