What this calculator works out
This calculator multiplies area by depth and density to give the weight of gravel needed in kilograms.
Gravel is one of the few garden materials sold by weight rather than volume, which is why density appears in the calculation. Get the density wrong and the order is wrong by the same proportion.
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Before you order
- Around 1,500 to 1,700 kg per cubic metre covers most decorative aggregates. Slate is lighter, around 1,300; some stone is heavier.
- 50 mm is the usual depth for a decorative bed or border. A path or drive needs 50 mm of gravel over a compacted sub-base, not 100 mm of loose gravel.
- Gravel on a slope migrates. On anything above a gentle gradient, use an angular grade that locks together, or a stabilising grid.
- A bulk bag typically holds 750 to 850 kg, so the calculated weight translates directly into a bag count.
Rounded pea shingle looks attractive and rolls underfoot, migrating towards edges and out of the area. Angular chippings interlock and stay where they are laid. For paths, drives and anywhere that gets walked on, angular is almost always the better choice despite the softer appearance of rounded stone.
How this calculator works
Volume converted to weight:
Kilograms = area × (depth ÷ 100) × density
Depth in centimetres, density in kg per cubic metreDividing depth by 100 converts centimetres to metres, giving a volume in cubic metres which the density then converts to weight. To go the other way — from a bulk bag to coverage — divide the bag weight by the density to get cubic metres, then by the depth.
Worked example: a 20 m² area
Using the default figures — 20 m² at 5 cm deep, density 1,600 kg/m³:
- Volume: 1 m³
- Weight: 1,600 kg, or 1.6 tonnes
1.6 tonnes is two bulk bags, and it is worth pausing on that figure — that is a substantial delivery to move by barrow. Gravel is cheap to buy and expensive in labour, and the difference between a 5 cm and a 7 cm depth on this area is another 640 kg of shifting for no visible benefit.
Common mistakes
- Laying gravel thicker for better coverage. Above about 50 mm it moves underfoot and looks worse.
- Using rounded gravel on a path. It migrates and is unpleasant to walk on.
- Laying gravel straight onto soil. It sinks in; use a membrane and a sub-base on anything trafficked.
- Using a generic density. Slate and stone differ by 25% or more.
- Forgetting edging. Without a restraint, gravel spreads into beds and lawn.
Frequently asked questions
What size gravel should I use?
10 to 20 mm suits most decorative areas and paths — small enough to walk on comfortably, large enough not to stick to shoes. Below 10 mm gets carried indoors; above 20 mm is uncomfortable underfoot. For a drive, 20 mm angular over a proper sub-base is the usual specification.
Do I need a membrane under gravel?
Under a path or drive, yes — it stops the gravel working down into the soil and keeps the sub-base separate. It also suppresses weeds, though seeds blowing into the gravel itself will germinate regardless. This is the one application where membrane is genuinely worthwhile.
Does gravel need planning permission?
For a front garden, possibly. Surfacing more than 5 m² of a front garden with an impermeable material requires planning permission unless the water drains to a permeable area. Loose gravel is generally considered permeable, which is one reason it is a common choice for front drives.
How do I stop gravel spreading?
Edging restraints — timber, metal or concrete — around the perimeter, and a stabilising grid on slopes or drives. Grids hold the gravel in cells so it cannot migrate, which also makes the surface easier to walk and push a wheelchair or pushchair over.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — permeable surfacing rules for front gardens and planning guidance
- Royal Horticultural Society — gravel gardens, mulches and hard landscaping
Sources are checked at publication and can change — how I choose and check references.