Ballast (20mm) — The Complete Guide

Ballast is the aggregate component of concrete. If you're mixing concrete on site — for footings, fence posts, or slabs — ballast is what gives it its bulk and strength.

What is Ballast (20mm)?

Ballast (or all-in aggregate) is a mixture of sharp sand and coarse gravel (typically 20mm stones) combined in proportions suitable for mixing concrete. It's essentially a pre-blended concrete aggregate — you just add cement and water. The 20mm gravel and sharp sand are combined in approximately a 2:1 ratio by volume. This saves time compared to buying sharp sand and coarse gravel separately.

Common uses

General purpose concrete mixing
Concrete foundations and footings
Fence post and gate post setting
Concrete slabs and oversite
Garden wall foundations
Small structural concrete pours

Sizes & grades

Standard ballast is 20mm all-in. The mix contains particles from dust up to 20mm stones. For reinforced concrete, specify clean separate aggregates rather than all-in ballast.

How much do you need?

Standard concrete mix is 1 part cement : 5 parts ballast (by volume) for general use, or 1:4 for stronger structural concrete. For a 100mm slab: floor area (m²) × 0.1 × 2.0 tonnes/m³. A 10m² slab needs approximately 2 tonnes of ballast plus 4–5 bags of cement.

Pro tip

For foundation concrete that will be in contact with sulphate-bearing soils, don't use all-in ballast — specify a sulphate-resistant concrete mix instead.