Rebar Spacing Calculator

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 = ceiling(Span / Spacing) + 1 This applies in each direction for a grid pattern.

Two parts, and both get missed. The +1 is the fence-post rule: bars sit at both edges of the mat as well as at every interval between, so a run with 13 gaps has 14 bars.

The ceiling is the part that decides whether you meet the spec. Spacing is a maximum, not a target. A 20 ft span at 18 inch spacing comes to 13.33 gaps. Round down to 13 and each gap opens to 18.5 inches, which is wider than the spacing you asked for. Round up to 14 gaps, 15 bars, and they close to 17.1 inches, which is inside it. The panel below prints the spacing you actually end up with so you can see it land.

Total rebar length: Total Length = (Bars in Direction 1 x Slab Width) + (Bars in Direction 2 x Slab Length)

Common rebar sizes:

Typical spacing guidelines:

• Residential slabs: 12-18 inches (30-45 cm) on center.
• Driveways and garage floors: 12 inches (30 cm) on center.
• Footings: 6-12 inches (15-30 cm) on center.

Foundation walls are the one case not to take from a rule of thumb. Bar size and spacing there come out of the IRC tables (R404.1.2) and depend on wall height, backfill depth and soil class, so the same 8 ft wall can call for #4 at 48 inches in one situation and #5 at 16 inches in another. Look it up or ask the engineer rather than picking a number.

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. The result panel shows the bare grid length and the same figure with 10% added for laps and cuts, and it is the second one you order from.

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.