The area of a square or rectangular garden is easy to calculate. Simply multiply the length by the width by the depth: For example: 5 metres long by 2 metres wide by 10cm (0.1 metres) deep is 5 x 2 x0.1 = 1m3
Circular areas use the formulae: Area = 3.1415 x radius x radius (where the radius is half the distance across the circle).
1cubic meter will cover 10square meters @ 100mm deep (10x1x0.1), or 20square meters @ 50mm deep (20x1x0.05)
If you need help, use this handy online calculator.