The formula
litres = length × width × depth ÷ 1,000, with the measurements in centimetres
Where it comes from
Compost is sold in litres and a planter is measured in centimetres, so the sum is a conversion: multiply the three measurements in centimetres and divide by a thousand. A planter of 100 × 20 × 20 is exactly forty litres. It is worth not filling to the brim: leave two or three centimetres clear so water does not run over when you water, and this calculator uses the depth of compost you give it, not the depth of the pot.
How to work it out by hand
- Measure the length, width and depth of compost in centimetres
- Multiply the three figures
- Divide by a thousand: those are the litres
- Multiply by how many identical planters you have
What is worth knowing
Potting compost is not soil: it is a light mix of peat, coir and perlite, made to hold water and stay open at the same time. It weighs around 400 kg per cubic metre against the 1,300 of topsoil, and that difference matters on a balcony or terrace, where weight counts. It also compacts over time and loses structure, so in permanent pots it is worth renewing the top layer every year or two. Large pots can be lightened at the bottom with expanded clay or drainage, which cuts both the compost needed and the total weight.