Concrete calculator: volume, cement, sand and gravel
The sketch takes your dimensions, the maths gives the shopping list.
Dimensions
Bags, mixer and price
A 160 L mixer yields about 110 L per load, a 120 L about 80 L, a 190 L about 130 L.
Concrete volume 2.1 m³
What you need
- To plan for, allowance included
- 2.31 m³
- Bags to buy
- 35 bags
- Cement
- 854 kg
- Sand
- 1.9 t 1.19 m³
- Gravel
- 2.67 t 1.78 m³
- Water
- 427 L
- Mixer loads
- 21 loads
An estimate for small works. Load-bearing work (suspended slab, beam, house foundation) calls for a professional and the applicable standard (EN 206).
How to work out your concrete
Pick the type of work
Slab, strip footing, pad or square column, round column, stairs, curb or ring. The sketch changes with the work.
Enter your dimensions
Lengths in metres, thicknesses in centimetres, or feet and inches. The drawing takes the proportions of your work and shows its dimensions: a typing mistake is visible at once.
Set the mix and allowance, then copy the list
Mixes are chosen by use: general reinforced work by default, leaner for simply setting a post. The waste allowance is 10% by default. The result gives the cement bags to buy, sand, gravel, water and the number of mixer loads.
When to use it
A patio or shed base
A 5 × 3.5 m slab, 12 cm thick, is 2.1 m³, so 2.31 m³ to plan for with a 10% allowance. Beyond 2 to 3 m³, ready-mix delivery becomes more sensible than a mixer.
Pads for a fence or a pergola
Enter one pad, then the number of identical works: ten pads of 40 × 40 × 50 cm make 0.8 m³.
Outdoor stairs
The calculation follows the real shape of concrete stairs: a sloped slab, the waist, carrying the steps. Stairs solid down to the ground would often take more than twice as much.
A job made of several works
Add each work to the job: volumes add up and the shopping list covers the whole.
How the calculation is done
- The default mix, 1:2:3, is the general-purpose rule for reinforced work: 1 volume of cement, 2 of sand, 3 of gravel. For 1 m³ the tool gives 370 kg of cement (15 bags of 25 kg), 821 kg of sand, 1,155 kg of gravel and 185 L of water.
- Mixes are given as volumes of cement, sand and gravel, with their equivalent in kilos of cement per cubic metre: 1:3:6 ≈ 222 kg/m³, 1:2:4 ≈ 317 kg/m³, 1:2:3 ≈ 370 kg/m³, 1:1.5:3 ≈ 403 kg/m³.
- The volume of dry materials is taken as 1.54 times the volume of wet concrete: sand fills the gaps between the gravel, and cement the gaps between the grains of sand.
- Water is counted at half the weight of the cement. That is a maximum: add it little by little, concrete that is too wet is weaker.
- Bags are rounded up: nobody buys a third of a bag.
- The number of mixer loads depends on what your mixer really yields, always less than its drum volume. The value can be changed.
Frequently asked questions
How many bags of cement for 1 m³ of concrete?
For general reinforced work (1:2:3), allow 370 kg of cement per cubic metre: 15 bags of 25 kg, or 9 bags of 94 lb. For plain concrete at 1:2:4, 317 kg is enough: 13 bags of 25 kg.
How thick should a slab be?
As a guide: 8 to 10 cm (3 to 4 in) for a footpath, 12 cm (5 in) for a patio or shed base, 15 cm (6 in) with steel mesh as soon as a vehicle drives on it. Beyond that, ask a professional.
Mixer or ready-mix?
Below 1 m³, a mixer is enough: about ten loads. Between 1 and 3 m³, it is a long day with helpers. Beyond that, ready-mix guarantees even concrete poured in one go, which matters for a slab.
Why allow for waste?
A sub-base is never perfectly flat, formwork bows a little, and some concrete stays in the drum and the barrow. 5 to 10% more saves running short ten minutes from the end.
How do I work out an area or convert units first?
The area converter and the volume converter handle those conversions; the percentage calculator helps adjust an allowance.