Rebar Calculator

Divide the clear span by the spacing and add one. The clear span is the full span less the cover at each end. At 150 mm centres across a 2.96 m clear width that is 19.7 spaces, so 21 bars — one at each edge and 19 between. Forgetting the +1 is the most common counting error in reinforcement and it under-orders every mat on the job.

Main bars run the shorter span, where the bending is, and carry the load. Distribution bars run the longer span at right angles to them: they spread concentrated loads, hold the main bars in position, and resist shrinkage and temperature cracking. Distribution bars are normally smaller and more widely spaced, typically 8 mm at 200 mm centres against 12 mm at 150.

A single-layer domestic slab mat usually falls between about 6 and 12 kg per square metre. It is a useful cross-check rather than a design figure — a result far outside it means either the spacing is wrong or a layer has been missed.

The concrete between the outside of the bar and the surface. It protects the steel from moisture and air, without which reinforcement corrodes, expands and spalls the concrete off. A slab takes 20 mm, a beam 25, a column 40, and a footing cast against earth 50. It is measured to the outside of the bar, and it reduces the length of every bar by twice the cover.

No. Chairs and spacers holding the top mat up, binding wire at roughly 8–10 kg per tonne, extra bars over supports and corner reinforcement all sit outside this calculation. For a full schedule with laps and bent shapes, use the bar bending schedule tool.