The calculation in three steps
Gravel is priced by the tonne but measured by volume, so the sum has one extra step compared with most measure-up jobs.
- Area. Length x width, in metres. A drive 10 m long and 3 m wide is 30 square metres.
- Volume. Multiply by the depth in metres. At 50 mm that is 0.05, so 30 x 0.05 = 1.5 cubic metres.
- Weight. Multiply by the density, about 1.6 tonnes per cubic metre for loose gravel. 1.5 x 1.6 = 2.4 tonnes.
The step people get wrong is the depth: 50 mm is 0.05 metres, not 0.5. A factor-of-ten slip there turns a 2.4 tonne order into 24 tonnes. The gravel calculator takes the depth in millimetres precisely so that mistake cannot happen.
Depths for different jobs
| Where | Gravel depth | Notes |
|---|---|---|
| Driveway, over a sub-base | 50 mm | Sub-base carries the load, not the gravel |
| Footpath | 40 to 50 mm | Thinner stays firmer underfoot |
| Garden border or decorative bed | 40 to 50 mm | Enough to hide the membrane |
| Shed or bin-store base | 50 mm | Over compacted hardcore |
| MOT Type 1 sub-base under a drive | 100 to 150 mm | Compacted in layers; heavier material |
The layer most people forget
A gravel driveway is not a layer of gravel. It is a compacted sub-base with a thin decorative layer on top, and the sub-base is what stops a car rutting it. If you are building a drive from scratch you need both calculations, and the sub-base is the bigger of the two.
For the same 10 m by 3 m drive with a 100 mm sub-base: 30 x 0.1 = 3 cubic metres of MOT Type 1. At roughly 2 tonnes per cubic metre compacted, that is about 6 tonnes, on top of the 2.4 tonnes of decorative gravel. Order them separately, and expect the sub-base to be delivered loose.
Between the two layers goes a geotextile membrane. Without it the gravel works its way down into the sub-base over a couple of winters and the surface goes patchy.
Bulk bags or a loose load
A bulk bag holds around 850 kg, so 2.4 tonnes is about three bags. Bags are easy to place and easy to handle, but per tonne they cost noticeably more than a tipped load. The crossover is usually around four bags: past that, if there is space for a lorry to tip and you can move the stone by barrow, loose is cheaper.
Round up rather than down. Gravel settles, and topping up a drive from a second delivery means paying a second delivery charge for a fraction of a load. Leftover gravel always finds a use in a border or a bin store.
Stone size matters more than you would think
For a driveway, 14 to 20 mm angular gravel is the usual choice. Angular stone knits together and stays put under tyres. Rounded pea shingle looks good in a border but rolls, so on a drive it migrates to the edges and collects in ruts.
Very small stone tracks into the house on shoes and gets thrown by tyres. Very large stone is uncomfortable to walk on and hard to push a wheelbarrow or a pushchair across. The 14 to 20 mm band is the compromise that most suppliers steer people to for good reason.
Quick reference
- Cubic metres = length x width x depth, all in metres.
- Tonnes = cubic metres x 1.6 for loose gravel.
- 50 mm is 0.05 m. Getting this wrong by a decimal place is the classic error.
- A bulk bag is about 850 kg and covers 10 to 11 m2 at 50 mm.
- Budget for the sub-base and the membrane, not just the gravel.
Run your own numbers through the gravel calculator. For a shed or a base underneath, the concrete calculator and shed base guide cover the alternative, and the Calc Garden guides have the rest of the outdoor jobs.