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Ballast Calculator

Works out the ballast for mixing your own concrete. Enter the dimensions of what you are pouring; ballast is the combined sand and stone that makes up most of a mix by volume.

Ballast is mixed into concrete rather than laid, so depth here usually means the thickness of the slab you are mixing for.

%

Covers uneven ground, spillage and the last barrow-load always being short. 10% is normal.

Optional. Yards quote by the tonne, by the cubic yard or by the bag — pick whichever matches your quote.

Ballast needed
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Includes the waste allowance — this is the figure to order.

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

As a volume —
Bulk bags — bags
Retail bags — bags
Area covered —
Ballast cost —
How to buy it —

The formula

volume = length × width × depth × (1 + waste) weight = volume × bulk density

The geometry is trivial; the bulk density is where the answer actually comes from. Ballast runs about 1700 kg per cubic metre loose, so a cubic metre weighs roughly 1.70 tonnes. Suppliers quote one or the other, which is why both are given.

What to watch out for

Ballast is pre-blended sand and aggregate — the all-in-one part of a concrete mix. You add cement and water.

  • A 1:5 mix — one part cement to five of ballast — suits general work such as paths and bases.
  • A 1:4 mix is stronger, for footings and anything structural.
  • Volume is roughly conserved. Cement fills the voids between the stones rather than adding bulk, so the finished concrete comes out close to the ballast volume, not the sum of the two.

Above about a cubic metre, mixing your own stops being worth it. Ready-mix is cheaper per cubic metre once you count the hire, the labour and the fact that a hand-mixed pour has to be finished before it starts going off.

Frequently Asked Questions

How much ballast and cement do I need?

For a 1:5 mix, take the ballast volume here and add one part cement for every five. A cubic metre of concrete needs roughly a cubic metre of ballast plus about 6–7 bags of 25 kg cement.

What is the difference between ballast and aggregate?

Ballast is a ready-blended mix of sharp sand and 20 mm aggregate, sold for making concrete. "Aggregate" on its own usually means the stone only, which you would then have to combine with sand yourself.

Is mixing my own concrete cheaper?

Below about half a cubic metre, yes. Above a cubic metre, ready-mix usually wins once mixer hire, labour and the risk of a poorly mixed batch are counted.

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