How Much Concrete Do You Need?

Concrete is a volume

Unlike paint or tiles, which cover an area, concrete fills a space, so you work in cubic metres. Multiply length by width by depth, all in the same unit. Keep everything in metres and convert thickness early — 100 mm is 0.1 m — because mixing centimetres and metres is the most common way these sums go wrong.

A worked example

For a slab 4 m long, 3 m wide and 100 mm thick, the volume is 4 × 3 × 0.1 = 1.2 cubic metres. A strip footing is the same idea in a long thin shape: a 10 m run, 0.3 m wide and 0.5 m deep is 10 × 0.3 × 0.5 = 1.5 m³. Break any awkward shape into rectangular blocks, find each volume, and add them up.

Add for waste, then choose bags or ready-mix

Ground is never perfectly level and some concrete is always lost, so add about 10% to the calculated volume. Then decide how to supply it: bagged concrete suits small jobs — divide your volume by the bag's yield, roughly 0.011 m³ for a 25 kg bag, so 1.2 m³ needs around 110 bags. Above a cubic metre or so, ordering ready-mix by the cubic metre is faster and more consistent.

Depth matters most

Because volume multiplies all three dimensions, a small change in thickness moves a lot of concrete. Going from 100 mm to 150 mm on that 12 m² slab lifts the volume from 1.2 to 1.8 m³ — half as much again. Use the right depth for the job: around 100 mm for a domestic slab, less for a light path, more for foundations that carry load.

Frequently asked questions

How many bags of concrete in a cubic metre?

Roughly 90 bags of 25 kg per cubic metre, since each makes about 0.011 m³ — always check your product's stated yield.

How much extra should I order?

About 10% over the calculated volume, to cover spillage, uneven ground and slightly deeper spots.

When should I use ready-mix instead of bags?

For anything above roughly a cubic metre — mixing that much by hand is slow and gives inconsistent strength.