What do pot sizes like 2 litre, 3 litre and P9 actually mean?
Nursery pots are sold by volume, garden centre pots by width, and neither number tells you what you expect. What each size really holds and why one step up buys so little.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 3 min
Short answer
A P9 is a 9cm square pot holding about half a litre. Litre sizes are the pot's own capacity, so a 2 litre is roughly 15cm across and a 10 litre roughly 26cm. Because volume follows width squared times depth, doubling a pot's width gives about eight times the compost — which is why a single step up the range changes almost nothing.
Two systems are in use at once. Plants raised for sale are sold in litre-rated pots — 1, 2, 3, 5, 10 — because a nursery buys by volume of compost. Empty pots on a garden centre shelf are usually labelled by rim diameter in centimetres, because that is what a customer measures. The diagram sets the common sizes side by side so you can convert between the two by eye.
Step by step
- Read a litre size as capacity, not width.A 3 litre pot holds three litres filled level to the rim. Filled properly, with a watering gap of 25 to 40mm below the rim, it holds slightly less than that.
- Learn the four sizes you will meet constantly.P9 — a 9cm square, about 0.5 litre, the standard for perennials and plugs grown on. 1 litre, about 11cm. 2 litre, about 15cm. 3 litre, about 17cm, the usual size for a shrub. Above that, 5 litre is about 20cm and 10 litre about 26cm.
- Understand why the steps feel so small.A pot is close to a cylinder, so its volume rises with the square of its diameter. Going from 15cm to 17cm across looks like a real change and adds about a litre. Going from 15cm to 30cm across is eight times the compost.
- Apply that to potting on.Moving a plant up one nursery size buys weeks, not a season. Moving it up two or three sizes at once is fine for most vigorous plants in an outdoor container — the old warning about drowning a small plant in a big pot matters mainly for slow, drought-adapted and indoor plants.
- Use volume, not width, when you are buying compost.Compost is sold in litres too, so the arithmetic is direct: a 50 litre bag fills about five 10 litre pots, or one 45cm tub. Work out the total litres before you go, or you will be back for a second bag.
- Ignore the number entirely for a square or trough shape.Litre ratings assume a tapered round pot. A square planter of the same rim width holds substantially more, and a shallow bowl of the same width holds far less. Measure and calculate rather than guessing from a size class.
Tips
- Pot volume for a straight-sided container is width times depth times length in centimetres, divided by 1,000, to give litres. For a tapered pot, use the width halfway down rather than the rim.
- A plant sold in a 3 litre pot has a rootball of about 17cm across, which is the hole size to prepare — not the eventual spread of the plant.
- Nursery litre sizes are nominal and vary between suppliers by 10 to 20 per cent. Two 2 litre pots from different growers are often visibly different.
Common mistakes
- Assuming a bigger-looking pot holds proportionally more compost — It holds far more. That is good news for the plant and expensive news for the compost budget — people routinely underestimate a large planter by a factor of three.
- Stepping a plant up one size at a time each year — It is more work than it needs to be and the plant spends most of its life root-bound. For hardy outdoor plants, go up properly and repot less often.
Questions people ask
What does the 'C' in C2 or C3 mean on a plant label?
Container, followed by the litre size. C3 is a 3 litre container-grown plant. You will also see 9cm, P9 and sometimes 'plug' for smaller stock.
Is a bigger pot always better outdoors?
For most things, yes — more compost means more water and nutrient reserve. The exceptions are alpines, succulents, herbs like rosemary and thyme, and anything that hates sitting wet, where an oversized pot of damp compost causes root rot.