01 / The measuring tool
How much soil
does your pot need?
Measure the inside, leave a little room at the top, and turn dimensions into a practical shopping quantity.
Your potting estimate
Build a mix with this amount ↗Save a repotting plan ↗Loose volume estimate, not a guaranteed bag yield. Irregular shapes and compaction change the actual amount.
Measure once. Understand the result.
For a tapered pot we calculate a truncated cone. The width at the fill line is smaller than the rim when the pot narrows downward. We account for that before subtracting the root ball.
Worked example: inside diameters 24 cm at the top and 16 cm at the bottom, 22 cm high, 2 cm below the rim, a 1 L root ball and 10% extra need about 5.64 L of new mix.
See the calculation
Filled height h = H − headspace. Fill diameter f = bottom + (top − bottom) × h / H. Filled volume = π × h × (f² + f × bottom + bottom²) / 12. With centimetres, divide by 1,000 for litres. Subtract root volume, multiply by pot count, then add your extra percentage.
What do the volume units mean?
1 litre = 1,000 cm³. 1 US dry quart ≈ 1.10122 litres. 1 US liquid gallon = 3.785411784 litres. Dry quarts and liquid quarts are different: always read the bag label.
Before you buy a bigger pot
Check whether your plant needs more root space. Choose a container with drainage and a potting mix suited to the plant. A larger pot alone doesn’t solve yellow leaves or persistently wet soil.
Established tools such as Omni’s potting soil calculator also use these geometric methods. Our focus is making the assumptions and next steps easy to use.