European Tyre and Rim Technical Organisation


The European Tyre and Rim Technical Organisation exists to specify and harmonise sizes of rims and their associated pneumatic tyres across the European Union. ETRTO sizes apply to rims and tyres for vehicles of all types. It is probably most well-known for size standardisation of bicycle tires designations in millimeters, which replaces many older national methods for referring to tire sizes that are incompatible. The great advantage of ETRTO sizing is that it is unambiguous; previously, nominal dimensions were used which were interpreted in different ways by different countries and manufacturers - a problem for the end user.
ETRTO works in liaison with the International Organization for Standardization to develop relevant standards.
WWW etrto.org

Motivation

Previously, one could for example risk buying a 28 inch tire for a 28 inch rim, with the result that the tire would not fit since 28" can refer to rim bead diameters of either 635 mm or 622 mm. Thus, a tire for one type of 28" inch rim will not fit the other, and vice versa. Another example is the rim sizes 406 mm and 451 mm, which both traditionally have been referred to as 20 inch.
ETRTO solves these problems by precisely referring to rim bead diameters in millimeters.

Bicycle tire sizes

The following is a list of different types of bicycle tires listed by bead diameter according to ETRTO:
Rim bead diameterTraditional names
1378 x 1 1/4
15210 x 2
20312 1/2 x "anything"
25212 x 1 5/8 French
25414 x 1.75
288350a
29814 x 1 3/8, Moulton Mini
30516 x 1.75- x 2.125
31716 x 1 3/4
33516x 1 3/8 Polish children's bicycles
33716 x 1 3/8
340400a
34916 x 1 3/8
35518 x 1.5- x 2.125
36917 x 1 1/4
38720 x 2.5 Trials & Unicycle
390450a
40018 x 1 3/8,
18 x 1
40620 x 1.5- x 2.125
41920 x 1 3/4 Schwinn Juven
42820 x 2 Swedish
43820 x 1 3/8 NL, Dutch children's bicycles
440500a
45120 x 1 1/8;
20 x 1 1/4;
20 x 1 3/8;
OS20 x 1.6, 1.75, 1.85
45722 x 1.75;
22 x 2.1,
22 x 2.125,
22 x 2.3,
22 x 2.4
484550b
48922 x 1 1/8 NL, Dutch children's bicycles
22 x 1 3/8 NL
490550a
50122 x 1 3/8, British
22 x 1.00
50724 x 1.5,
24 x 2.125
52024 x 1,
24 x 1 1/8,
24 x 1 3/8,
24 x 1 3/4
52124 x 1 3/4 S7
53424 x 1.5 Dutch
54024 x 1 1/8,
24 x 1 3/8,
541600 A
54724 x 1 1/4,
24 x 1 3/8
55926", traditional size for mountain bikes, but also for fat bikes up to 127 mm tire width
57126 x 1,
26 x 1 3/4,
650c
583700d, special size made by GT Bicycles
584650b,
26 x 1 1/2,
27.5"
59026 x 1 3/8,
650a
59726 x 1 1/4,
26 x 1 3/8
59926 x 1.25,
26 x 1.375, old American size
60927 x 1 1/2, rare Danish size
622700c,
28 x 1 5/8, 28 x 1 1/2 and others,,
29 inch,
28 x 1 1/2 F.13 Canada
63027 x "anything"
642700a
68632 inch
78736 inch