Concrete Calculator

Concrete volume is length times width times depth, which is easy. The part that matters is what you do afterwards: add an allowance, and round bags up.

The allowance is not padding. Sub-base is never perfectly level, so the real depth is always a little more than the nominal one, and some concrete is always lost to spillage and what sticks in the mixer. Ten percent is the standard figure.

Running short is worse than it sounds. Stopping a pour and resuming later creates a cold joint — the two pours do not bond properly, leaving a permanent plane of weakness through the slab. You cannot fix it by adding more concrete afterwards; you have to break it out and start again.

The bag counts are worth a sanity check before you commit. A cubic metre is around 108 twenty-kilogram bags, which is well over two tonnes to carry, mix and place. Above roughly half a cubic metre, having ready-mixed delivered is usually cheaper and enormously less work.