Plant Calculators

Pot & Soil Volume Calculator

Work out a pot’s volume and exactly how much potting soil it needs.

How to measure your pot

Measure the inside of the pot, not the outside. Wall thickness can throw the numbers off on smaller containers. For the common sloped flower-pot shape, you need the top diameter, the bottom diameter, and the inside height.

The math behind it

Tapered round: frustum volume = (π·h ÷ 3) × (R² + R·r + r²)
Straight round: cylinder volume = π·r²·h
Rectangular: length × width × height

How much soil to buy

The soil estimate fills to about 90% to leave a watering gap, then converts to whatever bag size you entered so you know how many bags to buy.

Frequently Asked Questions

How do I measure a tapered (flower-pot) shape?

Measure the inside diameter across the top, then across the bottom, and the inside height. This calculator uses the proper frustum (truncated cone) formula so a normal sloped pot is measured accurately, not over-estimated.

Why is the soil amount less than the pot volume?

You never fill a pot to the very brim. You leave a watering gap at the top, and the rootball takes up space, so the calculator assumes you fill to about 90% to keep the bag estimate realistic.