Gravel and Topsoil Calculator

The builders' merchant sells by weight and the job is measured in area, so somewhere in between there is a density nobody tells you. Gravel is about 1.6 tonnes to the cubic metre; bark is a fifth of that. Guess it and you order twice what you need, or half — and a bulk bag is not something you can send back.

Material

Shape of the area

Length in metres

The longer side.

Width in metres

The shorter side.

Depth in millimetres

50mm is usual — 50mm is enough to cover; deeper and it is hard to walk on.

Extra allowed, per cent

For settling, spillage and awkward corners. Ten is sensible.

Bulk bag size, in cubic metres

A builder's bulk bag is about 0.6.

2.2 m³ · about 3.52 tonnes

40m² at 50mm, with 10% extra. That is 4 bulk bags, or 110 small bags of 20 litres. One bulk bag covers 12m² at this depth.

✓ The volume is the area times the depth — 40m² at 50mm is 2 cubic metres
✓ The extra is added on top rather than folded into the depth — 10% on 2 gives 2.2 cubic metres
✓ The weight follows from the density — gravel or shingle is about 1.6 tonnes a cubic metre — 3.52 tonnes here
✓ The bags hold at least what the job needs — 4 bulk bags at 0.6m³ holds 2.4 — the job needs 2.2
✓ The small bags come to the same volume — 110 bags of 20 litres, if you are carrying it rather than tipping it
✓ One bulk bag covers what it says — 12m² at 50mm — the same sum read the other way round

What this generator does

Works out the area, multiplies by the depth for a volume, adds an allowance, converts to tonnes at the material's density, and counts the bags that volume takes.

How to use this tool

Pick the material; the usual depth for it fills itself in.
Give the area as a rectangle, a circle, or straight in square metres.
Set the depth and how much extra to allow.
Read the cubic metres, the tonnes and the number of bags.

Understanding the controls

Material: Gravel, topsoil, bark, sharp sand, MOT type 1 sub-base, mixed concrete or compost. Each has its own density and its own usual laying depth, which fills in when you choose it.
Shape of the area: A rectangle by length and width, a circle by diameter, or the area straight in square metres if you have already measured it.
Depth in millimetres: How deep the material goes. Fifty millimetres covers a path in gravel; topsoil for beds wants a hundred and fifty; a sub-base under a patio a hundred.
Extra allowed, per cent: Added on top for settling, spillage and the corners that always take more than the drawing says. Ten per cent is sensible.
Bulk bag size: How much a bulk bag holds where you are buying. A builder's bulk bag is around 0.6 cubic metres, but they vary and some are sold by weight instead.

Common use cases

Ordering gravel for a drive or a path
Working out topsoil for raised beds
Buying bark chippings by the bag rather than the tonne
Sizing the sub-base under a patio or shed
Checking a quote that came back in tonnes when you measured in metres

How this generator works

Volume is area times depth, which is the easy half. The weight comes from the density of the material, which is the half people guess: 1.6 tonnes a cubic metre for gravel and sand, 2.1 for sub-base, 2.4 for concrete, but only 0.35 for bark. The allowance is added to the volume rather than folded into the depth, so the depth you asked for is the depth you get. The checks then read the sum back the other way: the bags bought must hold at least the volume needed, and one bulk bag's coverage in square metres times the depth must come back to the bag's own volume.

Randomness and fairness

Nothing is random. The same area and depth always give the same order.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

Densities are typical figures for loose material; wet topsoil and damp sand weigh appreciably more.
Sub-base and sand compact when whacked down, by roughly a fifth — the allowance covers some of that, not all.
Bulk bag sizes vary between suppliers, and some are sold by weight rather than volume.
This is quantities, not construction advice: how deep a base needs to be depends on what goes on top of it.

Privacy and your data

The quantities are worked out in your browser. No measurements are transmitted or stored anywhere.