Concrete
Concrete Calculator
Enter your measurements to get the volume of concrete to order, and the cement, sand and aggregate for a site mix.
Results update as you type. Nothing to sign up for.
Measurements
Of the slab, footing, path or column.
A domestic path is often 100 mm; a driveway 150 mm or more.
Material
Site mixes show a cement, sand and aggregate breakdown.
You need approximately
1.65m³
of concrete to order
Measured volume
1.5m³
Waste allowance
10%
In litres
1,650L
In cubic yards
2.16yd³
Site-mixed quantities are an estimate for general work. For anything structural — foundations carrying load, reinforced slabs, columns — use a specified mix design from your engineer or supplier.
Volume
- Measured volume
- 1.5 m³1.5 m³
- With 10% waste
- 1.65 m³
- In litres
- 1,650 L
- In cubic feet
- 58.3 ft³
Materials for a 1:2:4 mix
- Dry material needed
- 2.541 m³wet volume × 1.54
- Cement
- 11 bags of 50 kg550 kg
- Sand
- 0.73 m³about 1162 kg
- Aggregate
- 1.45 m³about 2178 kg
- Water
- 275 L0.5 × cement mass
What you measured
- Length
- 5 m
- Width
- 3 m
- Depth
- 10 cm
What to buy
Cement (50 kg bags)
11 bags
Sand
0.73 m³
Aggregate
1.45 m³
Water
about 275 L
Add gradually — workability first, strength suffers if you overdo it.
Want a cost too?
Add your prices in the cost section of the form. We never guess prices — a made-up figure is worse than none.
How to measure a concrete pour
- Measure the longest and widest points inside the formwork, then take several depth readings because ground is rarely level.
- Use the intended finished depth, not the loose excavation depth; add sub-base separately rather than treating it as concrete.
- For steps, footings or irregular pours, split the job into simple rectangles and calculate each one.
Metric: Use metres for length and width, and millimetres or metres for depth.
Imperial: Use feet for plan dimensions and inches or feet for depth; units can be mixed safely.
How to work out how much concrete you need
- Measure the length and width of the pour, and the depth.
- Multiply all three together for the volume. Keep the units consistent — the calculator handles that for you.
- Add a waste allowance, usually 5–10%, because ground is never perfectly level.
- For ready-mix, order that volume. For a site mix, split the dry volume by the mix ratio.
- Work out cement bags from the cement volume, then water from the cement weight.
Assumptions
These are the defaults behind a new calculation. You can change the relevant values in the calculator; the result always shows the assumptions it used.
Dry-volume allowance: 1.54× finished volume
Industry rule of thumb — Site-mixed ingredients need more loose volume than the finished concrete they make.
Water-cement ratio: 0.5 by weight
Our default — Add water gradually and follow the mix or engineer specification where one exists.
The concrete formula
Volume = length × width × depth
Order volume = volume × (1 + waste %)
Dry volume = order volume × 1.54
Cement volume = dry volume × cement parts ÷ total parts
Cement bags = cement volume ÷ 0.0347 m³ per 50 kg bag
Water = cement weight × 0.5
Example: a 5 × 3 m slab, 100 mm deep
That is 1.5 m³ of concrete, or 1.65 m³ with a 10% allowance. For a 1:2:4 site mix the dry volume is 2.54 m³, which splits into 0.36 m³ of cement — about 11 bags of 50 kg — with 0.73 m³ of sand, 1.45 m³ of aggregate and roughly 275 litres of water.
Frequently asked questions
How many bags of cement do I need per cubic metre?
For a 1:2:4 mix, roughly 6 to 7 bags of 50 kg per cubic metre of finished concrete. A richer 1:1.5:3 mix needs around 8, and a leaner 1:3:6 mix around 4 to 5. The calculator works it out from your actual volume.
What does 1:2:4 mean?
One part cement to two parts sand to four parts aggregate, measured by volume. It is the usual general-purpose site mix. Lower numbers mean more cement and more strength.
How thick should a concrete slab be?
A garden path or shed base is commonly 75–100 mm. A driveway for cars is usually 150 mm, often with reinforcement. Anything carrying real load should be specified by an engineer.
How much water goes into concrete?
About half the weight of the cement, so a 50 kg bag takes roughly 25 litres. Add it gradually: extra water makes the mix easier to place and noticeably weaker.
Should I order ready-mix or mix on site?
Above about 1 m³, ready-mix is usually faster, more consistent and often cheaper once you count labour and hire. Below that, mixing on site is practical. Choose ready-mix above and the calculator gives you the order volume alone.