An 8-inch diameter alone does not tell you how much potting soil to buy. Measure the inside depth and shape as well. As a worked example, a tapered pot with an 8-inch inside diameter at the fill line, a 6-inch inside base and a 7-inch soil depth holds about 4.0 US dry quarts (4.4 liters) before subtracting a root ball. This is a calculated example, not a standard capacity for every 8-inch pot.
Measure the space that will actually hold mix
Use internal measurements rather than the outside rim size. Measure the diameter at your intended soil level, the bottom diameter and the vertical distance between them. Leave the watering space recommended for the plant and pot rather than filling level with the rim. A thick wall, false bottom or reservoir changes the usable space.
An eight-inch pot is often a step up for an existing plant, so subtract the approximate volume of its intact root ball. Do not add a gravel layer and then try to fit the same soil depth above it: that reduces root space without improving drainage.
Calculate a tapered round pot
For a pot with straight tapered sides, volume in cubic inches = π × depth × (top diameter² + top diameter × bottom diameter + bottom diameter²) ÷ 12. Keep all measurements in inches. This treats the pot as a truncated cone; it is an estimate for a pot with curved or irregular sides.
For the example above, π × 7 × (8² + 8 × 6 + 6²) ÷ 12 gives about 271 cubic inches. That is about 4.0 US dry quarts, 4.4 liters or 0.16 cubic feet. Use your own dimensions in place of the example.
A straight-sided cylinder uses π × radius² × depth. A rectangular container uses inside length × width × soil depth. If the manufacturer provides a usable soil capacity, compare that with your planned fill level instead of treating a decorative outer size as a volume.
Match the bag’s unit and subtract the root ball
US dry quarts and liquid quarts are different units. NIST’s conversion tables give one US dry quart as about 67.2 cubic inches. Check whether the bag lists dry quarts, liters or cubic feet, and do not use a kitchen liquid-quart measure as though it were a dry quart.
If you are comparing a six-inch and an eight-inch pot for repotting, do not assume that two extra inches of diameter means only two extra inches of root space. Both depth and taper can change. Choose the container for the plant, then calculate the amount of mix.
Subtract the approximate space already occupied by the plant and its existing mix. For example, if the empty pot needs 3 liters and the root ball occupies roughly 1 liter, the fresh-mix starting estimate is about 2 liters. You can keep a little extra available, but there is no universal 15–20% compression allowance for every bag and plant.
Fill for drainage and healthy roots
Choose a container potting mix appropriate to the plant and usable drainage holes. Moisten or prepare the mix according to its instructions and settle it gently around roots without packing it hard. Recheck the level after watering rather than deliberately compressing layers to fit more material.
Do not put gravel beneath the mix as a drainage improvement. Illinois Extension explains why that layer does not improve container drainage. Keep drainage outlets functional and empty unwanted standing water from outer pots or saucers as appropriate to the plant. Save the measured amount used for this particular pot; it will be a better guide next time than a diameter-only chart.
