How to use this calculator
Use internal measurements. Measure the upper diameter at the intended soil line, the bottom diameter, and the vertical distance between them. The same formula also handles a straight-sided round pot when both diameters match.
The formula
A worked example
A pot with a 40 cm top diameter, 30 cm bottom diameter and 30 cm fill height holds 29.06 liters. You need 15 whole 2-liter bags before allowing for an existing root ball.
Assumptions & limitations
Models a circular pot as a truncated cone with straight sides. It does not subtract an existing root ball, drainage material, or internal curves. A pointed cone may use zero for its bottom diameter.
Common questions
Why measure at the fill line?
The pot may be wider at the rim than where the soil ends. Using the rim diameter could overestimate volume.
Will this work for a rectangular planter?
No. Use the raised-bed calculator for a rectangular container.
Units reference
The calculation follows the method above. This reference supports the units and conversions.
NIST: units and conversion factors ↗Formula and worked example checked against automated test cases. About our methodology.