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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