Plant Pot Sizes: Litres to cm, How to Measure, Which Size to Use

Plant pots are sold by volume in litres or by width in centimetres, and the two do not convert neatly, because pots of the same volume come in different shapes. This guide gives the measurements from our pot pages, how to measure any pot, and which size to use at each stage and for tomatoes. Our plastic pots, Air-Pots, fabric pots, saucers and trays are all in the pots and trays section.

Plant pot sizes: litres to centimetres

Shape changes the measurements: the 20 litre Air-Pot is narrower and taller than the 20 litre Round Pot, and the 20 and 25 litre Round Pots share a 35.8 cm top. Square pots sit tight together, so more fit in a tent or tray: four 11l Square Pots fit across a metre with no gaps, their page says.

How to measure a plant pot

• Diameter: straight across the top, as the Round Pots page measures it; many pots narrow towards the base.
• Height: from the base to the rim.
• Square pots: along one side.
• Litres: the volume of growing medium it holds. To check an unmarked pot, line it with a plastic bag and fill it to the rim from a measuring jug.

Small pots and net pots go by width. Jiffy Botanian Peat Free Pots are 6 cm, with 8 cm and 10 cm versions, and Heavy Duty Net Pots for hydroponics are 80, 140 or 200 mm across. A Jiffy 7 Peat Pellet is 38 mm, swelling to 41 mm when watered.

Which pot size for each stage

The sizes are our rule of thumb. Pot on a size or two at a time, not straight into the final pot: a small plant in a big pot stays wet longest, as our root rot guide says.

1. Seeds and cuttings: a plug or a 6-8 cm pot. Our plugs include Jiffy 7 pellets, Root Riot cubes and Rockwool Propagation Cubes (25 or 36 mm square). Jiffy Botanian and Jiffy Coir Pots are planted whole, as our sowing seeds guide says.
2. Rooted seedlings and cuttings: 1 litre. The 1l Air Pots page calls them ideal for rooted seedlings but warns they dry out faster than standard pots; the solid-walled 1 litre Square to Round is the alternative.
3. Young plants: 2-3 litres, such as the 2 or 3 litre Square to Round. Use a light mix in the small pots: see our potting soil guide.
4. Final pot: by the size of the plant. Our tent size guide puts four to six plants on a square metre in 8-12 litre pots, or two to four in 15-25 litre pots. The Air-Pot pages give 3 litres for intensive coco and hydro systems with plants up to 60 cm, 9 litres as the most popular Air-Pot size for soil, and 20 litres for very large plants up to 200 cm.

When to pot on, and how

Pot on when roots show at the drainage holes or through the plug, as our sowing seeds guide says. As a rule of thumb, a pot that dries out very fast or a plant that topples over is due a move too.

1. Put fresh soil in the bottom of the new pot, enough for the plant to sit at the same depth as before. Tomatoes go deeper: they root along a buried stem.
2. Water the plant, then tip it out. The Square to Round page says the root ball slides out easily; Air-Pots come apart, so it comes out whole.
3. Set it in, fill round it, firm gently and water in.

At each move, the Great White page says to dust the transplant hole (see our mycorrhizae guide).

Pot size for tomatoes

Our rule of thumb is a final pot of 15-20 litres for a tall (cordon) variety and about 10 litres for a bush or tumbling one. A bigger pot holds more water, so the supply stays steadier: calcium moves with the water the plant drinks, and a shortage shows as blossom end rot, as our cal-mag guide explains.

• Cordon: the 15 or 20 litre Round Pots, the 20l Air Pots or the 16 or 20 litre RootMASS pots.
• Bush or tumbling: the 10 litre Round Pot, the 11l Square Pots or the 9l Air Pots.
• Hand watering? Solid plastic pots stay wet longer, which is easier to manage; Air-Pots and fabric pots dry faster, as our pots guide says.

Saucers and trays to match

The Round Saucers hold only a little run-off, their page says; to hold more, use a garden tray. Air-Pots and fabric pots drain through their walls too, the 5l Air-Pot and RootMASS pages say, so give them a saucer wider than the pot. Never leave a pot standing in run-off, as our root rot guide warns.

Quick answers

What size is a 2 litre plant pot in cm?

It depends on the shape. The pages for our 2 litre pots, the Square to Round and the RootMASS, give no measurements, so ask us. For scale, the 1l Air Pots are 10 x 14.5 cm and the 3l Air Pots 15 x 23.5 cm.

How do I measure a plant pot?

Across the top for the diameter and from base to rim for the height. For the litres, line it with a bag and fill it to the rim with water from a measuring jug.

What size pot for tomatoes?

Our rule of thumb: 15-20 litres for a tall cordon variety, about 10 litres for a bush or tumbling one.

What size pot for seedlings?

Our rule of thumb: a plug or a 6-8 cm pot, then 1 litre once rooted and 2-3 litres before the final pot.

More Growers' Guides

• Fabric, Air-Pot or plastic pots?
• Sowing seeds indoors
• Growing chillies indoors
• Which potting soil?

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.