Why gravel is quoted by weight
Concrete is sold by volume; aggregate is sold by weight. That mismatch is the reason this calculator exists — the figure the arithmetic produces is not the figure you can order with.
The conversion depends on the material. The default here is 1.6 tonnes per cubic metre, which is typical for dry crushed stone. Pea gravel runs slightly lighter, MOT Type 1 slightly heavier once compacted, and everything gets heavier when wet. That variation is around 10–15%, which matters on a large order and not at all on a small one.
Bulk bags are commonly described as "a tonne" but the actual weight varies between suppliers, with 850 kg being a very common real figure. If the total matters, order by weight rather than by bag.
| Use | Depth | Note |
|---|---|---|
| Decorative bed topping | 40–50 mm | Thin layers migrate and show the membrane |
| Footpath | 50 mm over sub-base | Needs edging or it spreads |
| Driveway wearing course | 50 mm | Over 150 mm of compacted sub-base |
| Driveway sub-base | 150–200 mm | MOT Type 1, compacted in layers |
| Shed base sub-base | 100 mm | Compacted in two layers |
Compaction changes the number
A loose cubic metre of MOT Type 1 does not stay a cubic metre. Compacted properly it loses roughly 20–25% of its volume, which means the hole you are filling needs more material than its dimensions suggest.
The calculator works on finished dimensions. If you are specifying a compacted sub-base, add that 20–25% on top, or lay it and compact it in layers and accept that you will need a second delivery if you cut it fine.
Compaction is not optional on anything load-bearing. Gravel laid loose under a slab settles afterwards, producing exactly the void the sub-base existed to prevent.
Edging and membrane
Loose gravel migrates. Without an edge restraint it ends up in the borders and on the lawn, and the area thins to the point where the sub-base shows through. Timber, steel or block edging all work; nothing at all does not.
A geotextile membrane underneath serves a different purpose from a weed barrier. Its real job is to stop the gravel and the soil beneath from mixing, which is what turns a clean sub-base into mud over a few winters. On a driveway it is worth the cost.
Common questions
How many tonnes of gravel do I need?
Multiply area by depth for the volume, then by about 1.6 for tonnes per cubic metre. The calculator does both and lets you adjust the density.
How deep should a gravel driveway be?
Around 50 mm of decorative wearing course over 150–200 mm of compacted MOT Type 1 sub-base.
Does a bulk bag hold a tonne?
Usually less. 850 kg is a common real figure. Order by weight if the total matters.