Concrete calculator
Choose the shape and units, enter the size and the waste allowance, and the page gives the concrete volume, the number of premix bags, and cement bags, sand and aggregate for a site mix.
- Volume and materials
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How to use it
- Pick slab, footing or round column, and metric or imperial units.
- Enter the sizes, how many, and the extra for waste.
- Read the volume, the bags of premix, and the cement, sand and aggregate for the mix you choose.
How the materials are worked out
Volume is length × width × thickness for a slab or footing, and π × radius² × height for a column, plus the waste percentage you choose (5 to 10% is usual). Premix bag counts use typical yields: 0.30 cu ft (0.0085 m³) for a 40 lb bag, 0.45 cu ft (0.0127 m³) for 60 lb and 0.60 cu ft (0.017 m³) for 80 lb.
For a site mix, the dry volume is about 1.54 times the wet volume. That dry volume is split by the mix ratio, such as 1 : 1.5 : 3 for M20, with cement at 1,440 kg per m³ in 50 kg bags. Water is estimated at a water-cement ratio of 0.5. Check quantities with your engineer for structural work.
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Questions
How many bags of cement are in one cubic metre of M20?
About 8 bags of 50 kg, from the 1.54 dry volume factor and the 1 : 1.5 : 3 mix. Enter 1 m³ with M20 to see cement, sand and aggregate.
How many 80 lb bags of concrete make a cubic yard?
About 45 bags, since each 80 lb bag makes roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
How thick should a concrete slab be?
Patios and paths are often 10 cm (4 in), driveways 12 to 15 cm (5 to 6 in). Follow local building rules for structural slabs.
How much extra should I order?
Add 5 to 10% for spillage, uneven ground and formwork, more for small or irregular pours.