Gravel Calculator
Works out the gravel for an area from its size and the depth you want to lay. Gives the answer in tonnes and cubic yards, plus bulk bags and retail bags, since yards quote in all of them.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Gravel runs about 1550 kg per cubic metre loose, so a cubic metre weighs roughly 1.55 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
Three things decide whether the answer is right:
- Depth, not area, is where the cost is. Doubling the depth doubles the order. Going from 50 mm to 75 mm on a 30 m² area adds most of a tonne.
- Bulk density varies with moisture. The figures here are for material as delivered. Wet stone weighs more per cubic metre, which matters if you are buying by weight and measuring by volume.
- Buy loose above about two tonnes. Bulk bags cost considerably more per tonne and every one has to be handled. Below a tonne the delivery charge on a loose load usually wipes out the saving.
Frequently Asked Questions
How much gravel do I need?
Multiply length by width by depth for the volume, then by about 1.55 tonnes per cubic metre. A 5 × 3 m area at 50 mm needs roughly 0.75 m³, which is about 1.3 tonnes once waste is allowed for.
How deep should gravel be?
50 mm for a decorative area or a path. Anywhere vehicles go needs 50 mm of gravel over 100–150 mm of compacted sub-base, and the sub-base is the part that stops ruts forming.
How much does a bulk bag of gravel cover?
A bulk bag is about 850 kg, roughly half a cubic metre. At 50 mm deep that covers around 10 m²; at 100 mm it covers 5 m².