How to use it
- Measure the area you want to cover and type it as Area to cover.
- Type the Depth you want the layer to be, and choose the Material.
- Read Weight to order, Volume and Bags of 25 kg, and enter a density from the supplier if you have one.
How the weight is worked out
The volume is the area multiplied by the depth. The weight is the volume multiplied by the bulk density of the material, which is its weight per cubic metre as it sits in a heap, with the air between the particles counted in.
The typical densities are taken from the density table of Engineering ToolBox, which gives the range for each material in pounds per cubic foot. The page uses the middle of each range, and shows the density used so you can change it.
volume (m³) = area (m²) × depth (m)
weight (t) = volume × density (t/m³)
bags of 25 kg = weight (t) × 1 000 ÷ 25, rounded upTypical densities
- Gravel: 75 to 85 lb/ft³ in the table, about 1.28 t/m³ at the middle.
- Sand, loose: 90 lb/ft³, about 1.44 t/m³. Wet sand is heavier, and dry sand lighter.
- Soil, dry: 65 to 80 lb/ft³, about 1.16 t/m³. Topsoil that is wet or clayey can weigh much more.
- Bark mulch: 10 to 20 lb/ft³, about 0.24 t/m³; wood chips: 20 to 30 lb/ft³, about 0.40 t/m³.
What this calculator does not include
- Waste, the compaction under a path, and the extra depth needed for a layer that will sink into soft ground.
- The delivery charge, the minimum order of a supplier, or the cost of spreading and hiring a skip.
- Stone of a set size, such as crushed rock for a base, whose density the supplier should give.
Worked example: 20 m² of gravel 5 cm deep
The volume is 20 × 0.05 = 1.00 m³. At 1.28 t/m³ the gravel weighs 1.28 t.
That is 1 280 kg ÷ 25 kg = 51.2 bags, so 52 bags of 25 kg. The same area in sand (1.44 t/m³) weighs 1.44 t and needs 58 bags.
| Gravel | Sand | |
|---|---|---|
| Volume | 1.00 m³ | 1.00 m³ |
| Weight | 1.28 t | 1.44 t |
| Bags of 25 kg | 52 | 58 |
Frequently asked questions
How much gravel do I need for 20 m²?
For a 5 cm layer, 20 m² is 1 m³, which weighs about 1.28 tonnes of gravel, or 52 bags of 25 kg. Use a deeper layer for a drive, and ask the supplier for the density of their gravel.
What is the difference between a tonne and a cubic metre?
A cubic metre is a volume and a tonne is a weight. Gravel and sand are sold by weight or by volume depending on the supplier, so the density links the two.
Why is the density of wet sand higher?
Water fills the spaces between the grains and adds its own weight. So a load of sand after rain weighs more than the same volume dry, and the supplier's figure for wet sand is the one to use.
How many bags of mulch make a cubic metre?
Bark mulch is light, about 0.24 tonnes per cubic metre, so a cubic metre is about 10 bags of 25 kg. Mulch is also sold by volume, in litres, so the bag count here is a weight-based figure.
Does the tool store what I enter?
No. The calculation runs in your browser and nothing you type is sent to us or stored.
Sources
Figures and text last checked on Tuesday, November 10, 2026.