How to Calculate Volume

The unit used to describe volume is cubic metres (m3).  Some examples of general items measured in cubic metres are concrete, sand, topsoil, mulch etc.  To help visualise what a cubic metre is equal to, 4 x 44 gallon drums = 1.25m³ and 4 x 200 litre drums = 1.25m³.

To calculate volume you multiply the length, width and depth of what you need to get an estimate of the amount (in cubic metres) of the material you need.


For example, the estimated volume of concrete needed for a garage floor slab 7.5m long by 3.5m wide and 0.1m thick (100mm or 10cm), is calculated as follows:

Estimated Volume = 7.5m x 3.5m x 0.1m = 2.63m³.

Note: When estimating your concrete needs it is also wise to add an additional 10% to allow for wastage or any inconsistencies (length x width x depth + 10%)

If you need to calculate how many bags of concrete are required for a job, the bags generally have information on the volume that the contents of each bag will produce.

The number of bags is calculated by dividing the estimated concrete volume by the volume produced by one bag.  In the example below let’s say a 40kg bag will produce .018m³. 

Note: Talk to your local hardware representative or other supplier to confirm the how much concrete can be obtained from a bag.

To continue using the garage floor concrete example above:

The estimated volume is 2.63m³ plus wastage of 10% (0.26m³) therefore 2.63m³ + 0.26m³ = 2.89m³.  For the sake of simplicity let us round it to 3.0m³.

Note: Talk to your supplier about how much ‘brickies’ sand and gravel you will also need.

Odd Shapes

Using the volume formula, the amount of mulch required to cover an area of garden 5m long by 2m wide by 10cm (0.1 metres) high is 5m x 2m x 0.1m = 1m3

For odd-shaped gardens break the area down into smaller portions to get an estimate.

For a circle shaped garden you need to use the formula 3.1415 x radius x radius (where the radius is half the distance across the circle) x depth.
The radius, of a circle is the distance from its centre to a point on the edge of the circle.

Therefore the calculation for a circular garden with a radius of 2.5m and 10cm of mulch is:

3.1415 x 2.5 x 2.5 x 0.1 = 1.96m³ or close enough to 2 cubic metres of mulch.

Your concrete volumes would also use the above formula.

Other Examples

If a trench 20m long, 600mm deep and 550mm wide is excavated, how much soil is has been removed?

The volume is 20m x .600mm x .550mm = 6.60m3