A 6-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 a 6-inch inside diameter at the fill line, a 4-inch inside base and a 5-inch soil depth holds about 1.5 US dry quarts (1.6 liters) before subtracting a root ball. This is a calculated example, not a standard capacity for every 6-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.
For a small repot, the root ball may occupy much of the new container. A full-pot capacity is therefore usually an overestimate of the fresh mix needed around that plant. Measure the empty space rather than automatically pouring in a quart.
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 × 5 × (6² + 6 × 4 + 4²) ÷ 12 gives about 99 cubic inches. That is about 1.5 US dry quarts, 1.6 liters or 0.06 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.
For several identical six-inch pots, fill one to the intended level using measured portions of mix and record the amount. Multiply that amount by the number of empty pots, then adjust for plants already occupying them. This is often simpler than calculating every small pot separately.
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.
Pot capacity in quarts is different from fresh mix needed
For the measured example above, the roughly 99.5 cubic inches of planting space equals about 1.48 US dry quarts or 1.72 US liquid quarts. Both figures describe the same space using different units. When a soil bag says dry quarts, do not compare that number directly with a kitchen measuring cup marked in liquid quarts.
If you are moving an existing plant into this pot, subtract the space occupied by its root ball when estimating new mix. For example, a planting space of about 1.6 liters with a root ball occupying about 0.5 liter needs roughly 1.1 liters of fresh mix, with a little available for adjustment. Measure your own container and root ball rather than treating this as a fixed six-inch-pot allowance.
Choose the pot for the plant as well as its capacity
A six-inch label is not a repotting schedule. Check the plant’s root development, drainage and species-specific needs before changing containers. Do not choose a much larger pot solely to fit a full bag of mix, or bury the stem to force a root ball into inadequate space. The useful question is how the plant fits at its appropriate planting depth, not whether every six-inch pot takes the same number of quarts.
