Pot sizes, decoded
Pot sizing is a mess of inches, centimetres and "gallons" that are trade classes rather than measured volume. Here is the whole thing in two tables: what each diameter actually holds, what tends to thrive in it, and what the nursery trade sizes really mean. When you are ready to move a plant up, the repotting calculator picks the next size and the soil mix calculator fills it.
Volumes assume a standard-proportion pot (height equal to the top diameter) filled to the rim; real pots vary by maker and shape, and you leave a little watering space, so expect slightly less soil.
Diameter → volume → what lives there
| Diameter | Holds about | US measure | What thrives in it |
|---|---|---|---|
| 2" 5 cm | ~0.08 L | ~0.3 cup | Seed starts, plugs and rooted cuttings: a stopover, not a home. |
| 3" 8 cm | ~0.25-0.28 L | ~1.1 cup | Seedlings ready for their first real leaves; small succulents. |
| 4" 10 cm | ~0.6-0.67 L | ~1.3 pint | The classic shop size: young houseplants, windowsill herbs, pepper and tomato seedlings waiting to move on. |
| 5" 13 cm | ~1.2-1.3 L | ~1.3 quart | African violets (they like it snug), small ferns, established succulents. |
| 6" 15 cm | ~2.0-2.3 L | ~2.3 quart | Pothos, peperomia, small philodendrons, a basil clump, compact annual flowers. |
| 8" 20 cm | ~4.8-5.4 L | ~1.3 gal | Peace lily, young monstera, larger herbs like rosemary, strawberries (one plant). |
| 10" 25 cm | ~9.4-10 L | ~2.6 gal | Mature snake plants, bird of paradise starters, bush beans, compact chilli plants. |
| 12" 30 cm | ~16-18 L | ~4.5 gal | Fiddle-leaf fig, big monstera, determinate (bush) tomatoes, full-size peppers. |
| 14" 36 cm | ~26-29 L | ~7.2 gal | Indeterminate tomatoes, zucchini, dwarf citrus starting out. |
| 16" 41 cm | ~38-43 L | ~10.7 gal | Big feature houseplants, potatoes (3-4 seed pieces), small shrubs and patio roses. |
| 18" 46 cm | ~55-61 L | ~15.3 gal | Dwarf fruit trees, blueberries, olive and fig trees, anything staying put for years. |
Nursery "trade gallon" decoder
Nursery containers are sold by trade class (#1, #3, #5...), and each class is a range of volumes, not one size. The American Standard for Nursery Stock (ANSI Z60.1) sets the ranges below: a #1 "1-gallon" container may hold anything from 2.5 to 4.1 litres, so some hold less than a US gallon and some a little more. The standard covers trade between nurseries, not the label on a retail pot.
| Trade class | Volume range | US gallons | Cubic inches |
|---|---|---|---|
| #1 | 2.5-4.1 L | 0.7-1.1 US gal | 152-251 |
| #2 | 5.2-7.8 L | 1.4-2.1 US gal | 320-474 |
| #3 | 10-12 L | 2.7-3.2 US gal | 628-742 |
| #5 | 13-20 L | 3.4-5.4 US gal | 785-1242 |
| #7 | 22-29 L | 5.8-7.7 US gal | 1337-1790 |
| #10 | 34-43 L | 9.0-11.5 US gal | 2080-2646 |
| #15 | 45-61 L | 12.0-16.0 US gal | 2768-3696 |
| #20 | 74-84 L | 19.6-22.3 US gal | 4520-5152 |
| #25 | 95-112 L | 25.0-29.7 US gal | 5775-6861 |
Source: Container Class Table, American Standard for Nursery Stock, ANSI Z60.1-2004 (AmericanHort; copy published by the Indiana DNR). Litres and gallons are converted from the standard's cubic inches.
Same diameter, different depth
The one-step rule for repotting
Move up one size at a time - about 1-2 inches (2-5 cm) of extra diameter. A pot that is too big holds a reservoir of wet soil around a small root ball, and that is the fast road to root rot. Check whether it is time at all with the repotting calculator, then work out how much fresh mix the new pot needs with the soil mix calculator. Drainage holes are non-negotiable in every size.
How these volumes are worked out
The first table is geometry, not a catalogue. A tapered pot is a truncated cone, so its volume is π × height ÷ 12 × (D² + D × d + d²), where D is the inside diameter at the rim and d the diameter at the base. We take height = D and a base 0.7 to 0.8 times as wide as the rim, which gives the range shown; US measures are the middle of that range (1 US gallon = 3.785 L, 1 quart = 0.946 L, 1 pint = 0.473 L, 1 cup = 0.237 L). Measure your own pot and the same formula applies. The trade-size table is different: it repeats what nurseries label their containers, and the "gallon" there is a trade class, not a measured volume.
Pots for every size on the chart
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