Monday, 6 March 2017

The confusing world of plant pot sizes

A wet spring weekend day provides a great opportunity to clean, sort and tidy our (rather large) collection of plant pots.  We've acquired quite a collection over a couple of decades of gardening, both from buying in plants but also inheriting pots from friends and family.

One thing that struck me when cleaning the pots was the number of different sizing systems in use.  Ignoring the really old clay pots (that had their own sizing system based on the number of pots made from a given amount of clay) there are a number of sizing systems for the plastic pots in the UK. Thankfully I can mostly ignore the US pot sizing here in the UK, though they have an ANSI standard for them. Of course the East and West coast seem to do it differently (one measures mostly by pot diameter whilst the other uses US gallon volumes).  Down under they seem to do measurements by pints too. So lets just stick to the pots we see in the UK, OK?

The old UK imperial measurement was in inches across the large top diameter and still appears on lots of pots, including quite a few relatively modern pots.  These often have metric equivalent measurements shown on the bottom too, which can be non-integer numbers (eg 7.6cm equates to a 3" pot).  To complicate things slightly, as well as the top diameter, pots can differ in their height.  As well as the "normal" height for each diameter you can get shallow "half", "squat" or "dwarf" pots which as roughly half the height, and "long toms" which are much deeper.  The dwarf pots are good for seed sowing and growing of many shallow rooted plants, whereas the long toms are well suited to deep root plants such as roses, some shrubs/trees and tomatoes.  A reference to a 5" pot usually means the "normal" depth - planting instructions will usually specify a shallow/half/squat/dwarf 5" or a deep long tom 5" if required.

Then we get to we have always jokingly called the "metric" pot sizing.  These pots have a number and a letter, such as 13F or 6E.  Whilst its common to see some of these mentioned in gardening product catalogues and on the bottom of pots, finding the standard that they are made to was tricky.  I'd sort of assumed it was a British, EU or ISO standard, or at least one from a major horticultural organisation. However no luck tracking it down yet.  I've had to cobble the table below together by trawling through several pot manufacturers catalogues, and I know I've seen other codes on my pots (such as 9B and 14A).  If anyone knows what the actual standard is and where to get it from, please pop a note in the comments.

CodeTop diameter (cm)Height (cm)Volume (ml)

You may also see an angle in degrees printed on the base - this is the angle of the slope between the top and the slightly smaller base. Usual angles are 5 degrees for "normal" sized pots and 8 degrees for half/squat/dwarf styles.  Different manufacturers also have different hole sizing and spacing at the bottom of their pots, including some with base side drainage. Again these can appear in the coding, along with things like "R" and "RX" for deep and extra deep long tom versions of pots.

Of course this is all too simple, so just to complicate matters, pots are often also labelled by volume, usually in litres in the UK/EU.  However a 1 litre pot might have a variety of top diameter measurements, depending on the depth. The larger pots from 3 litres upwards are nearly all specified by volume - right up to over 100 litres for large, mature trees. Pots under 1 litre seem to be rarely referred to by volume, with the top diameter measurement being more commonly used.

Another common sort of pot that you may have kicking around are the square top pots.  These are handy for fitting into larger trays with no gaps between them - making them easy to fill and water en masse.  Most of these are "metric" sizes, but some have straight sides down to a (slightly) smaller square base, whilst others have sides that bring the square top down to a circular base.  The latter are known as "square/round" containers, and can more easily fit into existing marketing or "shuttle" trays designed for use with round pots in nurseries, garden centres and DIY stores.  There seems to be as many coding systems, wall designs and drainage hole layouts as there are manufacturers, so there's even less evident standardisation here than in the round plastic pots.

Sorting, stacking and storing your pots

Stacking and sorting this variety of pots can be tiresome at home, where a wide variety of different types and sizes will appear in your collections in small quantities. In commercial horticulture they often stick to a limited number of sizes from known suppliers, plus their stock will be constantly replenished as they sell plants to retail outlets and customers, so this isn't a problem for them.  A few tips though:
  • Start by sorting out "normal" round plastic pots from square, square/round, long toms, clay pots, modules, etc. Store/stack the latter separately.
  • Sort the remaining pots by the large top diameter.  Its easiest to do this in metric with a conversion guide to Imperial sizes to hand (having a large sheet of of paper or card with the sizes in metric & imperial written on it for everything from 6cm to 18cm can be useful for stacking - after a while a 9cm, 10cm and 11cm stack all begin to look the same! Put bigger pots to one side for storing separately (or even holding the other pots if you don't have too many).
  • Clean pots as you sort and stack so that they are ready for reuse.
  • Discard any pots with cracks or splits.
  • Stack dwarf/squat pots separately whilst sorting.  If you only have a few they can then go in the top of that size stack at the end, otherwise make a separate stack for them. If you intermingle them with "normal" pots, you'll end up with high stacks with far fewer pots than you could have.
  • Try to stack pots from the same manufacturer and/or with similar designs together within a size.  Again this reduces stack height.
  • Note that sometimes you need to just make a value judgement as to which stack an odd imperial equivalent metric size pot goes in. A 16.6cm pot could go in the 16cm pile or the 17cm pile - different manufacturers pot designs can stack better in one size than the other.
  • Stack no higher than about 50cm unless you've got a way to hold the stack together, otherwise they become unstable and fall over.
  • For smaller plastic pot sizes, old tights can be used to hang the stacks up out of the way in sheds, etc.  Don't do this with clay pots though as a fall from height can easily smash them.  Xmas tree nets are also useful (or indeed any net tube that's long and a bit stretchy).
  • Alternatively a large cardboard or plastic box is great for keeping sorted pots stacked neatly, and also makes it easy to transport them between storage areas and potting bench.
  • 13cm pots are a very handy size as they are a common 1 litre volume size that you often have to pot on many plants into.

So that's the fun that is plant pot sizes.  Then of course there are British Standard seed trays and the various modules that fit into those, as well as propagators and staging that they themselves fit on.  And the various marketing, carry, shuttle and propagation trays. And root trainer modules and...


  1. The most useful pot I ever used in the garden was the old 3 1/2 inch pot. However on metrication this has been missed out meaning the nearest sizes are just too big or small. If I'm wrong, I'd love to know!

    1. 3.5" is about 8.89cm, so the nearest "metric" sized pots would be one of the 9cm diameter ranges. Something like a 9F for example.