Concrete calculator
Most small concrete jobs — a garden path, a shed base, a strip footing for a wall — start with the same question: how many cubic metres, and how many bags of cement? This calculator gives the volume and the ingredients for site-mixed concrete in a mixer.
Choose the mix by use. The classes follow the European standard EN 206 (C8/10, C16/20, C20/25): the first number is the cylinder strength and the second the cube strength, in N/mm².
Your measurements
Result
- Concrete volume
- 0.90 m³
- Cement
- 270 kg
- Cement bags
- 11 pcs
- Sand (0–4 mm)
- 0.45 m³
- Gravel (4–16 mm)
- 0.72 m³
- Water
- 162 L
Mix proportions are guide recipes for site-mixed concrete. For structural parts of a house, the engineer's design sets the class and reinforcement.
How the calculation works
- Volume (m³) = length × width × thickness (thickness converted from cm to m).
- Cement (kg) = volume × the cement content of the chosen mix: 250 kg/m³ for C8/10, 300 kg/m³ for C16/20 and 350 kg/m³ for C20/25. Bags are rounded up.
- Sand and gravel (m³) = volume × 0.45–0.55 m³ of sand and 0.80–0.85 m³ of gravel per m³ of concrete, depending on the mix.
- Water (litres) = volume × 170–190 L/m³.
Worked example
A strip footing 5 m long, 60 cm wide and 30 cm deep holds 5 × 0.6 × 0.3 = 0.9 m³. For C16/20 you need 0.9 × 300 = 270 kg of cement (11 bags of 25 kg), 0.45 m³ of sand, 0.72 m³ of gravel and about 162 litres of water.
What the calculator assumes
- The mixes are guide recipes for concrete mixed in a drum mixer with dry aggregates; sand and gravel are loose bulk volumes.
- If you use an all-in ballast (a natural sand-and-gravel mix), the quantity is roughly the sum of the two volumes.
- Water already in wet aggregates should be subtracted from the mixing water — concrete that is too wet loses strength.
When to adjust the result
- For a house's foundations, columns, beams and slabs, the concrete class and the reinforcement are set by the structural design.
- Above 1–2 m³, ready-mix concrete from a plant is usually safer (consistent recipe, one continuous pour) and not much more expensive.
- Site losses are not included: add 3–5%, especially for trenches with irregular sides.
Frequently asked questions
How many bags of cement are in a cubic metre of concrete?
It depends on the mix: about 300 kg of cement for a standard C16/20, which is 12 bags of 25 kg or 7.5 bags of 40 kg. Leaner mixes for paths use less, structural mixes more.
How do I convert cubic metres to cubic yards?
1 m³ = 1.308 yd³, and 1 yd³ = 0.765 m³. Switch the calculator to imperial units to get the volume in cubic yards and the dimensions in feet and inches.
Which concrete mix should I use for a garden path?
A lean mix such as C8/10 is common for paths and blinding layers. For driveways that take car traffic, a stronger mix and a sub-base are needed; for anything structural, follow the engineer's specification.