How the gravel calculator works
Gravel is sold by weight, usually in bulk bags (sometimes called tonne bags) or loose by the tonne, but you work out what you need by volume. The calculator finds the volume of the area, converts it to weight using a typical density, then divides by the size of the bag:
volume (m³) = length × width × (depth in mm ÷ 1,000)
tonnes = volume × 1.7
bulk bags = weight in kg ÷ 850, rounded up
We use 1,700kg per cubic metre. UK suppliers such as Beesley & Fildes put a cubic metre of typical gravel, chippings or pebbles at about 1,680kg and use 1,700kg for estimates, and Breedon's guide of 1 tonne covering about 12m² at 50mm works out at the same density. Bulk bags of gravel are typically sold as about 850kg (some merchants use 800kg), so you can change both figures if your supplier quotes something different.
How deep should gravel be?
| Use | Typical depth |
|---|---|
| Borders, around plants, on top of a weed membrane | 30mm |
| Decorative gravel areas | 40 to 50mm |
| Paths and driveways | 50mm |
| Cobbles and large pebbles | 100mm |
Deeper is not always better. On a path or drive, gravel much deeper than 50mm is hard to walk on and lets car tyres sink in. For a driveway, the gravel goes on top of a compacted sub-base such as MOT Type 1, with a weed membrane underneath, so don't count the sub-base depth here.
How much does a bulk bag of gravel cover?
At 1,700kg per m³, an 850kg bulk bag holds 0.5m³ of gravel. That covers:
| Depth | Area covered by one 850kg bag | Area per tonne |
|---|---|---|
| 30mm | 16.7m² | 19.6m² |
| 40mm | 12.5m² | 14.7m² |
| 50mm | 10m² | 11.8m² |
| 75mm | 6.7m² | 7.8m² |
| 100mm | 5m² | 5.9m² |
That matches what merchants print on their bags: a typical 850kg bulk bag of 10mm gravel is sold as covering about 10m² at 50mm deep.
Worked example: a 5m × 4m driveway
- Area: 5 × 4 = 20m²
- Volume at 50mm: 20 × 0.05 = 1m³
- Weight: 1 × 1,700 = 1,700kg, or 1.7 tonnes
- Bulk bags: 1,700 ÷ 850 = 2 bags
If the same gravel came in 20kg bags, you'd need 85 of them, which is why bulk bags are nearly always cheaper and easier for anything bigger than a small border.
Garden path example
A path 10m long and 1m wide at 40mm deep needs 0.4m³, or 680kg. That's one 850kg bulk bag with some left over for topping up, or 34 bags of 20kg.
Tips for ordering gravel
- Measure odd shapes in sections. Split a curved border or L-shaped drive into rectangles, or use the square metre calculator to add up the area, then enter the total as the length with a width of 1.
- Order a little extra if the ground is uneven or you want to top up later. Gravel spreads and settles into the sub-base over the first few months.
- Check access for delivery. Bulk bags are usually dropped by a lorry with a crane arm, and a full bag can't be moved by hand.
- Wet gravel weighs more. Washed gravel delivered wet can be a little heavier per bag, so the volume you get from a bag can vary slightly.
Planting up the border afterwards? The topsoil calculator works out how much soil you need, and the concrete calculator handles a solid base for a shed or edging.
Frequently asked questions
How much gravel do I need for 20 square metres?
At 50mm deep, 20m² needs 1m³ of gravel, which is about 1.7 tonnes or two 850kg bulk bags. At 30mm deep for a border, it's 0.6m³, or about 1 tonne.
How many square metres does a tonne of gravel cover?
About 11.8m² at 50mm deep, or 19.6m² at 30mm deep, assuming 1,700kg per m³. Breedon quotes about 12m² per tonne at 50mm.
How much does a bulk bag of gravel cover?
An 850kg bulk bag holds about 0.5m³, which covers about 10m² at 50mm deep or 16.7m² at 30mm deep.
How many tonnes is a cubic metre of gravel?
Roughly 1.7 tonnes. Most gravel, chippings and pebbles weigh between about 1.6 and 1.8 tonnes per cubic metre, depending on the stone and how wet it is.
How deep should gravel be on a driveway?
About 50mm of gravel on top of a compacted sub-base. Much deeper and the stones move around under tyres and feet.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.