Last updated: 2026-06-21

Soil Amendment

A soil amendment calculator works out how much compost, manure, or other amendment to mix into a garden bed to reach a target share of the worked soil by volume. Enter the bed area, the depth you will dig or till, and the percentage of the finished mix that should be amendment, and it returns the volume to add, the total worked soil volume, and how much original soil remains. It is for gardeners improving a bed before planting rather than building one from scratch.

How the amendment volume is calculated

Amendment is usually added as a share of the soil you actually work, not as a fixed depth. First find the worked volume — the bed area times how deep you dig — then take the target share of it:

where $V$ is the worked soil volume, $A$ the bed area, $d$ the working depth, $p$ the amendment share as a fraction, and $V_a$ the amendment to add. The rest of the worked volume, $V\,(1-p)$, is the existing soil the amendment blends into. Expressing the amount as a share keeps the blend consistent whether you work the soil shallow or deep.

Worked example

A 10 m² bed dug to 15 cm, aiming for compost to make up a quarter of the mix:

So you would mix about 0.375 m³ of compost into 1.5 m³ of worked soil. Spread over the 10 m² bed before digging in, that compost forms a layer just under 4 cm deep — a useful check that the share you chose is realistic.

Choosing the share and depth

For improving an existing bed, compost at around a quarter to a third of the worked volume is a common, generous target, with lighter yearly top-ups of 5 to 10 percent to maintain it. A working depth of 15 to 20 cm suits most vegetables and flowers. Very high shares of pure compost hold too much water and settle as they break down, so blending into the native soil beats replacing it. To check that your compost itself is balanced, see Compost C:N Ratio Calculator, and Raised Bed Soil Calculator for filling a new bed.