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August 16th, 2008

Tyre Sizes from CTC.org.uk

SteveintheUK in Articles, Products, Tech Info, Tyres/Inner Tubes

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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