If you are not sure how much soil you will need for the job, using big strides, pace out the length and the width of the area in question.  When you have this, multiply the length (in metres), by the width (again in metres), by the required depth (in millimetres).  This is not the exact measurement, but it will give you a guide to the quantity you will require. NB one big stride is approx. one metre.

When using products that you plan to compact down like gravel fines for pathways or yellow sand for a base under paving, an adjustment needs to be made to the quantity ordered.  As a general rule only, 20% extra needs to be ordered to factor in the compaction of the product.