How to calculate potting soil volume
Choose an approximate shape, measure the intended fill height and subtract the volume already occupied by roots or other items. Use the capacity and price of your chosen bag.
- Measure inside and choose fill height.
- Subtract any occupied volume you know.
- Choose allowance and bag capacity.
Formula
Rectangle: length × width × height. Cylinder: π × radius² × height. Truncated cone: π × height × (top radius² + top radius × bottom radius + bottom radius²) ÷ 3. Convert cm³ to litres by dividing by 1,000.
A 50 × 20 × 20 cm planter
The internal volume is 20 L. Roots occupying 2 L leave 18 L to fill. A 10% allowance gives 19.8 L, requiring 2 bags of 10 L.
- Use internal dimensions and the actual fill height. For a tapered pot, measure the top diameter at the fill level, not at the rim if it sits above that level.
- Occupied volume must be measured or estimated separately.
- The tool does not recommend plant mixes or predict compaction and drainage.
Frequently asked questions
Should I use the full pot height?
Use the height you will fill. Do not include the empty space you intend to leave below the rim.
What if the pot narrows at the bottom?
Choose a truncated cone and enter the internal diameters at the base and at the fill level. The top diameter is not the rim diameter when the compost sits below it. An irregular pot remains an approximation.
Do I need an allowance?
Choose one based on your material and measurements. The 10% example is not a universal compaction recommendation.