How much gravel you need

Gravel is sold in bags, bulk bags or loose by the tonne, but it is spread by square metres and centimetres of depth. This calculator turns the size of your area into cubic metres, tonnes and bags, with a margin for compaction, and tells you what kind of use the chosen depth suits.

How the calculation works

• Volume: length × width × depth, in cubic metres.
• Extra: gravel settles and some is lost at the edges; 5-10% more is a common margin.
• Weight: volume × bulk density. Loose gravel often weighs 1.5-1.7 tonnes per cubic metre.
• Bags: the weight divided by the weight of one bag, rounded up.

How deep

A 5 × 3 m area, gravel at 1.6 t/m³, 10% extra:

Depth matters as much as area: going from 5 to 10 cm doubles the material. Below 3 cm the ground tends to show through; driveways are usually built in compacted layers, coarser at the bottom and finer on top.

The material changes the weight

Same area, 5 cm deep, 10% extra:

For the same volume, a light material such as lava rock weighs about half as much as gravel. When buying by the tonne, use the density given by the supplier.

The reading bands

• Below 3 cm: a thin layer, the ground tends to show through.
• 3 to 5 cm: a decorative layer, for beds and light footpaths.
• 5 to 10 cm: footpaths and light parking areas.
• Over 10 cm: driveways and sub-bases, usually in compacted layers.

These are common usage guidelines, not standards: for driveways and sub-bases, ground preparation matters most.

Limits of the calculator

The calculation applies to rectangular areas: for other shapes, split the area into rectangles and add the results. Real density depends on grading and moisture: wet gravel weighs more. The calculator does not include weed membrane, edging or ground preparation.

Frequently asked questions

How much gravel for 15 m²?

At 5 cm deep, with 10% extra and a density of 1.6 t/m³, about 0.83 m³, or 1.32 tonnes or 53 bags of 25 kg.

How much does a cubic metre of gravel weigh?

Often 1.5-1.7 tonnes, depending on grading and moisture.

How deep should gravel be?

Often 3-5 cm for a decorative layer, 5-10 cm for a footpath and more, in compacted layers, for a driveway.

Why add a margin?

Because gravel settles and some is lost at the edges: 5-10% more avoids running short.

How many 25 kg bags in a cubic metre?

With a density of 1.6 t/m³, about 64 bags.

Bags or loose?

Bags are handy for small areas; for large volumes, bulk bags or loose delivery are easier.

Do I need a membrane under gravel?

Often yes, to limit weeds and keep gravel separate from the soil, especially on soft ground.

Does it work for sand and crushed stone?

Yes: enter the material density given by the supplier.

How do I work out an irregular area?

Split it into rectangles, calculate each and add the results.