Measure inside the container
The available space is inside the pot. Thick walls can make external measurements noticeably different. Also measure the height you intend to fill, rather than automatically using the full height up to the rim.
For a rectangular planter filled to 50 × 20 × 20 cm, multiplying the dimensions gives 20,000 cm³. Divide by 1,000 to obtain 20 L. Keeping all measurements in centimetres prevents mixing different scales in one multiplication.
Choose a reasonable shape
A round pot with vertical sides can be approximated as a cylinder: π × radius² × height. The radius is half the diameter. Using diameter in place of radius would make the calculated cross-sectional area four times too large.
For a pot that narrows towards the bottom, choose a truncated cone with top and bottom internal diameters. These diameters should describe the section being filled. Bulging or irregular shapes will still be geometric approximations.
Subtract occupied space
If roots or other items occupy 2 L within a 20 L planter, 18 L remains for new material. Subtract that volume once. Do not subtract something that was already excluded by reducing the measured fill height.
This tool estimates the space available for compost. The mix, drainage and watering requirements depend on the plant. If you do not know the root-ball volume, compare several estimated values to see how much the final quantity changes.
- Measure the inside of the container.
- Use the actual height to the intended fill level.
- Keep diameter and radius distinct.
Frequently asked questions
Can I use a capacity printed on the pot?
Use it as a reference if it describes the volume you will actually use. Check whether it includes space you will leave empty or that is already occupied.
Is the result exact for an irregular pot?
No. The nearest shape gives a geometric approximation. The calculator cannot account for curves or cavities that are not represented by your inputs.