A 12-inch pot does not have one fixed soil capacity. The diameter, depth, taper and intended fill level all matter. A shallow 12-inch hanging basket can hold much less than a tall planter with the same rim width.
For a worked example, a round pot measuring 12 inches across the inside at the fill line, 8 inches across the inside base and 10 inches deep to that line holds about 0.46 cubic foot, 13.0 liters or 11.8 US dry quarts. This describes that example shape, not every pot sold as 12 inches.
Measure the space that soil will actually occupy
Measure inside the container, since an outside rim dimension includes the wall and lip. Leave the watering space you intend to use, then measure the depth only to that fill line. Check for an internal reservoir or raised base that reduces the planting compartment.
For a tapered pot, measure both the top and bottom interior diameters. For a straight-sided round pot, one diameter is enough. Basket liners, curved bowls and molded shapes can make a simple geometric calculation approximate; use a stated planting-volume specification when available.
Calculate a round tapered pot
The formula in cubic inches is π × height × (top diameter² + top diameter × bottom diameter + bottom diameter²) ÷ 12. For the example above, π × 10 × (144 + 96 + 64) ÷ 12 gives about 796 cubic inches.
Divide cubic inches by 1,728 for cubic feet. Divide by approximately 67.2 for US dry quarts, or multiply by 0.016387 for liters. Thus the example is approximately 11.8 dry quarts. Use the same unit on your bag when comparing quantities. The NIST volume conversions distinguish dry quarts from liquid quarts.
A straight cylinder instead uses π × radius² × height. Do not apply that full-cylinder volume to a deeply tapered hanging basket and expect an exact result. Also, a window box cannot be measured by length alone: its width and planting depth matter.
Subtract an existing root ball
Total planting-space volume and new mix required are different. When repotting, the existing root ball occupies some of the pot. If this example container needs 13 liters at the chosen fill line and the root ball occupies about 3 liters, the fresh-mix estimate is about 10 liters.
Use those numbers only as an example. Measure or estimate your own root ball, and keep a little extra mix for adjustments. There is no universal compaction percentage that applies to every bag. Do not press mix hard to force a calculated quantity into the container.
Choose the mix and plant position
Use a container growing medium suited to the plant. Keep drainage holes open, and do not put stones in the bottom to improve drainage. Illinois Extension recommends appropriate container drainage rather than a gravel layer.
Set the plant at an appropriate depth and leave space for watering. If the root ball leaves almost no room around it, reconsider the pot size instead of burying the stem to make it fit. If a plant is very small, a large wet volume of unused mix can make watering harder to manage.
How many bags should you buy?
Divide your fresh-mix estimate by the labeled bag volume, then round up to whole bags. For an empty example pot needing 11.8 dry quarts, one 16-dry-quart bag is sufficient. For the repotting example needing 10 liters, compare with liters on the package. Never assume that 12-inch diameter means 12 quarts.
