Why is there a compaction allowance?

Loose material settles when it is laid and rolled, so the delivered volume has to exceed the finished volume. Ten percent is typical and heavily compacted bases need more.

Construction & HomeoutdoorUnited States only

Gravel and Mulch Calculator

Gravel or mulch by volume and weight for a given area and depth. Depth is the input that decides everything and fifty millimetres is the usual minimum for coverage.

Also called: gravel calculator, mulch calculator, how much gravel.

Tonnes needed

4.13

4.13 tonnes, which is 2.75 m³ of gravel or mulch at 50mm deep over 50 m².

Volume (m³)

2.75

Area (m²)

In cubic feet

97.1

In 25 kg bags

165

Loads at 2.8 m³

0.98

How this is calculated

Method and background

How this is calculated

Area times depth gives volume, and bulk density converts it to the tonnes it is sold by. Below about fifty millimetres gravel does not cover the ground properly and weeds come through; above about seventy-five it becomes difficult to walk on. Mulch is lighter than gravel by a factor of three or more, so the same volume is a very different weight.

volume = area * depth, plus compaction allowance; mass = volume * bulk density

A: Area (m2)
d: Depth (metres)
density: Bulk density (kg/m3)

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

fifty millimetres over a driveway

Length (m): 10
Width (m): 5
Or the area directly (m²): 0
Depth (mm): 50
Bulk density (kg/m³): 1,500
Compaction and wastage: 10%

Tonnes needed4.13

50 * 0.05 * 1.1, at 1500 kg/m³

Open this example

mulch is far lighter for the same volume

Length (m): 10
Width (m): 5
Or the area directly (m²): 0
Depth (mm): 50
Bulk density (kg/m³): 400
Compaction and wastage: 10%

Tonnes needed1.1

boundary: the density input doing the work

Open this example

no area needs nothing

Length (m): 0
Width (m): 0
Or the area directly (m²): 0
Depth (mm): 50
Bulk density (kg/m³): 1,500
Compaction and wastage: 10%

Tonnes needed0

degenerate case

Open this example

Method and limits

What it assumes

A uniform depth over the whole area.

What it deliberately does not model

Bulk density varies widely by material: gravel around 1500, bark mulch around 400.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is there a compaction allowance?: Loose material settles when it is laid and rolled, so the delivered volume has to exceed the finished volume. Ten percent is typical and heavily compacted bases need more.