How it's calculated
bars = ⌊(span − 2 × cover) ÷ spacing⌋ + 1; rebar = (bars_L × L + bars_W × W) × 1.10
L, W = slab length and width, cover = edge clearance, spacing = on-center bar spacing, bars_L/bars_W = bar counts each way, ×1.10 = 10% lap/waste allowance. Weight = total length × bar mass per foot.
Common questions
What does on-center spacing mean?
It is the distance from the center of one bar to the center of the next. Tighter spacing means more bars and more steel.
What bar size do most slabs use?
Residential slabs commonly use #3 or #4 rebar, but the right size depends on the load and your local code.