Concrete Calculator
Concrete volume for a slab or footing, the weight, and the cement, sand and aggregate to mix it yourself.
At a glance
- What you enter
- Length, Width, Thickness, Waste allowance, Cement part, Sand part, Coarse aggregate part, Dry volume factor, Bulk density of cement, Cement bag size, Price per bag of cement
- What you get
- Concrete to order, Weight of the concrete, Cement bags to buy, Cement by weight, Sand, Coarse aggregate, Area of the slab, Cost of the cement
- Formula shown
- Yes, every step, right below your results.
- Cost and privacy
- Free, no account needed, and nothing you type is stored.
See worked examples ↓ Read the FAQ ↓ Share this calculator ↓
Results are estimates for general information only, always verify with a professional or official source before acting on them.
How to use the Concrete Calculator
- 1 Enter Length (mm), The longer dimension of the slab, footing or pad.
- 2 Enter Width (mm), The other dimension, at right angles to the length.
- 3 Enter Thickness (mm), 75 to 100 mm for a path, 100 to 150 mm for a shed base. A slab that carries a vehicle needs a thickness that has been specified.
- 4 Enter Waste allowance (%), Five per cent on a flat slab over a prepared base, ten per cent in a rough trench or over hardcore.
- 5 Enter Cement part, The first number of the mix ratio. A 1:2:4 mix uses 1 part cement.
- 6 Enter Sand part, The middle number of the mix ratio. Sharp sand is the usual fine aggregate.
- 7 Enter Coarse aggregate part, The last number of the mix ratio. 20 mm crushed stone is the usual coarse aggregate.
- 8 Enter Dry volume factor (times), Loose sand and stone contain air that the cement paste fills, so the dry ingredients bulk up. The standard allowance is 1.54 times the wet volume.
- 9 Enter Bulk density of cement (kg/m3), Turns the volume of cement into the weight the bag is sold by. About 1440 kilograms per cubic metre for ordinary Portland cement.
- 10 Enter Cement bag size (kg), 25 kg is the common bag in the UK and Europe, 50 kg in the US, and 20 kg in some ranges.
- 11 Enter Price per bag of cement, Optional. Used for the cement cost only, not for the sand or the stone.
- 12 Press Calculate. You get Concrete to order, Weight of the concrete, Cement bags to buy, Cement by weight, Sand, Coarse aggregate, Area of the slab, Cost of the cement, and every intermediate step stays visible under "How is this calculated?"
- 13 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.
Worked examples
Real numbers run through this calculator. Open any example to load it into the form above and change it.
A garden path in 1:2:4
A 3000 by 2000 millimetre path, 100 millimetres thick, with five per cent waste.
You enter
- Length
- 3,000 mm
- Width
- 2,000 mm
- Thickness
- 100 mm
- Waste allowance
- 5%
- Cement part
- 1
- Sand part
- 2
- Coarse aggregate part
- 4
- Dry volume factor
- 1.54 times
- Bulk density of cement
- 1,440 kg/m3
- Cement bag size
- 25 kg
- Price per bag of cement
- $0.00
You get
- Concrete to order
- 0.63 m3
- Weight of the concrete
- 1.51 t
- Cement bags to buy
- 8
- Cement by weight
- 199.58 kg
- Sand
- 277.20 litres
- Coarse aggregate
- 554.40 litres
- Area of the slab
- 6.00 m2
- Cost of the cement
- $0.00
Six square metres and 0.6 of a cubic metre of wet concrete, which is 0.63 with the waste allowance and 1.51 tonnes. The dry ingredients come to 0.97 of a cubic metre, so 200 kilograms of cement, which is 8 bags of 25 kilograms, plus 277 litres of sand and 554 litres of stone.
A shed base in 1:2:4 with ten per cent waste
A 4000 by 3000 millimetre base, 100 millimetres thick, over a rough excavated base.
You enter
- Length
- 4,000 mm
- Width
- 3,000 mm
- Thickness
- 100 mm
- Waste allowance
- 10%
- Cement part
- 1
- Sand part
- 2
- Coarse aggregate part
- 4
- Dry volume factor
- 1.54 times
- Bulk density of cement
- 1,440 kg/m3
- Cement bag size
- 25 kg
- Price per bag of cement
- $0.00
You get
- Concrete to order
- 1.32 m3
- Weight of the concrete
- 3.17 t
- Cement bags to buy
- 17
- Cement by weight
- 418.18 kg
- Sand
- 580.80 litres
- Coarse aggregate
- 1,161.60 litres
- Area of the slab
- 12.00 m2
- Cost of the cement
- $0.00
Twelve square metres and 1.2 cubic metres of wet concrete, which is 1.32 with the waste allowance and 3.17 tonnes. That needs 418 kilograms of cement, so 17 bags of 25 kilograms, with 581 litres of sand and 1162 litres of stone. The cement works out at 317 kilograms per cubic metre, which is the published figure for this mix.
A strip footing in a leaner 1:3:6
A 10 metre strip footing 600 millimetres wide and 200 millimetres deep, with ten per cent waste.
You enter
- Length
- 10,000 mm
- Width
- 600 mm
- Thickness
- 200 mm
- Waste allowance
- 10%
- Cement part
- 1
- Sand part
- 3
- Coarse aggregate part
- 6
- Dry volume factor
- 1.54 times
- Bulk density of cement
- 1,440 kg/m3
- Cement bag size
- 25 kg
- Price per bag of cement
- $0.00
You get
- Concrete to order
- 1.32 m3
- Weight of the concrete
- 3.17 t
- Cement bags to buy
- 12
- Cement by weight
- 292.72 kg
- Sand
- 609.84 litres
- Coarse aggregate
- 1,219.68 litres
- Area of the slab
- 6.00 m2
- Cost of the cement
- $0.00
The same 1.32 cubic metres of concrete, but the leaner mix spreads the dry ingredients over ten parts instead of seven, so the cement drops to 293 kilograms, which is 12 bags of 25 kilograms, while the sand and stone rise to 610 and 1220 litres. Twelve bags instead of seventeen for the same volume is what mass fill mixes are for.
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