What this calculator does
Gravel is usually bought and priced by weight, in tonnes, rather than by the volume figure most area-and-depth calculations naturally produce. Converting between the two needs a density figure, and gravel density varies noticeably by type, roughly 1.4 to 1.8 tonnes per cubic metre depending on the stone and how tightly it has settled, so a fixed, hidden number would routinely be wrong for whatever is actually being delivered.
This calculator works out how much gravel do I need for a given area and depth, in cubic metres, kilograms and tonnes, using a gravel density you can set yourself, plus an estimated bag count and cost. Enter the figure quoted for your specific gravel, or check the bag or delivery docket for it, rather than relying on a single assumed value.
The formula
Volume is length times width times depth, with depth converted from millimetres to metres. That volume, in cubic metres, is multiplied by the density to give weight in tonnes, which is then converted to kilograms and divided by the bag size to estimate how many bags are needed.
| Term | Meaning |
|---|---|
| Density | How much a cubic metre of the gravel weighs, in tonnes per cubic metre; varies by stone type and compaction. |
| Volume | The space the gravel needs to fill: area times depth, in cubic metres. |
| Bags needed | Total weight divided by the bag size, rounded up to a whole number of bags. |
The inputs explained
| Field | What to enter |
|---|---|
| Area length (m) | The length of the area to be covered. |
| Area width (m) | The width of the area to be covered. |
| Depth (mm) | How deep a layer of gravel to lay, in millimetres. |
| Gravel density (t/m³) | The density of the specific gravel being used, in tonnes per cubic metre. Check the supplier’s figure rather than relying on a guess; 1.5 to 1.7 t/m³ is a commonly cited range. |
| Bag size (kg) | The size of the bags being bought, in kilograms. |
| Price per tonne ($) | The price charged per tonne, for an estimated total cost. |
When to use it
Working out how to work out how much gravel I need for a driveway
A driveway or path is usually specified as a depth in millimetres over its full length and width; entering those figures directly gives the volume, weight and bag count needed to cover it.
Comparing bagged versus bulk delivery
The bag count shows what buying in small bags would take, which can then be compared against a bulk tonne price to see which is cheaper for the quantity needed.
Adjusting for a specific gravel type
Crushed rock, river pebble and decorative gravel all settle to different densities; changing only the density figure re-runs the same area and depth against the correct material.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much gravel is needed as depth increases
A fixed 6 m by 3 m area, laid at a range of depths.
| Depth | Gravel needed | Bags needed |
|---|---|---|
| 25 mm | 0.720 tonnes | 36 × 20 kg bags |
| 50 mm | 1.440 tonnes | 72 × 20 kg bags |
| 75 mm | 2.160 tonnes | 108 × 20 kg bags |
| 100 mm | 2.880 tonnes | 144 × 20 kg bags |
| 150 mm | 4.320 tonnes | 216 × 20 kg bags |
| 200 mm | 5.760 tonnes | 288 × 20 kg bags |
How the weight needed changes with gravel density
The same area and depth, across the density range typical of different gravel types.
| Gravel density | Gravel needed | Bags needed |
|---|---|---|
| 1.4 t/m³ | 1.260 tonnes | 63 × 20 kg bags |
| 1.5 t/m³ | 1.350 tonnes | 68 × 20 kg bags |
| 1.6 t/m³ | 1.440 tonnes | 72 × 20 kg bags |
| 1.7 t/m³ | 1.530 tonnes | 77 × 20 kg bags |
| 1.8 t/m³ | 1.620 tonnes | 81 × 20 kg bags |
| 2 t/m³ | 1.800 tonnes | 90 × 20 kg bags |
Questions
What density should I use for gravel?
It depends on the type: crushed rock and road base commonly sit around 1.6 to 1.8 t/m³, while lighter decorative or rounded pebble gravel can be closer to 1.4 to 1.5 t/m³. Check the supplier’s specification sheet or delivery docket for the exact figure where possible, since it varies by product.
How much gravel do I need for a path?
Multiply the path’s length by its width to get the area, decide on a depth (50 to 75 mm is common for a decorative path, more for one that needs to bear vehicle weight), and enter both here along with the gravel’s density.
Should I add extra for compaction or wastage?
Gravel settles and compacts somewhat after laying, so it is common to add roughly 5 to 10% to the calculated depth for a finished result that stays at the intended depth once it has settled in.
How is this different from a soil or mulch calculator?
A soil calculator is sized for garden beds and potting mix, and a combined mulch, soil and gravel calculator gives only a shared volume-based estimate. This calculator is built specifically around gravel’s weight-based buying convention, converting volume to tonnes with an editable density rather than assuming one fixed figure.
For garden bed soil or potting mix instead of gravel, see the soil calculator. For a concrete slab or footing, see the concrete volume calculator.