Rebar Calculator: Quantity, Spacing, Weight & Cost for Any Project

For a standard residential slab using #4 rebar at 12 inch spacing both ways, estimate about 1 linear foot of rebar per square foot of slab area. A 20x20 ft slab needs about 880 linear feet, or 44 bars of 20-ft #4 rebar before waste, weighing about 588 lbs.

For residential slabs, #4 rebar at 12 inches on center both directions is the most common starting point. ACI 318-19 limits slab reinforcement spacing to the lesser of 3 times slab thickness or 18 inches, so a 4-inch slab has a 12-inch maximum spacing reference.

Common rebar weights are #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, and #8 = 2.670 lb/ft. A 20-ft #4 bar weighs about 13.36 lbs.

A lap splice is the overlap where two bars transfer force between pieces. For #4 bars, ACI Class B tension lap references are about 19 inches at 3,000 PSI and 17 inches at 4,000 PSI. Many residential crews use 24 inches as a conservative practical lap.

Residential driveways commonly use #4 rebar at 12 to 18 inches on center. For heavier vehicles or poor subgrade, engineers may specify #4 at 12 inches or #5 at 12 inches with higher-strength concrete.

Calculate vertical and horizontal bars separately. Vertical bars equal wall length divided by vertical spacing plus one. Horizontal bars equal wall height divided by horizontal spacing plus one. For double-face reinforcement, multiply both by two.

National average material costs are roughly #3 at $0.22/ft, #4 at $0.38/ft, #5 at $0.60/ft, #6 at $0.85/ft, and #8 at $1.40/ft. Installation labor commonly adds about $0.30-$0.50 per linear foot for residential work.