Rebar Calculator

Rebar is bought by weight but ordered by length, and the two only agree if you know the unit mass: 8 mm bar is 0.395 kg/m, 10 mm is 0.617, 12 mm is 0.888, 14 mm is 1.21, 16 mm is 1.58. A ton of 8 mm is about 2530 metres, a ton of 16 mm is about 630 metres, so the same budget buys four times the length in the thinner bar.

The calculator converts between the two so the order can be placed in the unit the supplier uses. Getting this wrong is expensive in both directions: too many bars is scrap, too few is a cold joint the day the concrete arrives.

Every lap adds length. A 12 mm bar laps about 40 diameters, roughly 480 mm, and on a slab with many laps that is 5 to 10 % extra bar beyond the centreline length. Chairs, spacers and tying wire are separate purchases again, and their absence is what leaves reinforcement sitting on the ground and doing nothing.

Cover matters as much as quantity. Bars need the specified cover on every face, which means spacers sized to the cover, and a slab laid on the ground needs a membrane under it so the steel is not sitting in wet soil.

A mesh sheet at 5 kg/m² is a crack-control layer, not structural reinforcement. If the design calls for bars, the mesh does not replace them, and adding mesh to a properly reinforced slab is a small cost that is often worth paying for crack control at the surface.

The quantity this page returns is a material estimate. Whether the section is adequate — bar diameter, spacing, laps and cover — is a structural decision and cannot be inferred from an area.