How to measure a pot
- Diameter: measure across the top, inside the rim, in inches.
- Soil depth: most people fill to about an inch below the rim so water doesn’t spill over. Measure from the bottom of the pot to that line.
- Shape: choose straight sides for cylinders and grow bags, tapered for classic flowerpots that are narrower at the bottom, and square / trough for boxes and window planters.
The tapered option assumes the bottom is about three quarters as wide as the top, which suits most standard flowerpots. A tapered pot holds roughly 23% less than a straight-sided pot of the same top width and depth.
The formulas behind it
straight sides: cubic inches = 3.14 × (diameter ÷ 2)² × depth tapered: cubic inches = 3.14 × depth ÷ 3 × (R² + R×r + r²), R = top radius, r = bottom radius square / trough: cubic inches = length × width × depth dry quarts = cubic inches ÷ 67.2 cubic feet = cubic inches ÷ 1,728 liters = cubic inches × 0.0164
Worked example (the default): four 12-inch straight-sided pots filled 11 inches deep.
- One pot: 3.14 × 6 × 6 × 11 = 1,244 cubic inches = 18.5 dry quarts
- Four pots: 74.1 dry quarts = 2.88 cubic feet
- With 10% extra: 81.5 dry quarts = 3.17 cubic feet = 89.7 liters
- Bags: 6 bags of 16 quarts, 11 bags of 8 quarts, 4 bags of 1 cubic foot or 2 bags of 2 cubic feet
Potting soil chart by pot size
Single pot, no extra. Quarts are US dry quarts, as printed on potting mix bags.
| Pot top diameter | Soil depth | Straight sides | Tapered pot |
|---|---|---|---|
| 6 in | 5 in | 2.1 qt (2.3 L) | 1.6 qt (1.8 L) |
| 8 in | 7 in | 5.2 qt (5.8 L) | 4.0 qt (4.4 L) |
| 10 in | 9 in | 10.5 qt (11.6 L) | 8.1 qt (8.9 L) |
| 12 in | 11 in | 18.5 qt (20.4 L) | 14.3 qt (15.7 L) |
| 14 in | 13 in | 29.8 qt (32.8 L) | 23.0 qt (25.3 L) |
| 16 in | 14 in | 41.9 qt (46.1 L) | 32.3 qt (35.6 L) |
| 18 in | 16 in | 60.6 qt (66.7 L) | 46.7 qt (51.4 L) |
| 20 in | 18 in | 84.1 qt (92.7 L) | 64.9 qt (71.4 L) |
| 24 in | 22 in | 148 qt (163 L) | 114 qt (126 L) |
Notice how fast big pots grow: a 24-inch pot takes about eight times the soil of a 12-inch pot.
Gallon pots, buckets and grow bags
Containers sold by the gallon are easy to convert, because one US gallon is 231 cubic inches.
| Size | Cubic feet | Dry quarts | Liters |
|---|---|---|---|
| 1 gallon | 0.13 | 3.4 | 3.8 |
| 3 gallons | 0.40 | 10.3 | 11.4 |
| 5 gallons | 0.67 | 17.2 | 18.9 |
| 7 gallons | 0.94 | 24.1 | 26.5 |
| 10 gallons | 1.34 | 34.4 | 37.9 |
| 15 gallons | 2.01 | 51.6 | 56.8 |
| 20 gallons | 2.67 | 68.7 | 75.7 |
| 25 gallons | 3.34 | 85.9 | 94.6 |
Nursery pots labelled “#1”, “#5” and so on are trade sizes, and many hold less than the gallon number suggests. Grow bags are usually closer to their label. When in doubt, measure the container and use the calculator.
Window boxes and troughs
| Inside size (L × W × depth) | Dry quarts | Liters | Cubic feet |
|---|---|---|---|
| 24 × 8 × 8 in | 22.9 | 25.2 | 0.89 |
| 30 × 8 × 8 in | 28.6 | 31.5 | 1.11 |
| 36 × 10 × 10 in | 53.6 | 59.0 | 2.08 |
| 48 × 12 × 12 in | 103 | 113 | 4.0 |
Potting mix bag sizes
Bags of potting mix are labelled in dry quarts or cubic feet. One cubic foot is 25.7 dry quarts.
- 8 qt ≈ 0.31 cubic feet ≈ 8.8 liters
- 16 qt ≈ 0.62 cubic feet ≈ 17.6 liters
- 1 cu ft ≈ 25.7 qt ≈ 28.3 liters
- 2 cu ft ≈ 51.4 qt ≈ 56.6 liters
Some bags of compressed potting mix expand when loosened and say so on the label (“makes up to”). Use the loosened volume.
Potting mix is not garden soil
Use a potting mix for containers, not topsoil or garden soil from the bed. Soil from the ground tends to settle into a dense block in a pot, which holds too much water and too little air. For raised beds, where the soil is in contact with the ground or is much deeper, a soil-based blend works better and costs far less; see the raised bed soil calculator and topsoil vs garden soil.
Potting mix settles after the first few waterings. The default 10% extra covers that; top up with fresh mix if the level drops more than an inch.
Questions gardeners ask
- How much potting soil do I need for a pot?
- Measure the inside diameter at the top and the depth you will fill, in inches. For straight sides, volume = 3.14 × radius × radius × depth in cubic inches; divide by 67.2 for dry quarts or by 1,728 for cubic feet. A 12-inch pot filled 11 inches deep holds about 18.5 dry quarts.
- How much potting soil for a 5 gallon pot or bucket?
- A 5-gallon bucket holds about 0.67 cubic feet, which is about 17 dry quarts or 19 liters. Nursery pots sold as 5-gallon usually hold a little less than that.
- How much potting soil for a 10 gallon pot?
- Ten US gallons is about 1.34 cubic feet, 34 dry quarts or 38 liters. One 1.5 cubic foot bag fills it with a little to spare, if the pot really holds 10 gallons.
- How many quarts are in a cubic foot of potting soil?
- About 25.7 US dry quarts. An 8-quart bag is about 0.31 cubic feet and a 16-quart bag about 0.62 cubic feet.
- Can I use garden soil or topsoil in pots?
- It is not recommended. In a container, soil from the ground tends to pack down and drain poorly. Potting mixes are made light and open so roots get air.