Rebar Spacing Calculator
Calculate rebar spacing, grid bar count, linear feet, and total weight for concrete slabs, walls, and footings.
Covers #3 through #6 bar sizes.
Rebar (reinforcing bar) strengthens concrete by resisting tension forces. Proper spacing ensures the concrete structure can handle the loads it was designed for.
Rebar spacing formula:
Number of Bars = floor(Span / Spacing) + 1
This applies in each direction for a grid pattern.
The floor matters because you cannot buy two thirds of a bar. A 20 ft span at 18 inch spacing works out to 13.33, which becomes 13 full spaces and 14 bars, with the last space running a little tight rather than a little wide.
Total rebar length:
Total Length = (Bars in Direction 1 x Slab Width) + (Bars in Direction 2 x Slab Length)
Common rebar sizes:
| Size | Diameter | Weight per foot |
|---|---|---|
| #3 | 3/8 inch (9.5 mm) | 0.376 lb (0.17 kg) |
| #4 | 1/2 inch (12.7 mm) | 0.668 lb (0.30 kg) |
| #5 | 5/8 inch (15.9 mm) | 1.043 lb (0.47 kg) |
| #6 | 3/4 inch (19.1 mm) | 1.502 lb (0.68 kg) |
Typical spacing guidelines:
- Residential slabs: 12-18 inches (30-45 cm) on center.
- Driveways and garage floors: 12 inches (30 cm) on center.
- Foundation walls: 12 inches (30 cm) horizontal, 12-16 inches (30-40 cm) vertical.
- Footings: 6-12 inches (15-30 cm) on center.
Overlap requirement: Where bars need to be joined, overlap by at least 40 bar diameters. For #4 rebar, that means 20 inches (50 cm) of overlap. That overlap is real steel you have to buy. On a slab wide enough to need splices, budget an extra 10% of total length rather than ordering the bare grid figure, which is what the result panel below assumes.
Tie the intersections rather than relying on friction, and support the mat on chairs or dobies so the steel ends up in the middle third of the slab thickness. Rebar lying on the ground does nothing at all: it is the position within the pour that carries the tension, not the presence of the bar.
Always follow your local building codes and engineering specifications for structural applications.
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