A pot that's too small squeezes the roots until the plant stalls, and a pot that's too large holds damp soil the roots can't reach yet, which is where root rot usually starts. Getting the size right should be simple, but British planters are sold two different ways: by diameter in centimetres and by volume in litres, and the two don't convert in your head. The Pot Size Calculator takes the pot you've got now and the state your plant is in, and turns that into a new diameter and a new volume to look for, with the reasoning behind the number shown alongside it.
What you enter, and what you get back
You put in the diameter of the current pot in centimetres and a description of the plant's condition: root-bound (roots have filled the pot and circled the inside of it, often visible at the drainage hole or when you ease the plant out), healthy and growing normally, or recently repotted. The calculator returns a recommended diameter in centimetres and a matching volume in litres, so you have both figures a retailer might use.
Diameter and litres aren't the same measurement
Two pots with the same diameter can hold different volumes depending on how deep they are and how the sides taper, which is why a diameter on its own doesn't tell you how much soil, and how much room for roots, you're actually buying. Giving you both figures means you can match either kind of listing without doing the conversion yourself.
Why the recommended size is a small step
The standard advice is to move up one pot size when you pot up (repot into a larger container), and in practice that means no more than 2 to 3 centimetres more in diameter. A much larger pot sounds generous, but it surrounds roots that haven't grown into the new space yet with soil they can't draw water from, and that soil stays wet for longer after every watering. The calculator applies that same step by default, because it's the size change a healthy root system can actually make use of.
How plant condition changes the number
A plant that's root-bound has already outgrown its pot, so the step up can reasonably be a little larger than the default, since the roots are ready to use the extra space straight away. A plant that's recently been repotted needs no change at all; going up again before the roots have filled the current pot just repeats the overpotting problem the first move was meant to avoid. The condition you select is what moves the recommendation between these cases.
Where the calculator can get it wrong
The recommendation is built on the diameter and condition you enter, so an inaccurate measurement or a guess at the plant's condition will carry straight through into the answer. It also can't see the shape of the pot you're considering, the plant species, or how you tend to water, all of which affect whether a given volume is the right amount of soil for that plant in your home. Treat the figures as a starting point for what to look for, not a guarantee of the right fit, and check the result against what you can see when you ease the plant out of its current pot.
What to do with the number
Once you have a diameter and a volume, the next decision is usually material and drainage. The planter materials guide covers how ceramic, terracotta, plastic and the rest affect how quickly that extra soil volume dries out between waterings, and the drainage guide covers what to do if the pot you've chosen doesn't have a hole. For the sizing rules behind the calculator itself, including how to measure a root ball and when potting up is worth doing at all, see the guide to choosing planter size.