Formula
Each component = total volume × its percentage ÷ 100. Example: 32 cubic feet at 60/30/10 gives 19.2 cu ft topsoil, 9.6 cu ft compost and 3.2 cu ft aeration material. Percentages must total 100%.
How to use it, step by step
First find your total volume with the soil volume calculator. Then choose percentages: a common starting mix is 60% topsoil, 30% compost and 10% aeration material such as perlite or coarse vermiculite. If your topsoil is sandy, reduce aeration; if it is heavy clay, increase compost. Enter the percentages and check the calculator's warning if they do not total 100%.
Worked example in detail
For a 32-cubic-foot bed at 60/30/10, you need about 19 cu ft topsoil, 10 cu ft compost and 3 cu ft perlite. In bags, that is roughly 10 bags of 2-cu-ft topsoil, 5 bags of compost and 2 bags of perlite — rounding each component up separately is sensible, because you cannot buy part of a bag. Mix in layers as you fill, rather than filling with topsoil first and compost on top, or the layers will stay separate.
What affects the right mix
Climate, crop and topsoil quality all matter. Wet climates benefit from more aeration and drainage; dry climates benefit from more compost for water-holding. Root crops prefer a looser, stone-free mix than leafy crops. Pure compost as a bed fill shrinks and holds too much water, while pure topsoil compacts — the mix is a compromise, and the percentages above are a starting point to adjust after one season, not a law. See the raised bed soil mix guide for the full explanation.
Frequently asked questions
What counts as aeration material?
Perlite, coarse vermiculite, pumice or coarse sand in small amounts. Do not use fine sand in clay soil — it can make compaction worse.
Can I use only compost and topsoil?
Yes, at roughly 70/30, if your topsoil is already well-structured. Aeration material helps most in heavy or compacted topsoil.
Should I mix before or after filling?
Mix as you fill, in layers, and water lightly between layers to settle. Mixing a whole bed afterwards is hard work and disturbs the structure you just built.