How much gravel do you need?
Gravel is laid by volume but sold by weight, which is why a simple area measurement is never enough. This gravel calculator turns the area and the depth you want into cubic metres, then into tonnes, bulk bags and 25kg bags — using the right density for gravel, slate, chippings, sand or sub-base.
The formula
Volume = area × depth in metres. Weight = volume × density. A 5m × 4m area at 50mm deep is 20 × 0.05 = 1 m³, and gravel weighs about 1.6 tonnes per cubic metre, so that is 1.6 tonnes before any wastage.
Why density differs
Slate chippings are lighter than gravel — around 1.3 tonnes per cubic metre against 1.6 — so the same area needs fewer tonnes. MOT type 1 sub-base is heavier still, about 2 tonnes, because it compacts down with fines filling the gaps.
Worked example: a 5m × 4m area at 50mm
20 m² of gravel with 5% added for settling and uneven ground.
| Material | Density | Volume | Weight | Bulk bags |
|---|---|---|---|---|
| Gravel / shingle | 1.6 t/m³ | 1.05 m³ | 1.68 t | 2 |
| Decorative chippings | 1.5 t/m³ | 1.05 m³ | 1.58 t | 2 |
| Slate chippings | 1.3 t/m³ | 1.05 m³ | 1.37 t | 2 |
| MOT type 1 sub-base | 2.0 t/m³ | 1.05 m³ | 2.10 t | 3 |
A bulk bag holds about 850kg, so roughly 34 bags of 25kg. Ordering 1.68 tonnes as small bags means 68 of them to carry.
How deep should it go?
Depth changes everything
Depth drives both the quantity and how the surface behaves. The same 20 m² area needs 1.34 tonnes of gravel at 40mm, 1.68 tonnes at 50mm and 2.52 tonnes at 75mm. One tonne covers about 15.6 m² at 40mm, 12.5 m² at 50mm and 8.3 m² at 75mm.
Paths, borders and drives
Around 50mm suits paths and decorative borders — deep enough to hide the membrane, shallow enough to walk on. Drives need a compacted sub-base with about 50mm of gravel on top, not a deep loose layer: loose stone over 50mm ruts under tyres and pushes out to the edges.
Ordering: bulk bags, loose loads and small bags
Bulk bags
A bulk bag is the usual unit for anything over a few hundred kilos, and it is far cheaper per tonne than small bags. Check the lorry can reach where you want it dropped — a bag moved by hand is a long afternoon with a barrow.
Loose tipped loads
For several tonnes, a tipped load is cheaper again, but it arrives as a pile on the drive and has to be moved before anyone parks. Make sure there is somewhere for it to go.
Small bags
25kg bags suit small jobs and topping up. They cost much more per tonne, so they only make sense when you need a fraction of a bulk bag or cannot get a delivery close to the work.
Preparing the ground
Membrane and edging
A weed membrane under the gravel stops weeds pushing through and keeps stone from sinking into soil. Edging — timber, steel or a mowing strip — is what stops the gravel migrating onto the lawn and into the house on shoes.
Sub-base first on drives
A drive needs excavation and a compacted layer of MOT type 1 before the decorative layer goes down. Work out the sub-base as its own layer in the calculator, then run it again for the gravel on top. For a concrete base instead, use the concrete calculator.
Getting the area right
An awkward shape is easiest split into rectangles and circles: measure each, add them, and use the total here. The square meter calculator does that and converts between metres and feet. If the gravel borders a deck, the decking calculator covers the boards and frame beside it.
Densities vary between quarries and rise when the material is wet, so treat the tonnage as a close estimate and check the supplier’s own coverage figure before a large order.