Project guide
How much gravel do I need? Tonnes and bags
To work out how much gravel you need, multiply the area by the depth to get cubic metres, then by the weight of a cubic metre of your gravel to get tonnes.
The short answer
Tonnes = length x width x depth x density. Decorative gravel weighs about 1.5 tonnes per m³ and 10 to 20mm gravel or shingle about 1.6. Lay most gravel 50mm deep, where one 800kg bulk bag covers about 10 m². A 5 x 2m path needs about 0.8 tonnes: 1 bulk bag or 32 x 25kg bags.
The formula
- Volume (m³) = length x width x depth (50mm = 0.05m)
- Tonnes = volume x density x 1.05 (5% for settling and spillage)
- Bulk bags = tonnes / 0.8; 25kg bags = tonnes x 40
- Weed-control fabric = area + 10% for overlaps (50 m² rolls)
Worked example
A 5 x 2m path in decorative gravel, 50mm deep:
- Volume: 5 x 2 x 0.05 = 0.5 m³
- Weight: 0.5 x 1.5 x 1.05 = 0.79 tonnes
- Bags: 1 bulk bag (about 800kg) or 32 x 25kg bags
- Weed-control fabric: 11 m², so 1 roll of 50 m²
Materials checklist
- Gravel or chippingsSold loose by the tonne, in bulk bags of about 800 to 850kg, or in 20 to 25kg bags.
- Weed membraneA permeable geotextile under the gravel to stop weeds and stop the stones sinking into the soil.
- EdgingTimber, metal or brick edging keeps the gravel in place.
- Gravel grids (optional)Honeycomb panels that stop gravel moving on slopes and drives.
You'll also need
- Spade and rake
- Wheelbarrow
- Tape measure
- Membrane pegs
Step by step
- MeasureMeasure the length and width. Split odd shapes into rectangles and add them up.
- Choose the depthUse 50mm for most gravel, 70mm for large pebbles and about 100mm for cobbles.
- Work out the tonnesArea x depth x density, plus about 5%.
- Prepare the groundDig out, level, fit edging and lay weed membrane with overlapping joints.
- Spread and rakeTip the gravel in heaps and rake it level to the full depth.
Tips from the manufacturers
- Stone Warehouse and IBT Merchants both give about 10 m² per 800kg bulk bag at 50mm.
- Density varies with the stone and how wet it is, so treat any calculator as a guide and round up.
- Driveways need a compacted sub-base under the gravel, not just a deeper layer of gravel.
- Pea shingle is comfortable underfoot but moves about; angular gravel locks together better on paths and drives.
Mistakes to avoid
- Laying gravel too thin, so the membrane shows through.
- Laying it too deep, so it is hard to walk on.
- Ordering by m³ when the merchant sells by the tonne, or the other way round.
- Leaving out edging, so the gravel spreads onto the lawn.
Frequently asked questions
How much does a tonne of gravel cover?
At 50mm deep, about 12 to 13 m² for typical gravel, and a little less for heavier stone.
How much gravel do I need for 10 m²?
At 50mm, about 0.75 to 0.8 tonnes, which is one bulk bag.
How deep should gravel be?
About 40 to 50mm for most decorative gravel and chippings, 70mm for pebbles up to 50mm, and about 100mm for cobbles.
How many 25kg bags of gravel make a tonne?
40 bags. At 50mm, a 25kg bag covers roughly a quarter to a third of a square metre.
Do I need a membrane under gravel?
It is strongly recommended. A permeable weed membrane stops weeds growing through and stops the gravel working into the soil.
Sources
- Stone Warehouse gravel calculator (recommended depths and bulk bag coverage)
- IBT Merchants: laying decorative aggregates (1 bulk bag covers about 10 m² at 50mm)
- Aggregate Calculator: gravel planning densities
Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.
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