Concrete Calculator

How to use the concrete calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

Concrete is ordered by volume, so the whole calculation is length × width × depth. What catches people out is depth: it is in inches or centimetres while everything else is in feet or metres, and mixing the two silently produces an answer that is wrong by an order of magnitude. Enter every measurement in the same unit here and the conversion is handled.

Ready-mix is sold in cubic metres or cubic yards and delivered in fixed increments, so the practical figure is the rounded-up one. Bags are the alternative for small pours, and the bag count is worth reading before you decide — beyond about half a cubic metre, mixing by hand from bags stops being sensible.

The waste allowance is not padding. Cuts, breakages, spillage and the offcuts around edges and openings are real losses on every job, and a second delivery costs far more than a spare bag or board. With concrete the allowance also covers the ground taking more than you measured, which it usually does: a sub-base that is a centimetre low across a slab adds real volume.

A pour that runs short cannot be topped up later without a cold joint, which is a permanent weakness. Ordering slightly over is the cheap mistake; ordering under is the expensive one.

Common uses

• Ordering ready-mix for a shed base or driveway.
• Working out how many bags a small footing needs.
• Checking a quoted volume before accepting it.

Frequently asked questions

How much concrete do I need for a slab?

Length × width × depth. A 4 × 3 metre slab at 100 mm deep is 1.2 cubic metres before any waste allowance.

How many bags is a cubic metre?

Around 108 twenty-kilogram bags, which is why bags stop making sense much above half a cubic metre. Ready-mix is cheaper and far less work at that point.

What happens if I order too little?

You get a cold joint where the second pour meets the first, and that is a permanent weak line. Ordering slightly over is the cheaper error.