How the gravel calculator works
The calculator works out the volume of your area and multiplies it by the typical loose density of your chosen material to get the weight. Different stones weigh different amounts per m³, which is why you'll need more tonnes of ballast than slate for the same area.
Typical densities and coverage
| Material | Density (t/m³) | 850kg bag at 50mm |
|---|---|---|
| 20mm gravel | 1.65 | ~10.3 m² |
| Pea shingle | 1.6 | ~10.6 m² |
| Limestone chippings | 1.5 | ~11.3 m² |
| Slate chippings | 1.4 | ~12.1 m² |
| Sharp sand | 1.65 | ~10.3 m² |
| All-in ballast | 1.75 | ~9.7 m² |
Tips
- Always lay a weed membrane under decorative gravel, and edging to stop it migrating onto lawns.
- Angular gravel (e.g. 20mm crushed) locks together and is better for driveways than round pea shingle, which moves underfoot.
- Bulk bags are much cheaper per tonne than small bags, but check there's room for the delivery lorry's crane.
Worked examples
A gravel driveway, 6m × 4m at 50mm: 24m² × 0.05m = 1.2m³. 20mm gravel weighs about 1.65t/m³, so with 5% extra that's 2.08 tonnes, or 3 bulk bags of 850kg. One bag covers about 10.3m² at this depth. Buying the same amount in 25kg bags would take 84 bags and cost far more. You'll also want about 26m² of membrane.
A round slate border, 3m across at 30mm: 7.1m² × 0.03m = 0.21m³, about 0.31 tonnes of slate. That's a third of a bulk bag, so 13 × 25kg bags may be easier and similar in price. Choose "Circle" above to check your own bed.
Building a gravel drive that lasts
Gravel on its own over soil soon ruts and sinks. A driveway needs a dug-out base with 100–150mm of compacted MOT Type 1, a permeable membrane, and then 40–50mm of angular gravel. Firm edging (kerbs, sleepers or setts) stops the stone spreading onto lawns and the pavement, and plastic gravel grids stop it moving under tyres. The MOT Type 1 calculator sizes the sub-base, and the excavation calculator works out the dig and the spoil.
Gravel lets rain soak through. That's one reason it's popular for front gardens: in England, replacing more than 5m² of front garden with hard surfacing that doesn't drain usually needs planning permission, while permeable surfaces such as gravel generally don't.
Choosing the right stone
- Driveways: 14–20mm angular gravel. It locks together better than round stone and doesn't stick in tyre treads.
- Paths: 10–20mm gravel or self-binding path gravel, which compacts to a firm surface that's easier for wheelchairs, buggies and bikes.
- Borders and pots: any decorative stone, slate or chippings at 30–40mm over a membrane.
- Light-coloured limestone can mark and look dusty at first. Wash a sample before ordering a full load.
Common gravel mistakes
- Too deep. More than about 50mm of loose gravel is hard to walk on, and cars sink into it. More isn't better.
- No membrane or edging. Weeds come through and the stone wanders. Both are cheap compared with replacing lost gravel.
- Ordering by volume from a weight price. Merchants price by the bag or tonne. Use the tonnes figure above when comparing quotes.
For more on sub-base depths for paths, patios and drives, read our sub-base depth guide, or browse all concrete, sub-base and paving calculators.