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.
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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.
ISO | Imperial | Metric | American | Tyre Diameter | Circumference | |
---|---|---|---|---|---|---|
sect–dia | (mm) | (in) | (m) | |||
40–635 | 28×1½ | 700B | 715 | 28.1 | 2.25 | |
25–630 | 27×1 | 27×11/8 | 680 | 26.8 | 2.14 | |
32–630 | 27×1¼ | 27×13/8 | 694 | 27.3 | 2.18 | |
18–622 | 700×18C | 700×20C | 658 | 25.9 | 2.07 | |
23–622 | 700×23C | 700×25C | 668 | 26.3 | 2.10 | |
25–622 | 700×25C | 700×28C | 672 | 26.5 | 2.11 | |
28–622 | 28×15/8×11/8 | 700×28C | 700×32C | 678 | 26.7 | 2.13 |
32–622 | 28×15/8×1¼ | 700×32C | 700×35C | 686 | 27.0 | 2.16 |
37–622 | 28×15/8×13/8 | 700×35C | 696 | 27.4 | 2.19 | |
47–622 | 700×47C | 716 | 28.2 | 2.25 | ||
32–597 | 26×1¼ | 661 | 26.0 | 2.08 | ||
28–590 | 650×28A | 646 | 25.4 | 2.03 | ||
37–590 | 26×13/8 | 650A | 664 | 26.1 | 2.09 | |
28–584 | 650×28B | 640 | 25.2 | 2.01 | ||
40–584 | 26×1½ | 650B | 664 | 26.1 | 2.09 | |
20–571 | 26×¾ | 611 | 24.0 | 1.92 | ||
23–571 | 26×1 | 617 | 24.3 | 1.94 | ||
40–571 | 650C | 651 | 25.6 | 2.05 | ||
25–559 | 26×1.0 | 609 | 24.0 | 1.91 | ||
32–559 | 26×1.25 | 623 | 24.5 | 1.96 | ||
40–559 | 26×1.5 | 639 | 25.2 | 2.01 | ||
47–559 | 26×1.75 | 653 | 25.7 | 2.05 | ||
50–559 | 26×1.9 | 659 | 25.9 | 2.07 | ||
57–559 | 26×2.125 | 673 | 26.5 | 2.11 | ||
28–541 | 600×28A | 597 | 23.5 | 1.88 | ||
37–541 | 600×35A | 615 | 24.2 | 1.93 | ||
32–540 | 24×13/8 | 604 | 23.8 | 1.90 | ||
28–520 | 24×11/8 | 576 | 22.6 | 1.81 | ||
37–507 | 24×1.5 | 581 | 22.9 | 1.83 | ||
47–507 | 24×1.75 | 601 | 23.7 | 1.89 | ||
37–501 | 22×13/8 | 575 | 22.6 | 1.81 | ||
37–489 | 22×13/8 | 563 | 22.2 | 1.77 | ||
28–451 | 20×11/8 | 507 | 20.0 | 1.59 | ||
35–451 | 20×13/8 | 521 | 20.5 | 1.64 | ||
28–440 | 500×28A | 496 | 19.5 | 1.56 | ||
32–440 | 500A | 504 | 19.8 | 1.58 | ||
28–406 | 20×11/8 | 462 | 18.2 | 1.45 | ||
37–406 | 20×1.5 | 480 | 18.9 | 1.51 | ||
47–406 | 20×1.75 | 500 | 19.7 | 1.57 | ||
54–406 | 20×2.0 | 514 | 20.2 | 1.61 | ||
32–369 | 17×1¼ | 433 | 17.0 | 1.36 | ||
28–355 | 18×11/8 | 411 | 16.2 | 1.29 | ||
40–355 | 18×1.5 | 435 | 17.1 | 1.37 | ||
47–355 | 18×1.75 | 449 | 17.7 | 1.41 | ||
37–349 | 16×13/8 | 423 | 16.7 | 1.33 | ||
32–340 | 400A | 404 | 15.9 | 1.27 | ||
37–340 | 400×35A | 414 | 16.3 | 1.30 | ||
47–305 | 16×1.75×2 | 399 | 15.7 | 1.25 | ||
47–203 | 12×1.75 | 297 | 11.7 | 0.93 |
Table from CTC.org.uk
Recommended pressures
All tyres are marked with a pressure. Sometimes this is an absolute maximum that nobody should need to exceed, sometimes only a recommended maximum that may be exceeded by heavier riders (e.g. tandemists).
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