Concrete Calculator
Work out how much concrete you need in cubic metres for a slab, footing or post holes – plus the number of 25 kg bags of concrete mix or postcrete and an estimated cost in GBP.
Formulas shown on this page · Last reviewed October 2026
How the concrete calculator works
volume (m³) = length (m) × width (m) × thickness (mm) ÷ 1,000
bags = volume × (1 + waste) ÷ bag yield
A 25 kg bag of dry concrete mix makes roughly 0.0116 m³ of concrete, so you need about 86 bags per cubic metre. Check the yield printed on the bag – it varies a little between brands.
Typical slab thickness
| Project | Thickness |
|---|---|
| Garden path, shed base | 75–100 mm |
| Patio | 100 mm |
| Driveway | 100–150 mm on a compacted MOT Type 1 sub-base |
| Garage floor | 100–150 mm |
Bags vs ready-mix
Bags from builders’ merchants and DIY stores are convenient for post holes and small pads under about 0.5 m³. Above that, a ready-mix truck is usually cheaper and gives a more consistent pour. For bigger jobs, many people mix their own from ballast and cement – roughly 1 part cement to 5 parts ballast for general concrete.
At the default prices, 1 m³ costs about £602 in 25 kg bags versus about £130 delivered as ready-mix. Prices vary by region, so enter your local quotes above.
Frequently asked questions
How many 25 kg bags of concrete are in a cubic metre?
About 86 bags. A 25 kg bag yields roughly 0.0116 m³ once mixed.
How much concrete do I need for a 3 × 3 m slab?
At 100 mm thick, a 3 × 3 m slab needs 0.9 m³ – about 0.99 m³ with 10% extra, or 86 bags of 25 kg.
How much extra concrete should I order?
Add 5–10% for spillage, uneven ground and formwork bowing. Running short mid-pour is far more costly than a little left over.
How do I convert mm to metres for the formula?
Divide millimetres by 1,000: 100 mm = 0.1 m, 125 mm = 0.125 m, 150 mm = 0.15 m.