Rebar calculator

Count bars in each direction: width divided by spacing plus one, times the slab length — then the same the other way. Add 8–10% for laps and cuts: bars overlap at splices by at least 40 bar diameters.

Residential slabs on grade typically use #3 or #4 bar (10M) on a 12–18 in grid, or welded mesh for crack control only. Structural slabs and footings come from the engineer — spacing and size are load calculations, not rules of thumb.

#3 weighs 0.376 lb/ft, #4 — 0.668, #5 — 1.043 (metric: Ø10 = 0.617 kg/m, Ø12 = 0.888). Weight is how rebar is priced and delivered, which is why the calculator converts length to pounds and tons.

Plan roughly 1–1.5 lb of tie wire per 100 lb of rebar, tying every other intersection on a slab grid. Chairs go about every 2–3 ft — around 4 per square metre — so the mesh sits at proper cover, not on the ground.