Multiply length by width by depth, all in metres, and you have the volume in cubic metres. That's the number every merchant and ready-mix firm works from. To turn it into weight, multiply by a typical density: about 1.7 tonnes per m³ for gravel and about 1.5 tonnes for topsoil. For bagged concrete mix, allow roughly 2.2 tonnes of dry mix per finished m³.
The one sum you need
The common mistake is the depth. Depths are usually talked about in millimetres, so divide by 1,000 first: 100mm is 0.1m, 50mm is 0.05m, 150mm is 0.15m. Mix up the units and you'll be out by a factor of ten.
A cubic metre is also 1,000 litres, which is handy because topsoil bulk bags are sold in litres (500 or 730 is typical).
For odd shapes, split the area into rectangles and add them up. A curved border can be treated as a rectangle using its average width. For a round post hole, the volume is 3.1416 × radius² × depth.
Typical depths
These are rules of thumb for garden and DIY work. Anything structural, like an extension or garage foundation, needs to follow building control or an engineer.
| Job | Typical depth |
|---|---|
| Shed base (concrete) | 75 to 100mm, on 75 to 100mm of compacted hardcore |
| Patio or path base (concrete) | 100mm |
| Driveway or garage floor (concrete) | 100 to 150mm, more for heavy vehicles |
| Gravel path or drive | 50mm, on a compacted sub-base |
| Gravel in borders | About 30mm |
| Topsoil under a new lawn | 100 to 150mm |
| Topsoil for beds and borders | 150 to 200mm |
| Topsoil for root veg | 300 to 400mm |
With gravel, deeper isn't better. Much past 50mm and it's hard to walk on and tyres sink in. Don't count the sub-base depth when you work out the gravel.
From cubic metres to tonnes
Gravel and loose loads are sold by weight, so you need a density. These are the figures the site's calculators use:
| Material | Weight per m³ | 1 tonne is about |
|---|---|---|
| Gravel, chippings, pebbles | 1,700kg (1.7 tonnes) | 0.59m³ |
| Topsoil | 1,500kg (1.5 tonnes) | 0.67m³ |
| Bagged dry concrete mix | 2,200kg per finished m³ | 0.45m³ of concrete |
Treat them as estimates. Most gravel sits somewhere between 1.6 and 1.8 tonnes per m³. Topsoil varies a lot more with moisture and how compacted it is, and loose screened soil in a bag can be well under 1.5 tonnes. If you're buying by the tonne, ask the supplier how many cubic metres a tonne of their material makes.
Bulk bags or a loose load
- Bulk bags are dropped by a lorry with a crane arm, usually onto the drive. A gravel bulk bag typically holds 800 to 850kg, which at 1.7 tonnes per m³ is about 0.5m³. Topsoil bags are sold by volume. A full bag can't be shifted by hand, so pick the drop spot carefully.
- Loose loads are tipped from a tipper lorry and priced by the tonne or cubic metre. They can work out cheaper for big jobs, but you need somewhere to tip it and a barrow to move it.
- Small 20 or 25kg bags suit a top-up or a raised bed. For anything bigger the count gets silly: a 5m × 4m drive at 50mm deep is 85 bags of 20kg gravel.
Concrete: bags, mixer or lorry
Bagged ready-mix is a dry blend you add water to. A 20kg bag makes roughly 0.009m³ and a 25kg bag about 0.011m³, though brands vary, so read the yield on the bag. That works out at around 110 bags of 20kg per cubic metre.
- Under about 0.25m³ (post holes, small pads): bags in a barrow are fine.
- 0.25 to 1m³: hire a mixer, or mix your own from a bulk bag of all-in ballast and bags of cement, which usually beats bagged ready-mix on price.
- Over 1m³: get quotes for ready-mixed concrete by lorry or a mix-on-site service. Check the lorry can get close enough, or book a pump.
If you're mixing your own, follow the ratio printed on the cement bag for the job you're doing. As a rough guide, builders' merchants commonly quote 1 part cement to 2 parts sand to 3 parts coarse aggregate for foundations and heavier slabs, and 1:2:4 for paths and lighter bases. Add water a little at a time. Too much makes weaker concrete that cracks.
Worked examples
Shed base, 2.4m × 1.8m at 100mm
- Volume: 2.4 × 1.8 × 0.1 = 0.432m³
- Add 10% for waste and uneven ground: 0.475m³
- Dry mix at 2,200kg per m³: about 1,045kg
- That's 53 bags of 20kg or 42 bags of 25kg
You'll also want 100mm of compacted hardcore underneath, which is another 0.43m³.
Gravel path, 8m × 0.9m at 50mm
- Volume: 8 × 0.9 × 0.05 = 0.36m³
- Weight at 1.7 tonnes per m³: 612kg
- That's one 850kg bulk bag with some to spare, or 31 bags of 20kg
Topsoil for a new lawn, 6m × 5m at 150mm
- Volume: 6 × 5 × 0.15 = 4.5m³
- Add 10% for settling: 4.95m³, or 4,950 litres
- 500 litre bulk bags: 9.9, so 10 bags
- Weight at 1.5 tonnes per m³: about 7.4 tonnes
That's a full tipper load's worth of work, so check access before you order.
A few things that catch people out
- Topsoil settles once it's firmed and watered, so order at least 10% extra.
- Wet gravel weighs more than dry, so a bag of washed stone may hold slightly less volume than you expect.
- Large concrete slabs need expansion joints. A common guide is every 3 to 4m.
- Don't pour concrete if frost is forecast.
The concrete calculator, gravel calculator and topsoil calculator do these sums for you and let you change the density if your supplier quotes a different figure.
Last reviewed October 2026.