Raised Bed Soil Calculator — Volume & Soil Mix

Our raised bed soil calculator computes the volume of your bed based on its shape and dimensions, then recommends the ideal topsoil and compost mix quantities.

Volume formulas

• Rectangular: Length × Width × Depth
• Circular: π × Radius² × Depth
• L-Shaped: (Section 1 area + Section 2 area) × Depth

Soil mix

This tool defaults to the widely-used 70/30 topsoil-to-compost mix — a common UK landscaping convention rather than a single published standard (the RHS suggests bulking screened topsoil with around a third organic matter, plus sharp sand on heavier soils). You can change the proportion in Advanced Options:

• 70% screened topsoil — provides structure, minerals and moisture retention
• 30% compost or organic matter — adds nutrients, improves drainage and soil biology

Bulk bag sizing

Bulk (tonne) bags typically hold around 0.7–0.75 m³ (700–750 L), though sizes vary by supplier (some are 0.6–0.85 m³) — confirm the volume with yours and adjust it in Advanced Options. We round up to whole bags as suppliers do not sell part-bags. Topsoil should meet BS 3882:2015 for landscaping quality.