Construction Calculators — Concrete, Rebar & Roofing

How much waste allowance should I add?

It depends on the shape of the work: straightforward slabs usually allow around 5-10%, while cut-heavy work such as tiling or complex formwork can waste much more. Ordering slightly over is far cheaper than a second delivery mid-pour, so add the allowance deliberately rather than rounding up by feel.

Should I order bagged concrete or ready-mix?

Roughly, bagged mix becomes uneconomic past about a cubic metre — the bag count and mixing time grow faster than the cost of a truck. Below that, bagged mix is convenient and easier to place in small or awkward spots. The volume figure is what decides it, so calculate that first.

Do the calculators handle both metric and imperial?

Yes. You can enter dimensions in one system and read the material quantities in whichever units your supplier uses, because the underlying volume is converted rather than recalculated. That avoids the rounding drift you get from converting the dimensions first.