How it is calculated
Volume is length × width × thickness for a slab, or total length × width × depth for a strip foundation, plus a reserve for spills and uneven ground. To mix by hand, dry materials take about 1.54 times the finished volume because they shrink when mixed with water. They are split by the chosen ratio; cement weighs about 1,440 kg per m³.
Volume = length × width × thickness × (1 + reserve)Dry volume = volume × 1.54, split by the ratio cement : sand : gravelCement, kg = dry volume × cement share × 1,440Example
A 4 × 3 m slab 10 cm thick needs 1.26 m³ of concrete with a 5 % reserve. Mixed 1:2:4, that is about 400 kg of cement (16 bags of 25 kg), 0.55 m³ of sand and 1.11 m³ of gravel.
Tips from practice
- For more than 1–2 m³, ready-mix concrete delivered by a truck is usually cheaper and stronger.
- Keep fresh concrete damp for the first week so it does not crack.
- Ratios are a guide for small jobs. Structural work must follow a design.
Questions and answers
How many bags of cement per m³ of concrete?
With a 1:2:4 mix, about 320 kg, which is 13 bags of 25 kg per cubic metre.
Is ready-mix in bags a good option?
For small jobs, yes. A 25 kg bag gives about 12 litres of concrete, so 1 m³ needs over 80 bags.
How thick should a garden slab be?
Paths and shed bases are usually 8–10 cm. Garage floors and driveways need 15 cm or more with reinforcement.
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