How Much Rebar Do I Need for a Concrete Slab?

Rebar quantities come from grid geometry, not volume. For each direction of the grid, divide the slab dimension by your bar spacing and add one bar, because a grid always needs a bar at both edges. A 20 ft × 20 ft slab with bars at 18 in (1.5 ft) centres needs (20 ÷ 1.5) + 1 = 14.3, rounded up to 15 bars running each way.

Multiply the bar count by the length each bar spans: 15 bars × 20 ft = 300 linear feet in one direction, and another 300 ft in the perpendicular direction, for 600 linear feet total.

Add for laps. Where two bars meet end to end they must overlap, and the accepted lap is roughly 40 bar diameters — about 20 in for #4 (1/2 in) bar. On slabs longer than the standard 20 ft stock length this can add 5–10% to your order, so most estimators simply add 10% to cover laps, offcuts and the occasional bent bar. 600 ft becomes 660 ft.

Convert linear feet into pieces you can actually buy. Divide by the stock length you plan to use: 660 ÷ 20 = 33 pieces of 20 ft #4 bar. Add tie wire (about 1 lb per 150 ties) and rebar chairs to hold the grid at mid-depth — roughly one chair every 3–4 ft in both directions.

Bar size and spacing depend on the job. Residential patios, walkways and shed slabs typically use #3 (3/8 in) or #4 (1/2 in) bar at 18 in centres, or 6×6 welded wire mesh instead. Driveways and slabs carrying vehicles usually step up to #4 at 12–16 in centres. Structural slabs, footings supporting a building and anything on expansive soil should follow an engineer's drawing rather than a rule of thumb.

One placement rule matters more than the quantity: rebar must sit in the middle third of the slab depth with at least 3 in of cover from soil and 1.5 in from the finished surface. Steel laid flat on the ground does nothing.