Raised Garden Bed Soil Mix Calculator
Estimate raised garden bed fill volume and two soil-mix component quantities from internal dimensions, fill level and mix ratio.
Your results
Calculation breakdown
- Fill volume
- internal length × width × depth × fill fraction
Worked example
A 3 m × 1.2 m × 450 mm bed filled to 90% holds 1.458 m³, split into about 1.021 m³ and 0.437 m³ at a 70:30 mix.
Assumptions
- The usable bed is a rectangular prism.
- Component share is by loose volume and component B is the remaining percentage.
Sources
Rates effective 23 August 2026.
How this calculator works
Internal dimensions define the usable rectangular volume, while a fill percentage leaves room below the top edge for mulch and watering. The calculator then divides that filled volume into two user-defined components, such as soil and compost, using complementary percentages rather than prescribing a horticultural recipe.
Frequently asked questions
Why use internal bed dimensions?
External dimensions include sleepers, blocks or panels that do not hold soil and would overstate the actual fill volume.
What soil and compost ratio should I choose?
Select a mix suited to the crops, drainage, existing materials and supplier advice; the calculator only divides the nominated ratio.
Will fresh soil settle after filling?
Often yes. Discuss settlement with the supplier and either increase the fill percentage or add a separate settlement allowance if appropriate.