The method

How this calculator works

Volume is area × depth

Measure the length and width of the area in metres and multiply them to get the area in square metres. Then multiply by the depth in metres: 50 mm is 0.05 m. The answer is the volume in cubic metres (m³).

If the area is not a rectangle, split it into rectangles, work out each one and add them. For a circle, the area is about 3.14 × the radius × the radius.

Weight depends on the material

Gravel, sand and soil are usually sold by weight, in tonnes or bags, so the volume has to be converted using the material's density. A cubic metre of gravel weighs roughly 1.6 tonnes, while compacted sub-base such as MOT Type 1 is heavier at around 2.1 tonnes.

These are typical figures, not exact ones. Wet material is heavier, and stone size changes how tightly it packs. If your supplier gives a density or says how many m³ a bag covers, use their figure by choosing “Enter my supplier's density”.

Add a little extra, then round up to whole bags

Loose material settles, sub-bases are compacted, and ground is rarely perfectly level. A 10% allowance is common. The calculator adds it to the volume before working out the weight and the number of bags.

Bag sizes vary: bulk bags are often around 800–900 kg, while small bags are usually 20–25 kg. Enter the size your supplier sells and the calculator rounds up to whole bags.

Worked example

Example: a 5 m × 3 m area of decorative gravel, 50 mm deep

  1. Area: 5 × 3 = 15 m².
  2. Volume: 15 × 0.05 m = 0.75 m³.
  3. Add 10% for settling: 0.75 × 1.1 = 0.825 m³.
  4. Weight: 0.825 × 1.6 t/m³ = 1.32 tonnes.
  5. Bags: 1,320 kg ÷ 850 kg = 1.55, rounded up to 2 bulk bags.

Practical answer: Order about 1.32 tonnes, which is 2 bulk bags of 850 kg. You will have a little left over for topping up later.

Before you buy

Common measuring mistakes

  • Entering the depth in centimetres or metres when the field asks for millimetres.
  • Ordering by volume when the supplier sells by weight, or the other way round, without converting.
  • Using one density for every material: a tonne of sub-base covers much less than a tonne of topsoil.
  • Forgetting the sub-base under a driveway or path, which is ordered separately from the top layer.
  • Not checking that the delivery vehicle and bulk bags can reach where you need them.