Rebar Calculator
Estimate the rebar for a slab grid — number of bars, total linear feet and weight — from the slab size and bar spacing.
How it works
bars each way = floor(dimension ÷ spacing) + 1 · linear feet = barsX × width + barsY × length · weight = linear feet × lb per foot
A slab is reinforced with a grid of bars running both directions at a set centre-to-centre spacing. The count in each direction is the slab dimension divided by the spacing, plus one (both edges get a bar). Multiplying each set of bars by the length it spans and adding the two gives total linear feet, and multiplying by the bar’s weight per foot (0.668 lb/ft for #4) gives the weight of steel. Add about 10% because bars overlap where they splice.
Worked example
A 20 × 20 ft slab on a 12 in grid: bars each way = 20 ÷ 1 + 1 = 21. Linear feet = 21 × 20 + 21 × 20 = 840 ft over 42 bars. In #4 rebar at 0.668 lb/ft that is about 561 lb of steel — call it 620 lb with lap splices.
Frequently asked questions
What rebar size and spacing does a slab need?
#4 (1/2 in) bar on a 12–18 in grid is typical for residential slabs and driveways; #3 suits light-duty pads and #5 heavier structural slabs. Spacing and size come from the engineered design and load — this tool quantifies a grid you have already chosen.
Why add 10% for laps?
Rebar comes in fixed 20 ft lengths, so long runs are spliced by overlapping two bars — usually 40 bar diameters, around 20 in for #4. Those overlaps are extra steel the clean grid math does not count, so budget roughly 10% more.
Can I use welded wire mesh instead?
For thin slabs and light loads, welded wire reinforcement is a common, faster alternative to a tied rebar grid. For thicker structural slabs, footings and anything carrying real load, engineered rebar is usually specified. Follow the plan for your pour.
Related calculators
Estimate for planning. Add a waste margin and confirm against your supplier specs and local building codes.