GardenCalcs.uk
Coverage Rate Calculator — Material Quantities by Area
Calculate how much material you need based on coverage rates.
Gravel is priced by the tonne (≈1.5 t/m³); sand & slate come in ~0.5 m³ tonne bags, topsoil ~0.65 m³, bark ~0.8 m³, compost ~0.75 m³ bulk bags
Plan area of the bed, border or path — length × width in metres
Gravel paths 40-50 mm, bark mulch 50-75 mm, topsoil/turf bed 100-150 mm
Cost per tonne (gravel) or per bulk bag — leave at 0 to skip the estimate
Adds a margin for settling, compaction and material lost spreading — adjust the figure under Advanced options
Extra material to account for waste and spillage
How We Calculate This
Volume = Area × Depth. The depth is converted from millimetres to metres first (50 mm = 0.05 m), so 20 m² at 50 mm needs 20 × 0.05 = 1 m³ before wastage.
Gravel is sold loose by the tonne, so the volume is multiplied by its bulk density (≈1.5 t/m³ for decorative gravel) to give tonnes — shown to one decimal place because you pay for the exact amount, not a rounded-up tonne.
Bagged materials are divided by their real UK bulk-bag volume and rounded up to whole bags: sharp sand and slate chippings ≈0.5 m³ tonne bags, topsoil ≈0.65 m³, bark mulch ≈0.8 m³ (800 L) and compost ≈0.75 m³. The optional 10% wastage covers settling, compaction and material lost while spreading.
Frequently Asked Questions
Decorative gravel: 25-40mm. Driveways: 50-75mm over a sub-base. Paths: 40-50mm. Weed suppression: at least 50mm.
Apply bark mulch at 50-75mm depth. Less than 50mm allows weeds through. Over 100mm can prevent rain reaching roots.
The 10% wastage accounts for uneven spreading, compaction after settling, and material left in bags or lost during transport.
Measure length and width in metres and multiply them. For irregular areas, use our Odd Shape Area Calculator or Border Area Calculator.
Coverage rate is how much area a given quantity of material covers at a specified depth. For example, 1 tonne of gravel covers about 13 m² at 5cm depth.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.