How to estimate rebar quantity
Rebar forms a grid in the slab. In each direction, the number of bars equals the slab dimension divided by the spacing, rounded up, plus one edge bar. A 20 × 20 ft slab with 16 inch (1.33 ft) spacing has 15 bars per direction, each 20 ft long — about 300 linear feet before waste. Add laps and cuts (10% is common) and round to 20 ft pieces.
Spacing depends on load and code: 16 inches on center is typical residential; 12 inches for heavier driveways or structural slabs. Rebar should be chaired so it sits in the middle third of the slab, not on the ground — reinforcing only works when it is embedded in concrete. Always follow your engineering plan for critical work.
Rebar quick facts
- Standard length: 20 ft (also 10, 30, 40 ft)
- Common diameters: #3 (3/8"), #4 (1/2"), #5 (5/8")
- Lap length: usually 40× bar diameter, minimum 12 inches