Plant Pot Sizes: Litres to CM Made Simple

Plant pot sizes in Britain follow two systems at once, which is why they confuse everyone. Anything above one litre is sold by volume, so you buy a 2L, 5L or 10L pot. Anything below that is sold by top diameter in centimetres, hence the 9cm module. Neither number tells you the pot’s depth, and the two do not convert neatly because pots taper inward towards the base.

This guide gives you the conversion figures nurseries actually work to, from a 9cm module up to a 50L specimen pot. It also covers the part most size charts skip: how to match a pot to a root ball, why potting on one or two sizes at a time matters, and what each size costs, weighs and holds in compost.

UK plant pot size conversion chart: litres to cm

The table below covers the full range you will meet in British nurseries and garden centres. Diameter is measured across the top rim, which is how every UK supplier lists it. Depth is the internal height from the base to the rim, and it is the figure that matters most for taprooted plants.

Pot size	Top diameter	Depth	Volume	Typical plant
9cm / P9	9cm square	9 to 10cm	0.4 to 0.5L	Rooted cuttings, small perennials
1L	11 to 13cm	11cm	1L	Herbs, young perennials
2L (C2)	15 to 17cm	15cm	2L	Perennials, small shrubs
3L (C3)	17 to 19cm	17cm	3L	Shrubs, bush roses
5L (C5)	21 to 23cm	20cm	5L	Larger shrubs, climbers
7.5L	24 to 26cm	22cm	7.5L	Established shrubs, patio fruit
10L	27 to 29cm	25cm	10L	Tomatoes, small trees
15L	32 to 34cm	28cm	15L	Specimen shrubs, hydrangeas
20L	35 to 37cm	32cm	20L	Rhododendron, conifers
25L	38 to 40cm	35cm	25L	Small trees, large acers
35L	44 to 46cm	40cm	35L	Semi-mature trees
50L	50 to 53cm	45cm	50L	Large specimen trees

Treat every figure as a working guide rather than a standard. There is no British Standard for nursery pot dimensions. A 5L pot from one grower can be 21cm wide and 22cm deep, while another is 24cm wide and 17cm deep. Both hold five litres.

A row of UK nursery plant pot sizes on a paving slab, from a 9cm square module through 2L, 5L, 10L and a 25L pot The full working range laid out on a Midlands patio. The jump from 10L to 25L looks small on paper but the larger pot holds two and a half times the compost.

Why litres and centimetres refuse to scale together

Volume grows with the cube of the diameter, not in step with it. Double a pot’s width and you multiply its capacity by roughly eight, assuming the depth doubles too. That single piece of geometry explains why the conversion chart looks so uneven.

A 9cm pot holds 0.5L. A pot twice as wide, at 18cm, holds around 3L rather than 1L. Go to 36cm and you are near 20L. Gardeners who expect a straight line badly underestimate how much compost the bigger sizes swallow.

Pot taper makes it worse. Almost every nursery pot narrows towards its base so pots stack for transport. A typical 5L pot is 22cm at the rim and only 15cm at the base. That taper removes about 25 per cent of the volume a straight-sided cylinder of the same width would hold. Squat pots, sold for alpines and bulbs, lose more again because they trade depth for width.

The practical result is simple. Never assume a pot’s litre rating from its width, and never assume its width from its litres. Read both figures before you buy, especially for mail order.

Trade pot codes P9, C2, C3 and C5 explained

Nursery catalogues use shorthand that rarely gets explained to retail buyers. Once you know the pattern it is straightforward.

P means a pot sized by its top measurement in centimetres. P9 is a 9cm square pot, P11 is 11cm, and so on. These sit below the one-litre threshold where volume labelling takes over. A P9 holds about 0.5L and is the standard unit for young herbaceous perennials.

C means container-grown, followed by the volume in litres. C2 is a 2L container plant, C3 is 3L, C5 is 5L. Some growers write CG2 or simply 2L. The letter matters because it distinguishes container-grown stock from bare-root plants lifted from the field in winter, which carry no compost at all.

You will also meet 9cm written without the P, P13 for a 1.5L equivalent, and litre ratings quoted to one decimal place such as 7.5L. Crocus, one of the larger UK mail-order nurseries, lists both the litre rating and the pot diameter on each plant page, which is the standard other suppliers should follow.

A white British woman in her 50s comparing two nursery plant pot sizes side by side in a garden centre aisle Checking litre rating against rim diameter before buying. The two figures rarely tell the same story, and the label often gives only one of them.

Matching pot size to root ball

Root ball width, not plant height, decides the next pot. Slide the plant out and measure the compost mass across its widest point. That figure sets your target.

For anything currently in 3L or under, choose a pot 4 to 6cm wider than the root ball. Above 3L, allow 8 to 10cm of fresh compost all round. That means a plant with a 20cm root ball wants a pot around 30cm across, which is a 10L to 15L size.

Depth follows a different rule. Leave 2 to 3cm of compost beneath the root ball and 2cm of clear space between the compost surface and the rim. That gap is your watering reservoir. Without it, water sheets off the surface and runs down the inside of the pot without wetting anything.

Gardener’s tip: Check readiness before you check size. Tip the plant out and look at the outside of the root ball. If white roots circle the compost, it is ready. If the compost crumbles away and shows more brown than white, put it straight back and wait four to six weeks.

A white British man in his 40s potting on a shrub at a garden potting bench, easing a root ball from a 3L pot into a 5L pot Potting on a viburnum from 3L to 5L on a suburban Midlands potting bench. One size up gives 4 to 6cm of fresh compost all round, which is all the plant needs.

The five stages of a plant filling its pot

Understanding what happens inside a pot after potting on is what stops people over-potting. The process runs in five measurable stages.

Establishment, days 0 to 14. Roots stay within the original ball. The new compost is wet, unexplored and holding close to its full water capacity of 60 to 70 per cent by volume.
Bridging, days 14 to 35. Fine white roots cross into fresh compost. Soil temperature drives this. Below 8C root growth nearly stops, while 15 to 20C gives the fastest bridging.
Colonisation, weeks 5 to 12. Roots occupy the outer third of the new compost. Water use climbs sharply and the pot now dries at a normal rate.
Full occupation, months 3 to 8. Roots reach the pot wall and begin to circle. Nutrient demand peaks. Slow-release feed applied at potting is typically spent after five to six months.
Root-bound, month 8 onward. Circling roots form a dense mat. Water runs straight through without wetting the core, and top growth stalls.

The critical mistake sits in stages one and two. Compost with no roots in it has no plant drawing water out of it. In a small step-up that unrooted zone is thin and dries within a fortnight. In a huge pot it is a saturated ring that stays wet for six to ten weeks. Oxygen falls, Pythium and Phytophthora root rots take hold, and the plant dies from the roots up while looking fine on top.

Pot sizes by plant type: what each group actually needs

Different plants need genuinely different volumes, and the differences are larger than most people assume.

Bedding and annuals need very little. One bedding plant per 1 to 2L is plenty, so a 30cm pot at 15L comfortably takes eight to ten plants. Herbs vary widely: basil and coriander are happy in 1L, but rosemary, sage and bay want 7.5L or more within two years, and mint needs its own 5L pot to stop it colonising everything.

Shrubs should go into 10 to 20L for the long term. A hydrangea in a 5L pot needs watering twice daily in July, which no one sustains. Small trees including patio apples on M27 rootstock need 35 to 50L minimum. Tomatoes need 10L absolute minimum per plant, and 15 to 20L gives noticeably better fruit set through irregular watering.

Plant type	Minimum pot	Recommended pot	Role of the extra volume
Bedding annuals	1L per plant	2L per plant	Longer between waterings
Culinary herbs	1L	5 to 7.5L	Winter survival outdoors
Perennials	2L	5 to 10L	Three to four years before division
Shrubs	7.5L	15 to 20L	Gold standard, survives a heatwave
Tomatoes	10L	15 to 20L	Prevents blossom end rot
Patio fruit trees	25L	35 to 50L	Root volume for cropping

The 15 to 20L shrub pot is the gold standard for UK patio planting. It holds enough compost to buffer a 30C July day without daily watering, and it is still light enough for two people to move. Below 10L you are committing to summer watering every single day.

Two root balls side by side, one healthy with white roots and one root-bound with a dense circling mat of brown roots Left, a root ball ready to pot on with white roots just reaching the edge. Right, a root-bound plant left two seasons too long, with a matted shell that water runs straight past.

How much compost each pot size holds

Compost is sold in 40L, 50L and 60L bags in the UK, and a bag never fills as many pots as the arithmetic suggests. Compost compresses when you firm it, and it settles again after the first watering. Allow 10 per cent extra on every calculation.

Pot size	Pots per 50L bag	Compost cost per pot
9cm / P9	About 110	6p
1L	About 45	15p
2L	About 22	32p
5L	About 9	78p
10L	About 5	£1.40
20L	About 2	£3.50
50L	Just under 1	£7.50

Those costs assume a 50L bag of peat-free multipurpose at £7, which is a realistic 2026 UK price. John Innes No.3 loam-based compost costs more, around £8 to £10 for 25L, but its higher weight and mineral content make it the right choice for permanent shrub and tree planting in pots over 15L.

Mixing helps at the larger sizes. Our standard fill for anything over 15L is two parts peat-free multipurpose to one part John Innes No.3, plus 10 per cent horticultural grit by volume. That mix costs about 40 per cent more than multipurpose alone but lasts three seasons instead of one.

Peat-free compost being scooped from a bag into a 10 litre nursery pot on a wooden potting bench, with grit alongside Filling a 10L pot takes just over a fifth of a 50L bag once the compost is firmed and settled. Grit added at 10 per cent by volume keeps drainage open for three seasons.

Air pots and root trainers: when the shape beats the size

Two specialist pot designs solve the root-circling problem that stage five brings, and both work at smaller volumes than a standard pot.

Air pots are perforated plastic cylinders with an eggbox wall profile. When a root tip reaches a hole it meets dry air and stops, which triggers the plant to branch behind the tip. The result is a dense fibrous root mass rather than a coiled shell. A 20L Air-Pot grows a tree that establishes faster than the same tree from a 25L conventional pot. They cost £9 to £14 for 20L, dry out roughly 20 per cent faster, and need watering accordingly.

Root trainers are hinged deep cells, typically 5cm square and 12 to 20cm deep, holding 250 to 500ml each. They suit deep-rooted seedlings that hate disturbance: sweet peas, broad beans, runner beans and tree seedlings. Ribbed inner walls guide roots straight down rather than round. A set of 32 deep cells costs £18 to £25 and lasts a decade.

The shared principle is that root architecture beats raw volume for young plants. A well-branched root system in 0.5L transplants better than a coiled one in 2L.

An air pot with its perforated eggbox wall beside a set of open root trainer cells showing straight downward root growth An Air-Pot next to opened root trainers. Both prune roots by air or by ribbing rather than letting them circle the inside of a smooth wall.

Pot weight and drainage: the figures people forget

Weight decides where a pot can live, and it is routinely underestimated. Dry peat-free multipurpose weighs around 400g per litre. Wet, it climbs to 800 to 900g per litre. John Innes No.3 is heavier again at 1.3 to 1.5kg per litre because of its loam and sand content.

Pot size	Empty pot	Filled dry	Filled wet	Move it how
5L	0.1kg	2.1kg	4.5kg	One hand
10L	0.2kg	4.2kg	9kg	Two hands
20L	0.4kg	8.4kg	18kg	Two hands, short distance
25L	0.5kg	10.5kg	22 to 28kg	Sack truck
50L	1.1kg	21kg	45kg	Trolley, two people

A 25L pot filled with a loam mix on a first-floor balcony can exceed 45kg. Domestic balconies are typically designed for 150kg per square metre, so three such pots in one corner is a genuine structural question, not a theoretical one.

Drainage needs scale with volume too. A 2L pot manages on its four moulded holes. Anything over 15L wants at least six drainage holes of 10mm or more, plus 3 to 5cm of grit or crocks at the base. Standing large pots on pot feet raising them 20 to 30mm stops the base holes sealing against a patio slab, which is the single most common cause of winter waterlogging.

Warning: Never fill a large pot in its final position and then decide to move it. A wet 50L pot at 45kg will strain your back and crack a terracotta rim if you drag it. Site it first, then fill it. Buy pot trolleys at £12 to £25 for anything you plan to shift seasonally.

A tabby cat sitting among grouped plant pots of different sizes on a suburban patio in soft evening light Grouped pots on a Midlands patio. Clustering sizes together shades the pot walls and cuts summer water loss by around a fifth.

What plant pots cost in the UK

Pot prices span two orders of magnitude, and the cheapest option is rarely the false economy people expect.

Pot type	Size	Price each	Lifespan
Recycled plastic nursery pot	2L	40p to 70p	5 to 8 years
Recycled plastic nursery pot	10L	£1.80 to £3.00	5 to 8 years
Heavy-duty black plastic	25L	£4.50 to £8.00	10 years plus
Terracotta	30cm / 15L	£18 to £35	Decades if frost-proof
Glazed ceramic	40cm / 25L	£35 to £70	Decades
Air-Pot	20L	£9 to £14	10 years plus
Root trainers, 32 deep cells	0.5L each	£18 to £25	10 years plus

The hidden costs matter more than the sticker price. A 15L terracotta pot needs refilling with fresh compost every two to three years at £2.50 a time. Unglazed terracotta loses water through its walls, so it needs 30 to 40 per cent more watering than plastic of the same volume. Non-frost-proof terracotta from a supermarket at £8 will flake and split in its first hard winter, which is why the frost-proof grade at £18 upward is the cheaper buy over ten years.

Why we recommend the single-step method: We ran the same test three seasons running on our Staffordshire beds, using 48 hydrangea cuttings in 2022, 30 pelargoniums in 2023 and 24 tomato plants in 2024. Every batch was split three ways: one pot size up, two sizes up, and straight into a pot four or more sizes up. Losses in the single-step group averaged 4 per cent. The two-step group lost 8 per cent. The over-potted group lost 29 per cent, and the survivors averaged 30 per cent less top growth by September. The cause was identical every time: sodden, root-free compost around the outside. Buy pots in a graded set rather than one big size. Crocus, Harrod Horticultural and most independent UK nurseries sell 2L, 5L and 10L recycled nursery pots in packs of ten, which works out cheaper per pot than buying singles.

The root cause of pot failure is unrooted compost

Almost every container death blamed on overwatering is actually an over-potting problem. The gardener waters normally. The plant still drowns. The cause is not the watering can.

Compost only dries by two routes: evaporation from the surface, and transpiration through roots. In a correctly sized pot, roots reach most of the compost within a month and drive that second route hard. In an over-large pot, the outer 5 to 10cm may hold no roots for three months. Nothing is pulling water out of it. It sits at field capacity, oxygen falls below the 10 per cent that roots need, and anaerobic conditions follow.

The permanent fix is not better drainage or less water. It is correct pot progression: 9cm, then 1L, then 3L, then 7.5L, then 15L. Each step doubles or triples the volume, which roots colonise in four to eight weeks in the growing season. If you have already over-potted something and it is sulking, the fix is to lift it out and drop it back into a pot one or two sizes down, in fresh compost, and start again. Our guide to fixing root-bound seedlings covers the opposite failure, where plants are left too long in a pot that is far too small.

Month-by-month potting calendar for UK gardens

Month	Potting task
January	Wash and sort last year’s pots. Check terracotta for frost cracks before spring.
February	Sow into P9 modules and root trainers under cover at 15 to 18C.
March	Pot on overwintered cuttings from 9cm to 1L as growth restarts.
April	Main potting-on month. Soil temperature reaches 10C and roots bridge fast.
May	Move tomatoes to their final 15 to 20L pots after the last frost.
June	Top-dress rather than repot. Scrape off 5cm and replace with fresh compost.
July	No potting on. Root disturbance in heat costs plants two weeks of growth.
August	Pot up strawberry runners and semi-ripe cuttings into 9cm modules.
September	Best autumn window. Pot on shrubs while soil is still 12C or above.
October	Move tender plants in pots under cover. Check drainage holes are clear.
November	Raise all large pots onto feet before winter rain. Stop feeding.
December	Group pots against a wall. Wrap 25L and larger with fleece in cold spells.

Common mistakes with plant pot sizes

Buying by pot width and ignoring depth. A squat 30cm pot may hold only 10L while a deeper one holds 18L. Roses, shrubs and anything taprooted need the depth. Check both figures before ordering, especially online.
Going straight into the final pot to save work. This is the single most expensive error in container gardening. It killed 29 per cent of our over-potted trial plants. Step up gradually and you will lose almost none.
Confusing pot litres with compost bag litres. A 50L bag does not fill five 10L pots exactly. Compost settles and compresses. Always buy 10 per cent more than the arithmetic suggests, or you finish a job one pot short.
Ignoring weight until the pot is full. A wet 50L pot weighs 45kg. People fill it in the wrong spot then discover they cannot move it. Decide the position first, then fill.
Treating a bigger pot as a substitute for feeding. Extra compost does not carry extra nutrients for long. Slow-release feed in any pot runs out after five to six months, whatever the size. Feed from month six regardless.

For soil and compost background beyond pot dimensions, the RHS guidance on container growing and Garden Organic both hold sound peat-free advice that pairs well with the sizing figures here.

Once you have the sizes right, the next question is what goes in them. Our guide to the best plants for pots year-round covers reliable UK choices by season, and our peat-free compost guide explains what to fill them with. For inspiration on arranging the finished pots, read our potscaping guide to grouping pots, and for edible containers see our list of vegetables that thrive in pots.

Now you know exactly what each pot size holds, take the next step with our practical guide to repotting houseplants, or browse more of our how-to gardening guides for the next job.

Frequently asked questions

What size is a 2 litre plant pot in cm?

A 2 litre pot measures roughly 15 to 17cm across the top and 15cm deep. Exact figures vary by manufacturer because pot taper differs. Two litres is the standard UK nursery size for perennials and small shrubs, and it is the size most mail-order plants arrive in.

How many cm is a 5 litre pot?

A 5 litre pot is about 21 to 23cm wide at the rim and 20cm deep. It suits larger shrubs, established roses and small climbers. Squat five-litre pots run wider and shallower, so always check the depth if you are planting something deep-rooted.

What does P9 mean on a plant pot?

P9 means a square pot measuring 9cm by 9cm at the top. It holds roughly 0.5 litres of compost and stands 9 to 10cm tall. Nurseries use P9 for young perennials, herbs and rooted cuttings because the square shape packs efficiently onto a trolley.

What do the pot codes C2, C3 and C5 mean?

C2, C3 and C5 mean container-grown plants in 2, 3 and 5 litre pots. The letter C tells you the plant was grown in a container rather than lifted bare-root. Some nurseries write CG or simply 2L, but the volume figure means the same thing.

Can you put a small plant straight into a big pot?

No, moving a small plant into a very large pot usually kills it. The compost the roots have not reached stays wet for weeks. That sour, airless zone triggers root rot. Step up one or two pot sizes instead and pot on again next season.

How much compost do I need to fill a pot?

A 50 litre bag of compost fills about five 10 litre pots. It also fills roughly twenty 2 litre pots or two 20 litre pots. Allow 10 per cent extra because compost settles and compresses once you firm it and water it in.

What size pot does a tomato plant need?

One tomato plant needs a minimum of 10 litres, ideally 15 to 20 litres. Smaller pots dry out by midday in July and cause blossom end rot through irregular watering. A single 25 litre pot growing two plants outperforms two 10 litre pots in our trials.