Coverage Rate Calculator — Material Quantities by Area

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.