Most tyres are now marked according to the International Standards Organisation (ISO) in addition to more familiar markings. The ISO size comprises two numbers separated by a dash. The three-digit number after the dash is the most important. This is the bead diameter at which the tyre fits onto the rim. Check the bead diameter of the tyres on your bike. If you’re ever in desperate need of a tyre and can find one with the same bead diameter, it will probably fit even if every other thing about it is different.
That initial two-digit number is the tyre section and roughly indicates the tyre width. Its actual width is normally less, but varies according to how the tyre is designed and the width of rim to which it is fitted. But the height of the tyre usually corresponds quite closely to tyre section, so overall diameter approximately equals the bead diameter plus twice the section. That’s how the diameter and circumference (Circ. – equals distance rolled in one turn of the wheel) are calculated in the Tyre Size Table. These figures are useful for setting your cycle-computer and precisely calculating gears.
Half of the bead diameter (i.e. the bead radius) corresponds with the distance from the centre of the wheel to the centre of a brake block. By comparing this dimension you can see if the brakes will reach when a different size of rim is fitted to your bike. Switching from 630 to 622 for example, needs another 4mm of brake reach.
Imperial, Metric & American
Sometimes you can’t find any ISO size on a tyre, only the old familiar markings. Although they’re familiar they can be very confusing. Tyres of same size come with lots of different markings: e.g. 28×1¼×15/8 or 28×1¾×1¼ or 700×32C are all 32–622. Even worse, the same or very similar markings can sometimes be found on completely different sizes of tyre: e.g. 26×1½ is a whole inch bigger than 26×1.5. So use these columns to convert yourself to ISO and then have nothing more to do with those old markings!
Refer to the column headed Imperial, Metric or American in order to work out what actual (ISO) size you have. The first two headings need no explanation. The American column is for tyre sizes originating in the USA and where US markings may confuse. Note that American and Japanese branded road tyres tend to be smaller than described and often lack any ISO markings.
Two tyres with the same ISO markings are completely interchangeable, even if one seems to be metric and the other is also marked in inches: e.g. 650B and 26×1½ are both 38-584 and should be marked accordingly. Look for the ISO numbers and you always know exactly where you are. Some rims are also now marked according to ISO with their bead diameter and internal section (width where the tyre fits): e.g. 622–17 for the size usually fitted to touring and hybrid bikes. But some manufacturers give the overall width instead. Subtract 6mm from overall width to estimate the rim section, or simply measure the distance between the flanges. If the rim is not marked with its bead diameter you can read this from the tyre – it’s the one dimension they have in common.
The ideal relationship between tyre and rim section is about 1.8 to 1, but any tyre from 1.4 to 2.2 times should fit – always provided that the bead diameters correspond.
Nowadays nearly all rims have hooked flanges (required to retain folding tyres) that will hang onto tyres up to three times their width. This increases the cushioning effect: you get more comfort out of the same tyre at the same pressure when you fit it to a narrow rim – yet it still rolls just as easily. On the other hand it increases the stress on the sidewall of the tyre. Mountain-bike tyres are designed to take this extra stress but road tyres are not. Another reason not to exceed the 2.2 limit on road is that a more bulbous tyre can deflect sideways with a noticeable effect on control and roadholding. Conversely, fitting a tyre of less than 1.4 times the rim section will result in a harsh ride and more rolling drag than a larger tyre at the same pressure.
Tyre Size Table
The table below lists most of the sizes of tyre that are likely to be found in the UK, but some common tyre sections are omitted to save space. Where you see a block of two or more compatible sizes, having the same bead diameter, you can assume that a variety of intermediate sizes exist that will also fit the same rim. For example: between 18–622 and 25–622 there are tyres of 19, 20, 22 and 23mm section. Sizes highlighted in white seem most likely to be available in the long term. Anyone specifying a new cycle should try to avoid other ISO diameters.
All tyres are marked with a pressure. Sometimes this is an absolute maximum that nobody should need to exceed, or sometimes only a recommended maximum that may be exceeded by heavier riders (e.g. tandemists).
Off-road tyres are usually marked with a pressure range, from the minimum that will support a rider of average weight to the absolute maximum. Do not exceed the maximum when tyres are marked with a range.
What's the best cycle tyre pressure? Read our guide to tyre pressure  by Richard Hallett, the technical editor of Cycle magazine, to find out more. Join Cycling UK today  to receive 6 issues of Cycle magazine per year.
More technical cycling information can be found on our Cycling Advice  page.