How the calculation works
Each container's volume comes from its shape, then the calculator multiplies by how many you have, adds everything up and divides by the bag size, rounding up.
- Round pot: a straight pot is a cylinder, π × radius² × depth. For a tapered pot, enter the bottom diameter too and the calculator uses the frustum formula below.
- Square or rectangular planter, window box, raised planter: length × width × depth.
- Grow bag or bucket: the nominal gallon size on the label.
Tapered pot volume = π × depth ÷ 3 × (R² + R × r + r²), with R the top radius and r the bottom radius
Measure inside the rim and use the depth you will actually fill. Results come out in cubic inches first, then convert with the NIST tables: 231 cubic inches to the US gallon, 67.2 to the US dry quart, 1,728 to the cubic foot.
Quarts are not all the same
Potting mix is a dry product, and this calculator counts quarts as US dry quarts: NIST gives 25.7 of them to the cubic foot. A liquid quart is smaller, 29.9 to the cubic foot. Pot and grow bag sizes are given in gallons, which are liquid measure. That is why a 5 gallon grow bag shows as about 17.2 quarts here and not 20.
A worked example
The page opens with three straight 12 inch pots filled 10 inches deep, plus one 5 gallon grow bag.
- Each pot holds π × 6² × 10 = 1,131 cubic inches, or 16.8 quarts. Three hold 50.5 quarts.
- The grow bag holds 5 × 231 = 1,155 cubic inches, or 17.2 quarts.
- Together that is 67.7 quarts, 19.7 gallons, 74.5 litres or 2.63 cubic feet: 5 bags of 16 qt, 3 of 25 qt, or 2 of 2 cu ft.
Taper matters. A 12 inch pot that narrows to 9 inches at the bottom, 10 inches deep, holds about 13 quarts, almost a quarter less than the straight pot.
Bag sizes
Miracle-Gro sells its potting mix in 8, 16, 25 and 50 quart bags and 1 and 2 cubic foot bags; FoxFarm's Ocean Forest comes in 12 quarts and 1.5 cubic feet. Kellogg Garden says one 2 cubic foot bag of its raised bed and potting mix repots 5 to 7 one gallon plants. The table under the result shows how many bags of each common size you would need, so you can compare prices by the bag.
What size container for vegetables
Texas A&M AgriLife (Vegetable Gardening in Containers) and University of Maryland Extension give these sizes. They disagree on tomatoes: Texas calls 5 gallons the most suitable size for tomato, pepper and eggplant, while Maryland suggests 8 to 10 gallons.
| Crop | Texas A&M | University of Maryland |
|---|---|---|
| Tomato | 5 gal, 1 plant | 8 to 10 gal, 12 to 16 in deep |
| Pepper | 5 gal, 1 to 2 plants | 8 to 10 gal |
| Eggplant | 5 gal, 1 plant | 8 to 10 gal |
| Cucumber | 1 gal, 1 plant | 8 to 10 gal |
| Squash | 5 gal, 1 plant | 4 to 6 gal (summer squash) |
| Broccoli | 2 gal, 1 plant | 4 to 6 gal |
| Carrot | 1 gal, 2 to 3 plants | 4 to 6 gal, 8 to 12 in deep |
| Leaf lettuce, spinach | 1 gal, 2 plants | 1 to 3 gal, 4 to 6 in deep |
| Radish | 1 gal, 3 plants | 1 to 3 gal |
| Potatoes | 30 gal |
Texas A&M suggests 6 to 10 inch pots for herbs, and Maryland 8 inches of depth for strawberries.
Mixing your own
Texas A&M gives two recipes. The soilless mix is 1 bushel each of vermiculite and peat moss with 10 tablespoons of lime, 5 tablespoons of 0-20-0 and 1 cup of 5-10-10 fertilizer. The soil mix is equal parts of sphagnum peat moss or compost, pasteurized soil, and vermiculite or perlite. A bushel is 1.24 cubic feet, so the soilless recipe makes about 2.5 cubic feet, just short of the 2.63 in the example above.
For larger beds use the raised bed soil calculator, for in-ground beds the garden soil calculator, and to decide how many plants share a planter the plant spacing calculator.