Concrete Calculator
Volume = length × width × (thickness ÷ 100) × quantity — enter length and width in metres and thickness in centimetres, and get cubic metres, litres and 25 kg bags.
Reviewed by the OmniCalc teamMethod verified 2026-07-01
1.2m³
= 1,200 litres
Volume 1.2 m³, buy at least 110 bagsBuy at least 110 bags (25 kg each).
Show steps
- Volume = length × width × (thickness ÷ 100) × quantity = 4 m × 3 m × (10 cm ÷ 100) × 1 = 1.2 m³.
- In litres: 1.2 m³ × 1000 = 1,200 litres.
- Bags (25 kg, ~0.011 m³ each): ceil(1.2 ÷ 0.011) = 110 bags.
How to use the concrete calculator
- 1Enter the slab or footing length and width in metres (m).
- 2Set the thickness in centimetres (cm) and the quantity — how many identical pours you have.
- 3Read the volume in cubic metres, plus litres and 25 kg bags, with the working under Show steps.
Add a little for waste
An uneven sub-base and a little spillage mean you’ll actually need slightly more concrete than the perfect volume. Order about 5–10% extra— it’s far better to have a little left over than to run out of mix halfway through the pour.
Frequently asked questions
How much concrete do I need?
Multiply the slab's length by its width and by the thickness in metres, then by how many identical pours you have: volume = length × width × (thickness ÷ 100) × quantity. A 4 m × 3 m slab that is 10 cm thick needs 1.2 m³.
Why is thickness entered in centimetres?
Slabs and footings are usually specified in centimetres (a 10 cm slab, a 15 cm footing), while length and width are in metres. The calculator divides the thickness by 100 to convert it to metres before multiplying, so all three dimensions share the same unit.
How many 25 kg bags is that?
A 25 kg bag of pre-mixed concrete yields roughly 0.011 m³ once set, so the bag count is the volume divided by 0.011, rounded up. Bagged mix is convenient for small jobs; for anything above about half a cubic metre, ready-mix delivery is usually cheaper.
Should I add a waste allowance?
Yes. Uneven sub-base, spillage and over-excavation all eat into your mix, so most builders order 5–10% extra. Round the slab dimensions up slightly, or simply buy a bag or two more than the bare figure shown here.