🪨 Gravel Calculator
Work out how much gravel, sand, crushed stone or topsoil an area needs, in tonnes, cubic metres, cubic yards and bulk bags. Loose aggregate is sold by weight but laid by volume, which is why the bulk density matters as much as the area.
Gravel is sold by weight and laid by volume, which is where the confusion starts
You buy aggregate in tonnes and bulk bags; you lay it in square metres at a given depth. Converting between the two needs the bulk density of the material, which is how much a cubic metre of it weighs when loosely tipped. Gravel runs about 1.5 tonnes per cubic metre, which means one tonne covers roughly 13 square metres at 5 cm deep.
Tonnes = area (m²) × depth (m) × bulk density (t/m³)
| Material | Tonnes per m³ | m² covered per tonne at 5 cm |
|---|---|---|
| Gravel, shingle | 1.50 | 13.3 |
| Crushed stone, MOT type 1 | 1.60 | 12.5 |
| Sharp or building sand | 1.60 | 12.5 |
| Topsoil | 1.30 | 15.4 |
| Bark, wood chip | 0.90 | 22.2 |
Bark is the outlier and the one that surprises people: at 0.9 t/m³ it is lighter than water, so a tonne of bark goes nearly twice as far as a tonne of gravel. Buying bark by weight without checking the density is a common way to end up with far more than the bed needed.
Depth is a decision, not a detail
| Use | Depth |
|---|---|
| Decorative bed, no foot traffic | 4 cm |
| Path, occasional walking | 5 cm |
| Driveway gravel over a sub-base | 5 cm |
| Bark mulch for weed suppression | 7 – 10 cm |
| Sub-base under paving or gravel drive | 10 – 15 cm, compacted |
Under 4 cm the gravel shifts underfoot and the membrane shows through within a season. Over about 6 cm it feels loose and unpleasant to walk on, and on a drive the tyres push it into ruts. The instinct to go deeper for durability is wrong for the surface layer — depth belongs in the sub-base underneath, which is a different material and a different calculation.
A gravel drive is two layers, not one
A drive that takes vehicles needs a compacted sub-base of MOT type 1 or crushed stone, typically 10 to 15 cm, with the decorative gravel only 5 cm on top. Laying 20 cm of loose gravel straight onto soil produces ruts, migration and a permanent maintenance job, because nothing is spreading the load. The sub-base is the structure; the gravel is the finish.
Both layers go over a geotextile membrane, which separates the stone from the soil beneath. Without it the sub-base gradually mixes into the subsoil and the whole construction sinks. A weed membrane and a geotextile are not the same product, and on a drive it is the heavier geotextile that matters.
Order 5% to 10% more than the calculation gives. Loose aggregate settles and compacts, a wheelbarrow spills, and levels are never as even as a tape measure suggests. Running 200 kg short on the last square metre costs a delivery charge that dwarfs the extra material.
Bulk bags, loose loads and small bags
A bulk bag is nominally 850 kg, though suppliers vary between 750 kg and a full tonne — always ask, because the price per tonne can look very different once the actual weight is known. Loose tipped loads are cheaper per tonne above roughly five tonnes but need somewhere a lorry can tip. Small 25 kg bags cost two to three times as much per tonne and only make sense for under a quarter of a tonne.
For a concrete pour rather than loose aggregate, the concrete calculator works in cubic metres and bags of mix, and the volume calculator handles shapes this tool does not, such as a curved bed or a tapered trench.
Stone size changes how it behaves
10 mm gravel packs tightly and walks comfortably but scatters easily. 20 mm is the usual driveway size: large enough to stay put under tyres, small enough to walk on. 40 mm and above is decorative or drainage material and is genuinely unpleasant underfoot. Angular crushed stone locks together and stays where it is put; rounded pea shingle rolls and migrates, which is why it needs edging.
Edging matters more than most people expect. Gravel without a restraining edge spreads sideways into borders and lawns, and the bed thins from the middle outwards until bare patches appear — which reads as not enough gravel when it was actually enough gravel with nowhere to stay.
A worked example
A path 8 m long and 1.2 m wide, 5 cm deep in 20 mm gravel. Area is 9.6 m², volume is 0.48 m³, and with 5% for settling that is 0.504 m³, so 0.76 tonnes — one bulk bag with a little left over. A circular bed 3 m across at the same depth is π × 1.5² = 7.07 m², 0.353 m³, 0.56 tonnes. Both fit in one bag, so the sensible order is a single bulk bag for the pair rather than two part-bags at a worse price.
If this is one step in a larger decision, the concrete calculator is usually the next question, and the square footage calculator is worth a look while you are here.
Frequently Asked Questions (FAQs)
How much gravel do I need?
Multiply the area in square metres by the depth in metres, then by the bulk density — about 1.5 tonnes per cubic metre for gravel. An 8 by 3 metre area at 5 cm deep is 1.2 cubic metres, or about 1.8 tonnes.
How many square metres does a tonne of gravel cover?
Around 13 square metres at 5 cm deep, or 16 at 4 cm. Crushed stone covers slightly less because it is denser, topsoil slightly more, and bark almost twice as much because it is much lighter.
How deep should gravel be?
About 4 cm for a decorative bed and 5 cm for a path or the surface of a drive. Shallower and the membrane shows through; deeper and it feels loose and rutts under tyres. Extra depth belongs in the sub-base, not the top layer.
Do I need a sub-base for a gravel driveway?
Yes, if vehicles will use it: 10 to 15 cm of compacted crushed stone under the 5 cm of decorative gravel, over a geotextile membrane. Loose gravel straight onto soil will rut and migrate because nothing spreads the load.
How much does a bulk bag of gravel weigh?
Nominally 850 kg, but suppliers range from about 750 kg to a full tonne. Ask before comparing prices, because the quoted price per bag means little without the weight.
What size gravel should I use?
10 mm walks comfortably but scatters, 20 mm is the usual driveway choice, and 40 mm and above is decorative or for drainage and uncomfortable underfoot. Angular crushed stone stays put far better than rounded pea shingle.
Should I order extra?
Yes, 5% to 10% over the calculation. Loose material settles, levels are never perfectly even, and a small shortfall means paying a second delivery charge for a fraction of a bag.