How Much Compost for Pots & Containers?

Start with internal dimensions

A plant container is not sized by the amount of compost it holds. A pot sold as “40cm” may refer to an outside rim diameter, while the actual inner diameter can be smaller. Measure the usable inside and the depth from the base to the compost fill line.

Why a tapered pot needs a different calculation

Many pots flare outward towards the top. Treating one as a straight cylinder assumes the narrower lower part is as wide as the rim, which overstates volume. The dedicated calculator uses top and bottom diameters to model the pot as a frustum.

Leave a watering space rather than measuring to the brim

RHS guidance for planting up containers places rootballs so the finished compost surface is ideally around 5cm below the rim. Other RHS crop-specific container guidance uses smaller gaps, so the safest calculation is to measure the fill line you actually intend rather than hard-code a single deduction.

An existing plant displaces compost

If you are repotting, the rootball occupies part of the new container. RHS repotting guidance says to add compost beneath and back-fill around the rootball. That means the empty-container capacity is an upper limit, not automatically the amount of new compost you need.

Partial fills and existing material

For a planter that already contains usable compost, measure only the fresh depth you plan to add if you are top-dressing. For a complete refill, empty the container first and use the new fill line. If there is a deliberate drainage layer, measure from the top of that layer rather than from the physical base.

Worked examples

Which compost?

This guide is about quantity, not prescribing one growing medium for every plant. RHS distinguishes garden compost/organic matter from manufactured potting composts and provides different mixes for different uses. Choose a medium suited to the plant and container, then use the calculator to work out how much that chosen medium you need.