Rebar Calculator
Bar count, total length, weight and laps for a slab or footing mat.
Quantity take-off, not structural design. Bar size, spacing and cover for a loaded slab come from a structural engineer and the governing code.
How it works
Rebar quantity is a grid problem with two complications: the mat stops short of the edge by the cover distance, and bars longer than stock length have to be lapped, which adds steel that is doing nothing structurally but still has to be bought and carried.
Bar count in each direction is the clear span divided by the spacing, plus one — the fencepost rule. Forgetting the plus one is how sites end up one bar short in each direction on every pour.
Weight follows from the diameter alone: steel is 7850 kg/m³, so a bar weighs 0.006165 × d² kilograms per metre with d in millimetres. A 12 mm bar is 0.888 kg/m; a #4 is 0.668 kg per foot. Suppliers price by weight even when they deliver by length, so this is the number that shows up on the invoice.
Laps are the part people underestimate. A 40d lap on 12 mm bar is 480 mm, and a 20 m slab in 12 m stock needs a splice in every single bar. On a big pour the lap allowance alone can be 8–10% of the total tonnage.
Cover is not a tolerance, it is the whole durability strategy. Concrete protects steel by keeping it in a high-pH environment; once carbonation or chloride reaches the bar it rusts, and rust occupies several times the volume of the steel it came from, which is what spalls the face off a slab. Too little cover is the most common reason concrete fails early — and a bar chair costs a few cents.
Common questions
How much lap do I need?
40 times the bar diameter is the common rule for a tension lap in ordinary concrete — 480 mm on 12 mm bar. It varies with concrete grade, bar coating and whether the splice is in tension or compression; the drawings for a designed slab will state it.
How much waste should I allow?
5% on top of the calculated total is normal for offcuts. This already counts laps separately, so do not double up on them.
Where does the mat sit in the slab?
Wherever the tension is. In a ground-bearing slab that is usually near the bottom, on chairs, with the specified cover under it. Rebar lying on the ground and lifted with a hook as the pour goes in ends up at an unknown depth, which is close to useless.
Do I need to tie every intersection?
No. Every second or third intersection in a staggered pattern is standard practice and holds the mat perfectly well during the pour. Ties are for keeping the grid in place, not for structural continuity.