How to Calculate Rebar Quantity for Concrete
Bar spacing and grid dimensions, not concrete volume, drive the count
Rebar count is a spacing calculation, not a volume calculation
Unlike concrete, rebar quantity doesn't come from a volume formula — it comes from the reinforcement design: how far apart the bars are spaced (say, every 12 or 18 inches) in each direction across a slab or footing, per the structural drawing or local code requirement for that application.
Counting bars in a grid
For a rectangular slab reinforced with a grid of bars in both directions, the number of bars running one way is (width ÷ spacing) + 1, and the number running the other way is (length ÷ spacing) + 1. Each bar's length matches the dimension it spans, typically with a small overlap allowance where bars are lapped/spliced together.
Total lineal feet, then stock lengths
Add up the total lineal footage of bar needed in both directions, then divide by the standard stock length rebar is sold in (commonly 20 ft, though this varies by supplier) to determine how many full-length bars to order, rounding up and accounting for lap splices.
Worked Example
A 10 ft × 20 ft slab reinforced on 18-inch (1.5 ft) centers each way.
Bars across the 10 ft width (spanning 20 ft each): (10 ÷ 1.5) + 1 ≈ 8 bars × 20 ft = 160 ft
Bars across the 20 ft length (spanning 10 ft each): (20 ÷ 1.5) + 1 ≈ 14 bars × 10 ft = 140 ft
Total lineal feet: 160 + 140 = 300 ft
Assumptions & Limitations
- This is a simplified single-mat grid — real structural designs may specify double mats, different top/bottom spacing, or specific lap-splice lengths that a licensed engineer's drawing should always govern.