Find the area of the section you wish to cover 

Multiply the average width by the average length in metres. For mulch in a garden you may have to divide up the garden into the various beds, work out their individual areas and then add them together.

Then decide what depth of the product you need to have. For mulch this is usually 100 to 150 millimetres. Remember the deeper the mulch the less likely that weeds will develop. For gravel on a path usually 100 millimetres will suffice. 

Then multiply the area which will be in metres squared by the depth in millimetres and divide by 1000 to get the amount you need in cubic metres. 

For example you have a garden bed which is about 3 metres long by 2 metres wide you will need 3 x 2 x 150 divided by 1000 equals .9 of a cubic metre. If you decide that 150 millimetres is too deep the calculation is 3 x 2 x 100 divided by 1000  equals .6 of a cubic metre. The decision you then have to make is whether the bed is slightly bigger or smaller than your estimate and whether the area is largely weed free.