UK Plant Pot Sizes and Measuring Guide

ame internal measurements as the size shown online.

If you’re replacing an old pot, repotting a growing plant, or finding a decorative planter for an existing grow pot, a couple of quick measurements can save a lot of guesswork.

Here’s how to measure a plant pot, how UK pot sizes work, and what to look for when choosing one.

How Are Plant Pots Measured in the UK?

In the UK, plant pots are usually sized in either centimetres or litres.

When you see a pot listed as 15cm, 20cm, or 30cm, this will normally refer to one of its physical dimensions, most often the diameter across the top.

Litre measurements are different. A 5-litre or 10-litre pot refers to roughly how much compost or growing medium the pot can hold.

These measurements aren’t directly interchangeable. Two pots could both hold 10 litres but look completely different. One might be tall and narrow, while another could be much wider and shallower.

If you need a pot to fit into a particular space or around another container, it’s worth checking the full dimensions rather than relying on the stated size alone.

How to Measure a Plant Pot

So, the big question is – how do you measure a plant pot properly?

It depends on the shape, but you’ll usually need the width across the top, the height, and sometimes the internal opening. All you need is a tape measure or ruler.

How to Measure the Diameter of a Plant Pot

For a standard round plant pot, measure straight across the top from one side to the other, passing through the centre.

That measurement is the diameter. So, if your pot measures 20cm straight across the top, you’d generally describe it as a 20cm pot.

Measuring around the outside gives you the circumference, not the diameter, so make sure you measure straight across.

How to Measure Square or Rectangular Pots

For a square pot, measure across the top from one side to the opposite side.

For rectangular pots, troughs, or window boxes, you’ll usually need both the length and width. For example, a planter might be listed as 40cm long x 20cm wide.

This is especially useful if the pot needs to fit onto a windowsill, balcony, shelf, or narrow patio.

How to Measure Plant Pot Height

Measure from the base of the pot to the top rim. Height matters if you’re choosing a deeper pot or placing one pot inside another. A decorative planter may have the perfect width but still be too short, leaving the grow pot sticking out above the top.

Don’t Forget the Internal Opening

If you’re buying a decorative pot to go around an existing grow pot, the internal opening is one of the most important measurements. So, measure from one inner edge of the opening to the other.

The external width can be misleading. A pot with thick sides or a chunky rim may look wide enough but still have an opening that’s too small. It’s also worth checking whether the pot tapers towards the bottom. Your grow pot could fit through the top but become wedged further down.

What Do Plant Pot Sizes in Litres Mean?

Litre measurements tell you the approximate volume of the pot rather than its exact dimensions. This is common with larger nursery pots, shrubs, trees, and outdoor plants, which may be sold in sizes such as 1, 2, 5, 10, or 20 litres.

A larger litre size generally means more room for roots and compost, but it doesn’t tell you a specific width or height. A tall 10-litre pot and a wide 10-litre pot can have very different dimensions, so it’s best to check both the capacity and the measurements if the exact size matters.

A Simple UK Plant Pot Size Guide

TOP TIP: If you’re replacing an existing pot, diameter is often the best place to start. If you’re fitting one pot inside another, focus on the internal diameter.

What Size Decorative Pot Do I Need?

Start by measuring the widest part of the grow pot. Your decorative planter should have an internal opening that’s slightly wider, giving you enough room to lift the plant in and out. It should also be deep enough to hide the grow pot comfortably.

Try not to choose an exact fit. Leaving a little space around the sides makes watering and removing the plant much easier.

What Size Pot Should I Move My Plant Into?

If your plant has outgrown its current home, you might be tempted to move it straight into a much larger pot. However, it’s often better to size up gradually. A pot that’s just slightly larger gives the roots more room to spread without leaving them sitting in a large amount of damp compost.

You may need to repot if:

• Roots are growing through the drainage holes
• The roots have become tightly packed
• The plant dries out much faster than it used to
• Growth has slowed
• The plant has become top-heavy

Before buying a replacement, measure the current pot and look for a slightly larger option. You can browse our range of planters if you’re ready to give your plants a bit more growing room.

Does the Shape of a Plant Pot Matter?

It can do. Two pots with the same diameter can still offer different amounts of growing space. For example, tall pots can provide more depth without using too much floor space, while wide planters give plants more room to spread.

Troughs and rectangular planters are handy for windowsills, walls, balconies, and patio edges, particularly if you want to grow several plants together.

But the best choice usually comes down to what your plant needs and where you want the pot to go.

Do Plant Pots Need Drainage Holes?

If you’re planting directly into an outdoor pot, drainage is important. Holes in the base allow excess water to escape instead of sitting around the roots.

Decorative indoor pots are a little different. Many are designed as covers rather than containers for direct planting. In that case, the plant stays in its grow pot, which can be lifted out for watering or whenever you need to empty excess water.

Once you’ve chosen the right container, make sure you use a suitable compost for whatever you’re growing.

Common Plant Pot Measuring Mistakes

It only takes a small measuring error to end up with a pot that doesn’t quite work. Here are some mistakes you should avoid: 

• Measuring around the pot instead of across it: for diameter, measure in a straight line across the centre.
• Only checking the outside measurement: a thick rim can make the internal space smaller than you expect.
• Forgetting about height: the width might be perfect, but the inner grow pot could still stick out.
• Assuming litres equal a fixed diameter: they don’t. Pot shape affects both dimensions and capacity.
• Buying a pot that’s much too large: when repotting, moving up gradually is usually the better approach.

Choosing the Right Plant Pot

Once you know what to measure, plant pot sizing becomes much easier.

Whether you’re repotting a houseplant, refreshing your patio, or getting ready for the next growing season, take a look at our wider range of gardening products and accessories for everything you need to get started.