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Concrete Volume Calculator

Calculate the volume of concrete needed for slabs, footings, columns and post holes, including a waste allowance and approximate bag quantities.

Calculate concrete

Measurement system
Metric uses metres, millimetres and cubic metres. Imperial uses feet, inches and cubic yards.
Waste allowance
A waste allowance can help account for uneven ground, formwork variation, spillage and material left in the mixer.
Concrete to order
1.65 m³
Net volume
1.50 m³
Litres
1,650 L
Approximate weight
3.96 tonnes
25 kg bags
150 bags

Includes a 10% waste allowance.

Bag quantities are estimates. Check the yield stated on your chosen product before buying.

For larger pours, compare the cost and effort of bagged concrete with ready-mix delivery.

This calculator provides an approximate material estimate only. Concrete requirements depend on ground conditions, project design, concrete specification, reinforcement and local building requirements. For structural work, foundations, retaining walls or work requiring Building Regulations or code approval, consult a qualified professional.

How to use this calculator

  1. Choose the project type, such as a slab, footing, column or post holes.
  2. Select Metric or Imperial measurements.
  3. Enter the relevant dimensions.
  4. Choose a waste allowance and bag option, if needed.
  5. The calculator estimates the concrete volume to order, plus approximate weight and bag quantities.

How to calculate concrete volume

For a rectangular slab or footing, multiply length by width by depth. Make sure all measurements use the same unit before calculating.

concrete volume = length × width × depth
  • A circular column uses π × (diameter ÷ 2)² × height.
  • Post holes use π × (diameter ÷ 2)² × depth × number of holes.
  • Concrete to order = net volume × (1 + waste percentage ÷ 100).

Example: a 5 m × 3 m slab, 100 mm deep

  1. 100 mm is 0.1 m, so 5 × 3 × 0.1 = 1.5 m³ net volume.
  2. With a 10% waste allowance, 1.5 × 1.10 = 1.65 m³ to order.
  3. That is 1,650 litres and roughly 3.96 tonnes of concrete.

Metric and imperial concrete measurements

The calculator converts every measurement into a single internal volume before showing results in the selected measurement system.

Equivalent metric and imperial units used for concrete estimating
MetricImperial
Metres, centimetres and millimetresFeet and inches
Cubic metres (m³)Cubic yards (yd³)
LitresCubic feet (ft³)
20 kg and 25 kg concrete bags40 lb, 60 lb and 80 lb concrete bags
Kilograms and tonnesPounds and US tons

How much concrete waste should I allow?

A small waste allowance can help account for uneven ground, formwork variation, spillage and material left in a mixer. For straightforward work, 5% to 10% is often used as a starting estimate. Consider a higher allowance for irregular excavations, trenches, post holes or uneven ground.

Ready-mix concrete or bags?

Bagged concrete can be practical for small repairs, pads and individual post holes. For larger pours, ready-mix delivery may be more practical than transporting, mixing and placing many bags. Compare the total cost, delivery requirements, access and available labour before ordering.

Frequently asked questions

How do I calculate the amount of concrete I need?

For a rectangular slab, multiply length × width × depth. Convert all dimensions into the same unit first. The result is the net concrete volume before allowing for waste.

How do I calculate concrete in cubic metres?

Measure length, width and depth in metres, then multiply them together. For example, 5 m × 3 m × 0.1 m = 1.5 m³.

How do I calculate concrete in cubic yards?

Measure the area in feet, calculate the volume in cubic feet, then divide by 27. One cubic yard contains 27 cubic feet.

How many litres are in one cubic metre?

One cubic metre equals 1,000 litres.

How many cubic feet are in one cubic yard?

One cubic yard equals 27 cubic feet.

How many 25 kg bags of concrete make one cubic metre?

It depends on the stated yield of the product. As a general estimate, a 25 kg ready-mix bag may yield around 0.011 m³, so approximately 91 bags are needed for 1 m³ before allowing for waste. Always check the packaging for the actual yield.

How many 80 lb bags of concrete make one cubic yard?

It depends on the product yield. An 80 lb bag is commonly estimated to yield around 0.60 cubic feet, meaning approximately 45 bags are needed for 1 cubic yard before allowing for waste. Check the product packaging for the actual yield.

Should I add extra concrete for waste?

A small waste allowance can help cover uneven ground, formwork variation, spillage and material left in the mixer. For straightforward work, 5% to 10% is a practical starting estimate.

Can I use this calculator for foundations?

You can use the calculator to estimate concrete volume for a footing or trench. Foundation design, depth, reinforcement and concrete specification should be based on the project requirements and professional advice where needed.

What thickness should my concrete slab be?

The right slab thickness depends on the intended use, loads, ground preparation, reinforcement and local requirements. This calculator estimates material volume only and does not provide structural design advice.