Pot sizes, decoded
Pot sizing is a mess of inches, centimetres and "gallons" that are not gallons. Here is the whole thing in two honest 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 close to diameter) filled to the soil line. Real pots vary by maker and shape - treat these as good working numbers, not lab measures.
📏 Diameter → volume → what lives there
| Diameter | Holds about | US measure | What thrives in it |
|---|---|---|---|
| 2" 5 cm | ~0.1-0.2 L | a coffee-cup | Seed starts, plugs and rooted cuttings - a stopover, not a home. |
| 3" 8 cm | ~0.3 L | ~½ pint | Seedlings ready for their first real leaves; small succulents. |
| 4" 10 cm | ~0.5-0.6 L | ~1 pint | The classic shop size: young houseplants, windowsill herbs, pepper and tomato seedlings waiting to move on. |
| 5" 13 cm | ~0.9-1 L | ~1 quart | African violets (they like it snug), small ferns, established succulents. |
| 6" 15 cm | ~1.5-2 L | ~½ gallon | Pothos, peperomia, small philodendrons, a basil clump, compact annual flowers. |
| 8" 20 cm | ~4-6 L | ~1-1.5 gal | Peace lily, young monstera, larger herbs like rosemary, strawberries (one plant). |
| 10" 25 cm | ~8-11 L | ~2-3 gal | Mature snake plants, bird of paradise starters, bush beans, compact chilli plants. |
| 12" 30 cm | ~12-19 L | ~3-5 gal | Fiddle-leaf fig, big monstera, determinate (bush) tomatoes at the small end of comfort, full-size peppers. |
| 14" 36 cm | ~19-23 L | ~5-6 gal | Indeterminate tomatoes (this is the honest minimum), zucchini, dwarf citrus starting out. |
| 16" 40 cm | ~27-38 L | ~7-10 gal | Big feature houseplants, potatoes (3-4 seed pieces), small shrubs and patio roses. |
| 18"+ 46+ cm | ~38-60 L | ~10-15+ gal | Dwarf fruit trees, blueberries, olive and fig trees, anything staying put for years. |
🏷️ Nursery "trade gallon" decoder
Nursery containers are sold by trade size (#1, #3, #5...) and the "gallons" are labels, not liquid measure: a "1-gallon" pot holds roughly three quarters of a real US gallon, and the industry has never standardised it. If a recipe or guide says "5-gallon container", it almost always means a #5 trade pot (~14 litres), not 19 litres of actual volume.
| Trade size | Typical diameter | Actually holds |
|---|---|---|
| #1 (“1 gallon”) | ~6.5" / 16 cm | ~2.8-3 L (0.7-0.8 US gal) |
| #2 (“2 gallon”) | ~8.5" / 22 cm | ~6 L (1.5-1.6 US gal) |
| #3 (“3 gallon”) | ~10.5" / 27 cm | ~9-10 L (2.3-2.6 US gal) |
| #5 (“5 gallon”) | ~10.5-12" / 27-30 cm | ~13-15 L (3.5-4 US gal) |
| #7 (“7 gallon”) | ~14" / 36 cm | ~20-22 L (5.3-5.8 US gal) |
| #10 (“10 gallon”) | ~15-16" / 38-40 cm | ~28-30 L (7.5-8 US gal) |
| #15 (“15 gallon”) | ~17-18" / 43-46 cm | ~42-45 L (11-12 US gal) |
| #25 (“25 gallon”) | ~21-24" / 53-61 cm | ~85-95 L (22-25 US gal) |
🏺 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.
Free to cite with a link back. Figures are typical for standard-proportion pots and vary by manufacturer.