One cubic metre of concrete at a 350 kg mix takes about ten 35 kg bags of cement, 800 kg of sand, 1,000 kg of gravel and 175 litres of water. The calculator applies that mix to your volume and adds the waste allowance you set.
The assumptions behind this calculation
- A mix of 350 kg of cement per cubic metre. That is the usual mix for a patio slab, a shed base or a path, none of them structural.
- Proportions used per cubic metre: 350 kg cement, 800 kg sand, 1,000 kg gravel, 175 litres water.
- Waste defaults to 10 percent. It covers an uneven sub-base, what stays in the mixer and what lands next to the form.
- The calculation assumes a level sub-base. On ground dug by hand, real thickness often runs several centimetres over the planned figure in places.
This calculator works in metres and kilograms. Bag sizes, mix ratios and the names of the materials differ between markets, so figures from a site written for another country will not transfer directly.
What this calculator does not replace
A slab carrying a building, a foundation, a retaining wall or a suspended slab are structural calculations, not volume calculations. The mix, the reinforcement, the soil and the frost depth are engineering questions, and they may be subject to inspection where you live. This calculator gives you a quantity of material; it does not tell you whether your structure stands up.
Frequently asked questions
Should I count 25 kg or 35 kg bags?
Both exist. The calculator lets you enter the real weight of the bag you buy. At the same mix, the number of bags changes, the amount of cement does not.
Is ready mixed concrete cheaper?
From roughly one and a half cubic metres upwards, a delivered truck often becomes competitive, and above all it removes the time pressure: beyond that volume, mixing on site takes several people and an organisation few private jobs have.
Why is waste set to 10 percent by default?
Because that is the order of magnitude on a sub-base dug by hand. On a formed and levelled surface, 5 percent is enough. On uneven ground, 15 percent is not excessive.