How to calculate concrete
- Measure the area. Measure length and width in metres and thickness in millimetres. For round shapes measure the diameter.
- Convert millimetres to metres. Divide by 1,000. A 100 mm slab is 0.1 m thick.
- Multiply to get cubic metres. Length × width × thickness gives the volume in m³. For a circle use π × radius² × thickness.
- Add waste. Multiply by 1.05 to 1.10 to add 5–10% for spillage and uneven ground.
- Convert to bags. Divide cubic metres by the bag yield (about 0.0094 m³ for a 20kg bag) for the number of bags, rounding up.
Worked example: a 4 × 3.5 m patio, 100 mm thick
- Volume: 4 × 3.5 × 0.1 = 1.40 m³.
- Plus 5% for waste: 1.47 m³.
- 20kg bags at 0.0094 m³ each: 1.47 ÷ 0.0094 = 157.0, so 158 bags (3160 kg to carry).
Concrete formulas by shape
All measurements in metres; divide millimetres by 1,000 first. Results are in cubic metres.
- Slab
- length × width × thickness
- Round slab
- π × (diameter ÷ 2)² × thickness
- Continuous footing
- total length × width × depth
- Pad footing
- length × width × depth × number of footings
- Monolithic slab
- length × width × thickness + perimeter band × (edge depth − thickness), where perimeter band = length × width − (length − 2 × edge width) × (width − 2 × edge width)
- Bell footing
- π × r² × pier height + π × bell height ÷ 3 × (R² + R × r + r²), with r = pier radius and R = bell radius
- Wall
- length × height × thickness
- Column / pier
- π × (diameter ÷ 2)² × height
- Post hole
- (hole area − post area) × depth; round hole π × (diameter ÷ 2)², square hole width²
- Tube / ring
- π × [(outer ÷ 2)² − (inner ÷ 2)²] × height
- Curb & gutter
- [curb depth × (curb height + flag thickness) + gutter width × flag thickness] × length
- Steps (solid, n steps)
- width × rise × run × (1 + 2 + … + n). With a landing, the landing is the top step: width × rise × [run × (1 + … + (n − 1)) + landing depth × n]
1 m³ = 1,000 litres.
How many bags of concrete per cubic metre
| Bag size | Yield per bag | Bags per m³ |
|---|---|---|
| 20kg | 9.4 litres | 107 |
Frequently asked questions
How many 20kg bags of concrete make 1m³?
Bags usually say about 10 litres for 20kg; we count 9.4 litres (0.0094 m³), what a bag really gives once mixed, so you don't run short. One cubic metre takes about 107 bags, just over 2 tonnes to carry.
How much concrete do I need for a 3m x 3m slab?
A 3 m × 3 m slab 100 mm thick is 0.9 m³. Adding 5% for waste brings it to about 0.95 m³: 101 bags of 20kg concrete.
How much extra concrete should I order?
Plan on 5–10% extra. Uneven ground, formwork that bows out and spillage all use concrete, and running short in the middle of a pour is far worse than having a few bags left over.
Should I use bags or ready-mix?
Ready-mix suppliers usually have a minimum delivery of around 1 m³ and may charge for part loads, so small jobs are normally done with bags. For bigger jobs, a mix-on-site (volumetric) truck mixes the concrete at your door and you pay for what is used.
How much does a cubic metre of concrete weigh?
Normal concrete weighs about 2,400 kg per cubic metre, so a 0.5 m³ pour is roughly 1.2 tonnes.
How do I convert millimetres to metres?
Divide by 1,000. A 100 mm slab is 0.1 m thick, a 225 mm footing is 0.225 m deep.
Can I enter feet and inches?
Yes. Type them with their marks, like 10' 6" or 6ft, and the calculator converts them to metres or millimetres. Results stay in cubic metres and kilograms, the units concrete and bags are sold in.
How we estimate prices
We don't show UK price estimates yet. Open "Use your own prices" in the calculator, enter what your local merchant charges and every cost appears with your numbers.
We will only show our own estimates once they are checked against real merchant prices, with the source and date.