Estimate total rebar (reinforcing bar) length, weight and number of 20-foot bars needed for a concrete slab, driveway, footing or structural pour. Enter your slab dimensions, bar spacing, lap splice length and bar size to get a complete material list.
Enter slab dimensions to calculate.
For a standard two-way grid, bars run in both directions at regular spacing. The total linear footage is calculated as:
Bars along length = (Width ÷ Spacing) + 1
Bars along width = (Length ÷ Spacing) + 1
Each bar length includes a lap splice allowance where bars overlap. The total weight equals total linear footage multiplied by the pounds-per-foot rating of the bar size. A 10% waste allowance covers cutting losses.
#4 bar at 12 inches on center is the most common specification for residential concrete slabs. Always verify with your engineer or local building code.
Correct placement is as important as quantity. Key rules for residential concrete:
For most residential applications like driveways and patios, #4 rebar at 12–18 inches on center is recommended. A slab without rebar can crack and shift significantly under load.
#4 (1/2-inch) rebar at 12 inches on center in both directions is a common specification for a 4-inch residential driveway slab.
Wire mesh (welded wire reinforcement) controls shrinkage cracks in thin slabs but provides less structural strength than rebar. For driveways and structural slabs, rebar is preferred.