Concrete calculator
Enter the length and width in metres and the depth in millimetres to get the volume in m³ and the number of 25kg bags of ready-mix.
Estimates only. Check the manufacturer's coverage rates.
How this is calculated
The volume is length x width x depth. Depth is entered in millimetres, so it is divided by 1,000 to turn it into metres: volume (m³) = length (m) x width (m) x depth (mm) / 1000. A 3 m x 2 m base at 100 mm deep is 0.6 m³.
An allowance is then added for uneven ground, spillage and formwork movement. The default is 5%, so 0.6 m³ becomes 0.63 m³. You can change it to suit the job.
To turn volume into bags, the calculator divides by the bag yield, the volume of concrete one bag makes. The default assumes concrete weighs 2,400 kg per m³ and each bag weighs 25 kg, so one bag makes 25 / 2,400 = about 0.0104 m³ (10.4 litres). The result is rounded up to a whole bag. The total weight is the volume with allowance x 2.4 tonnes per m³, which is useful when planning how to move the bags.
Bag yield varies by product, so check the bag and enter its stated yield if it differs. For more than about 1 m³, consider ready-mixed concrete delivered to site, which saves a lot of mixing.
- Bag yield varies by product, so check the bag.
- For more than about 1 m³, consider ready-mixed delivered concrete.
Frequently asked questions
How much concrete does a 25kg bag make?
About 0.0104 m³ (10.4 litres), assuming concrete weighs 2,400 kg per m³. Yield varies by product, so check the bag.
How do I work out concrete volume?
Multiply length (m) by width (m) by depth (mm), then divide by 1,000 to get m³.
Why add an allowance?
Uneven ground, spillage and formwork movement all use extra concrete. The calculator adds 5% by default.
When should I order ready-mixed concrete?
For more than about 1 m³, ready-mixed concrete delivered to site is usually worth considering.