About this calculator
Enter the size in feet or metres and choose the concrete grade.
How to use it
- Choose feet or metres and enter the dimensions.
- Choose the mix or materials.
- Read the quantities to order.
The formula
Dry volume = wet volume × 1.54; cement = dry × cement share ÷ 0.0347 m³ per bag
- 1.54
- allows for voids in dry materials and bulking
- 0.0347 m³
- volume of one 50 kg bag of cement
- M20 = 1:1.5:3
- cement : sand : aggregate
Worked example
A 12 × 10 ft slab, 5 inches thick, M20
- Volume = 12 × 10 × 5/12 ft = 50 cft = 1.42 m³.
- Dry volume = 1.42 × 1.54 = 2.18 m³.
- Cement = 2.18 ÷ 5.5 = 0.396 m³ = 11.42 bags, so 12 bags; sand 21 cft, aggregate 42 cft.
What the result means
One cubic metre of M20 concrete needs about 8 bags of cement.
Assumptions
- Nominal mix by volume.
Limitations
- Structural concrete should follow an engineer’s design; steel reinforcement is not included.
Frequently asked questions
How many cement bags for 1 m³ of M20?
About 8 bags.
What is M20 mix ratio?
1 : 1.5 : 3 (cement : sand : aggregate) as a nominal mix.
Why multiply by 1.54?
Dry materials shrink when mixed with water; 1.54 converts wet volume to the dry materials needed.
Last reviewed on 3 October 2026. Found a mistake? Tell us.