How Many Bags of Concrete Do I Need per m³?
Short answer: a 25 kg bag of ready-mixed concrete makes about 11 litres, so you need about 91 bags per cubic metre. With 20 kg bags it is about 111. Below are bag counts for common shed bases and slabs.
Bags per cubic metre
| Bag size | Typical yield | Bags per m³ |
|---|---|---|
| 25 kg | about 11 litres | 91 |
| 20 kg | about 9 litres | 111 |
Yields vary a little between brands, so check the bag. A cubic metre of concrete weighs about 2.4 tonnes, which is why bags only make sense for small jobs.
The formula
Bags = volume × 1.10 (waste) ÷ 0.011, rounded up
Bags of 25 kg concrete for common bases (with 10% waste)
| Base | Depth | Volume with waste | 25 kg bags |
|---|---|---|---|
| 2 × 2 m | 100 mm | 0.44 m³ | 40 |
| 3 × 3 m | 100 mm | 0.99 m³ | 90 |
| 3 × 4 m | 100 mm | 1.32 m³ | 120 |
| 4 × 4 m | 100 mm | 1.76 m³ | 160 |
| 3 × 3 m | 150 mm | 1.49 m³ | 135 |
A 100 mm slab on 100 mm of compacted MOT Type 1 is normal for a shed or garden building base. Use 150 mm for a driveway or anything carrying a vehicle.
Worked example: a 3 × 4 m shed base
3 × 4 × 100 ÷ 1,000 = 1.2 m³. Adding 10% gives 1.32 m³, and 1.32 ÷ 0.011 = 120 bags of 25 kg. That is about 3 tonnes of bags to move and mix, so compare the price of a mix-on-site delivery.
Bags, mix your own or ready-mix?
- Under about 0.5 m³: bags are simplest.
- 0.5 to 1 m³: mixing your own from cement and ballast is usually cheaper than bags.
- Over about 1 m³: a ready-mix or mix-on-site lorry is usually cheaper and much quicker, and the slab sets evenly.
Need the sub-base too? Use the gravel calculator with MOT Type 1.
Try the concrete calculator (m³ and bags)
Enter your own numbers below, or open the full concrete calculator (m³ and bags) with the formula and more examples.
This guide is general information with example numbers, not financial, tax or building advice. Your costs and results will differ. Terms
Frequently asked questions
How many 25 kg bags of concrete make a cubic metre?
About 91, as each 25 kg bag makes roughly 11 litres of concrete.
How many bags of concrete for a 3 x 3 m base?
About 90 bags of 25 kg for a 100 mm base, including 10% waste.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes.
How thick should a concrete shed base be?
100 mm of concrete on about 100 mm of compacted hardcore is normal for a garden shed.