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 28622), 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.
ISO Size  Imperial Size  Metric Size  American Size 

47203  12 x 1.75  
47305  16 x 1.75 x 2  
32340  400A  
37340  400 x 35A  
35349  16 x 1.35  
37349  16 x 1 3/8  16 x 1.50  
28355  18 x 1 1/8  
40355  18 x 1.5  
47355  18 x 1.75  
32369  17 x 1 1/4  
28406  20 x 1 1/8  
35406  20 x 1.35  
37406  20 x 1.5  
47406  20 x 1.75  
54406  20 x 2.0  
28440  500 x 28A  
32440  500A  
28451  20 x 1 1/8  
35451  20 x 1 3/8  
37489  22 x 1 3/8  
37501  22 x 1 3/8  
37507  24 x 1.5  
47507  24 x 1.75  
28520  24 x 1 1/8  
32540  24 x 1 3/8  
28541  600 x 28A  
37541  600 x 35A  
25559  26 x 1.0  
28559  26 x 1.2  
32559  26 x 1.25  
35559  26 x 1.35  
40559  26 x 1.5  
47559  26. x 1.75  
50559  26 x 2.0  
57559  26 x 2.125  
20571  26 x 3/4  
23571  26 x 1  
40571  650C  
28584  650 x 28B  
30584  27.5 x 1.2  
32584  650 x 32B  27.5 x 1.25  
37584  650 x 37B  27.5 x 1.35  
40584  26 x 1 1/2  650B  27.5 x 1.5 
42584  27.5 x 1.6  
50584  27.5 x 2.0  
57584  27.5 x 2.25  
60584  27.5 x 2.35  
65584  27.5 x 2.6  
70584  27.5 x 2.8  
28590  650 x 28A  
37590  26 x 1 3/8  650 x 37A  26 x 1 3/8 
42590  650 x 40A  26 x 1 5/8  
32597  26 x 1 1/4  
18622  700 x 18C  
23622  700 x 23C  
25622  700 x 25C  28 x 1.0  
28622  28 x 1 5/8 x 1 1/8  700 x 28C  28 x 1.1 
30622  700 x 30C  28 x 1.2  
32622  28 x 1 5/8 x 1 1/4  700 x 32C  
37622  28 x 1 5/8 x 1 3/8  700 x 35C  28 x 1.4 
40622  700 x 38C  28 x 1.5  
42622  700 x 42C  
47622  700 x 47C  28 x 1.75  
50622  29 x 2.0  
57622  29 x 2.25  
60622  29 x 2.35  
25630  27 x 1  27 x 1 1/8  
32630  27 x 1 1/4  27 x 1 3/8 

40635  28 x 1 1/2  700B  28 x 1 1/2 