How to use the rebar calculator
- Enter slab length and width.
- Enter bar spacing and the edge cover from your plans.
- Pick the bar size and the stick length your yard sells.
- Leave lap blank to use 40 bar diameters, or enter the lap your plans require.
Formula
Bars each way = ⌊(slab dimension − 2 × cover) ÷ spacing⌋ + 1. Runs longer than one stick need n = ⌈(run − lap) ÷ (stick − lap)⌉ pieces, adding (n − 1) laps. Weight uses standard bar weights per ASTM A615: #3 0.376, #4 0.668, #5 1.043 lb/ft.
Worked example
A 10 × 10 ft pad with #4 at 12 in and 3 in cover: 10 bars each way, each 9.5 ft long = 190 ft. With 5% waste, 199.5 ft → 10 twenty-foot sticks, about 133 lb.
Tips for using the rebar calculator
- Keep bars off the ground on chairs or blocks so they sit in the middle third of the slab.
- Order a few extra sticks for laps at odd lengths and for bars you cut short around edges.
- Follow your engineer’s or local code spacing when the slab carries structural loads.
Rebar Calculator FAQs
How far apart should rebar be in a slab?
Residential slabs commonly use 12–18 in spacing each way, but the correct spacing depends on slab thickness, loads and soil. Follow your plans or engineer.
How long should rebar laps be?
Lap length depends on bar size, concrete strength and the governing code. 40 bar diameters (20 in for #4) is a widely used rule of thumb; your drawings take precedence.
Does rebar sit on the ground?
No. Bars should be supported on chairs or dobies so they end up within the slab thickness, typically near mid-depth for slabs on grade unless the design says otherwise.
