Concrete for a foundation slab

How to calculate concrete for a foundation slab

The volume of concrete is simply length × width × thickness. You enter the thickness (layer) in centimetres, so we convert it to metres:

volume (m³) = length (m) × width (m) × thickness (m)

Example: a 6 × 4 m slab, 15 cm thick → 6 × 4 × 0.15 = 3.6 m³. We recommend adding a ~5% reserve for waste and subgrade unevenness.

By hand, or ready-mix?

Up to roughly 1 m³, mixing by hand is still worthwhile, but it's laborious. For larger volumes (a foundation slab is usually 2–5 m³), delivering ready-mixed concrete from a plant is usually cheaper than the time and effort, and above all more consistent and higher quality. The calculator gives an approximate comparison of material cost for hand mixing versus ready-mix including delivery.

How to mix concrete by hand

A common ratio for foundation concrete (roughly C16/20) is 1 part cement : 3 parts sand : 3 parts gravel (8–16 mm grain size), plus water. Water is added at roughly half the volume of the cement – the mix should be workable, not runny.

1. Put some of the water in the mixer first.
2. Add the cement and let it mix into a thin paste.
3. Gradually add the sand and gravel.
4. Top up with the remaining water to the desired consistency.
5. Mix for 2–3 minutes until uniform.

Use the concrete within ~1 hour. After placing it, compact it (a vibrating screed or tamping) and let it cure – keep it moist for the first few days so it doesn't crack.

How many bags of cement and sand

For 1 m³ of concrete at a 1 : 3 : 3 ratio you need approximately 280–300 kg of cement (i.e. roughly 11–12 bags of 25 kg), 0.5 m³ of sand, and 0.5 m³ of gravel. The calculator computes this automatically from your volume. You can adjust the material and delivery prices.