Understanding Concrete Volumes: How Much Do You Need?
Updated: July 2026
Concrete is sold by volume (cubic metres or cubic yards), but projects are measured in dimensions. Getting the volume right is essential — running out mid-pour is a serious problem, since fresh concrete cannot wait. This guide explains how to calculate what you need.
The basic volume formula
Concrete volume is simply length × width × depth, all in the same unit. For a slab 4 m long, 3 m wide and 0.1 m deep: 4 × 3 × 0.1 = 1.2 cubic metres. The depth is where people most often go wrong, so measure it carefully.
Bags vs ready-mix
For small jobs, bagged concrete is convenient. A standard 25 kg bag yields roughly 0.011 m³, so you need about 90 bags per cubic metre — which is why ready-mix delivery makes sense for anything larger than a few hundred litres.
Footings and posts
For fence posts, the hole is usually a cylinder: volume = π × radius² × depth. For strip footings, treat them as long thin slabs (length × width × depth). Round holes and irregular shapes are where a calculator saves the most time.
Always order extra
Order about 5–10% more than your calculated volume. Ground is never perfectly level, forms bulge slightly, and some concrete is always lost to spillage. Running short during a pour creates a cold joint and a weak point, so a small surplus is cheap insurance.
Frequently asked questions
How do I calculate concrete volume?
Multiply length × width × depth, all in the same unit. For example, a slab 4 m × 3 m × 0.1 m deep needs 1.2 cubic metres of concrete. Always add 5–10% extra for spillage and uneven ground.
How many bags of concrete are in a cubic metre?
Roughly 90 standard 25 kg bags make one cubic metre. For anything larger than a few hundred litres, ready-mix delivery is usually more practical and economical than mixing bags.
Why should I order extra concrete?
Ground is never perfectly level, forms can bulge, and some concrete is lost to spillage. Running short mid-pour creates a weak cold joint, so ordering 5–10% extra is worthwhile insurance.