How this calculator works
Multiply the inside footprint by the filling depth and bed count. Ingredient shares are volume fractions, which should be measured before mixing. The displayed mixes are comparison recipes, not universal prescriptions; extension advice generally favours soil with a suitable compost proportion and a soil test. The equal-thirds option has no topsoil and splits the other portion into coir and aeration material. Shrinkage, existing fill and settling are excluded.
Formula
Total V = length × width × depth × bed count. Ingredient volume = total V × ingredient fraction. Reverse dimensions divide V by the other three factors.
Worked example
Two beds each 2 m × 1 m × 0.5 m need 2 m3. A 60/30/10 split gives 1.2 m3 topsoil, 0.6 m3 compost and 0.2 m3 other material.
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Frequently asked questions
Are the percentages by weight?
No. They are loose ingredient volumes; materials with different densities would have different mass shares.
Can too much compost cause problems?
Yes. Salts, nutrient accumulation and settling can become limiting. Use a suitable tested blend for the intended crop.
Should I fill to the top of the frame?
Allow watering headspace if required. Enter the intended soil depth rather than the outside frame height.