Tyre Sizes Explained

Over the years cycle tyre sizes seem to have become one of the most confusing aspects of cycling to a point where it can actually be really easy to buy the wrong tyres for your bike if you’re doing anything other than replacing old tyres with the exact same make / model.

Markings on the tyre aren’t always going to be of much help as sometimes tyres that look different can be the same, and some that appear to be the same are different. Much of this comes from the differences between the traditional Imperial and American systems of defining tyres sizes. If you look closely at the chart at the end of this article you will notice that 27 x 1 1/4 (imp) and 27 x 1 3/8 (U.S.) are actually the same size, conversely 26 x 1 1/2 (imp) and 26 x 1.5 (U.S.) are actually completely different! When you add in to the mix the Metric system it becomes even more confusing

To address this situation most makes of tyres are now marked with the International Standards Organisation (ISO) system of markings alongside any other legacy marking that may also be on the tyre.

The ISO system uses two sets of numbers separated with a dash (such as 28-622), the first number is the sectional width of the tyre and is approximately the same as the tyre width, although in practice it’s usually a little less and can vary according to the rim on which it’s fitted. The height of the tyre and it’s section are usually pretty close which helps to calculate tyre diameter and thus circumference.

The three numbers after the dash are the bead diameter for where the tyre fits on the rim, this is important in making sure the tyres fit on the wheel and is probably more important that the width in making sure that a tyre will fit correctly.  So to use the example above any tyre with a ‘622’ bead diameter will fit any wheel of that size in an emergency. (Note ‘622’ is about the most common size of wheel around at the moment, and the reason we’ve used it as the example n this article).

Wheel / Tyre compatibility

In essence any tyre with the same ISO designation will work on the same wheel, even if their alternate markings appear to be contradictory, and many wheel rims are now being marked with their ISO size marked on them which will make selecting the right tyre that much easier.

Tubeless tyres are now quite common and are always sized using the ISO designation, and in general follow the sizings in the table – for the best sealing effect we recommend using tyres and rims that are made for use together as there is no standard convention for fitment of tubeless tyres.

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