Concrete volume calculator

Order too little concrete and you're mixing a second batch mid-pour, with the first batch already going off. Order too much bagged mix and you're carrying spare bags back to the merchant. This calculator gives a working figure — cubic metres and bag count — from the dimensions of the pour.

Why the waste allowance matters

Ground is never perfectly flat, formwork isn't perfectly sealed, and some mix is lost to spillage and compaction. A 10% allowance is a sensible default for most domestic pours; push it higher on rough or uneven ground, or lower it for a well-prepared, formed base where you're confident in the depth.

Bags versus ready-mix

Bagged mix works fine for small jobs — post holes, small bases, patching. Past around a cubic metre, the bag count climbs into the hundreds and mixing by hand or in a small mixer starts eating the whole day. At that point it's usually worth getting a price for ready-mixed concrete delivered by the lorry-load instead — it's often cheaper once you count the labour, and the whole pour goes in at once with a consistent mix.

A worked example

A 3m × 2m shed base at 100mm deep, with a 10% waste allowance, comes to 0.66m³ — 53 bags at 25kg, or 67 bags at 20kg. Go to 150mm deep on a 5m × 4m base and that jumps to 3.3m³, well into ready-mix territory.

What this doesn't cover

This assumes a simple rectangular pour and a standard general-purpose mix. Reinforcement, formwork, sub-base preparation and mix strength (for load-bearing or structural work) aren't part of this figure — get those right separately before you order.