What do size marks on bicycle tyre package mean, how to “read” them, which bicycle tyre size can fit a given rim? This post answers all those questions, providing an easy, uniform, standardized way to tell a tyre (tube and rim) size using ISO (ETRTO) standards. For detailed tube sizing gude, see: Bicycle tubes – types, valves and sizing explained. Effect of tyre size on speed and comfort is explained here: Are narrower tyres “faster” – rolling resistance.
- Expressing (noting) bicycle tyre dimensions
Table 1: standard bicycle tyre sizes
- Tyre widths
Table 2: which tyre width fits which rim width
- Frame fit room (maximum width)
At first, tyre dimensions were expressed through mounted (and inflated) tyre outer diameter. That is why today we still often see MTB tyres noted as 26″ tyres, or 28″ tyres for road bicycles. Rim diameter of a MTB is 559 mm. When a 2″ wide tyre is mounted on such rim, the outer diameter of such wheel with inflated tyre is around 660 mm (26″). To further complicate things, differrent standards were made in different countries. That led to situations that same sized tyres were differently noted, or that a same dimension note was given for tyres that differed in size. Does this sound complicated and confusing already? 🙂 Here’s a picture that takes 26″ “size” as an example:
That is why ISO standard notation was introduced and is in use today. Standard was introduced by ETRTO (The European Tyre and Rim Technical Organisation). Diameter of wheel tyre bead seat in millimetres preceded by tyre width in millimetres. Tyre width is also a bit non-exact measurement. It is width of an inflated tyre mounted on a rim of standard width for that tyre size. However, the most important information: wheel bead diameter is noted exactly in millimetres, so there can be no confusion. 559 wheel and tyre are always compatible (at least in terms of diameter, see about tyre and rim width compatibility below).
When bead seat diameter (BSD) is known, a tyre of appropriate dimensions in mm is required. It is as simple as that.
For example, MTB wheel diameter is 559 mm and it can take practically any tyre of that BSD, whether wider, or narrower, such as 47-559 (26×1.75″ by old nomination), 60-559 (2.35×26″ by old nomination) and so on. All the producers today mark tyres with ISO dimension standards, sometimes adding old nomination as well, but ISO is always noted and can be used as a safe(st) refference.
Standard road bicycle wheel size today is 622 mm and it usually takes tyres of 23-622, or wider 25-622. Today when off road bicycles with wheels larger than standard MTB wheels started to be popular, it’s wheels are marketed as 29″, but in fact are 622 wheels with wider tyres (so that outer diameter is close to 29″). Tyres for those bicycles are always marked with ISO standard (sometimes along with the “old” standard 29″) saying something like: 57-622, 60-622 and similar, depending on tyre width.
In case a tyre has no ISO marks, here is a cross reference:
|ISO Bead Seat Diameter||Traditional Designations|
|787 mm||36 inch|
|686 mm||32 inch|
|635 mm||28 x 1 1/2, 700 B|
|630 mm||27 x anything except “27 five”|
|622 mm||700 C, 28 x (two fractions), 29 inch, 28 x 1 1/2 F.13 Canada Road 28″ wheels and MTB 29″.|
|599 mm||26 x 1.25, x 1.375|
|597 mm||26 x 1 1/4, 26 x 1 3/8 (S-6)|
|590 mm||26 x 1 3/8 (E.A.3), 650 A|
|587 mm||700 D|
|584 mm||650B, 26 x 1 1/2, 27.5″ MTB 27,5″ wheels|
|571 mm||26 x 1, 26 x 1 3/4, 650 C|
|559 mm||26 x 1.00- x 2.125, also fatbike tyres up to 5 inches wide MTB 26″ wheels.|
|547 mm||24 x 1 1/4, 24 x 1 3/8 (S-5)|
|540 mm||24 x 1 1/8, 24 x 1 3/8 (E.5), 600 A|
|520 mm||24 x 1, 24 x 1 1/8|
|507 mm||24 x 1.5- x 2.125 – Big kids bikes with 24″ wheels.|
|490 mm||550 A|
|457 mm||22 x 1.75; x 2.125|
|451 mm||20 x 1 1/8; x 1 1/4; x 1 3/8|
|440 mm||500 A|
|419 mm||20 x 1 3/4|
|406 mm||20 x 1.5- x 2.125 – Kids small 20″ wheels, BMX.|
|390 mm||450 A|
|369 mm||17 x 1 1/4|
|355 mm||18 x 1.5- x 2.125|
|349 mm||16 x 1 3/8|
|340 mm||400 A|
|337 mm||16 x 1 3/8|
|317 mm||16 x 1 3/4|
|305 mm||16 x 1.75- x 2.125 – Kids 16″ wheels.|
|254 mm||14 x 2.00|
|203 mm||12 1/2 X anything. Small kids 12″ wheels.|
|152 mm||10 x 2|
|137 mm||8 x 1 1/4|
After explaining tyre dimensions, a few words on tyre widths. Generally, narrower tyres are good for fast road riding on pavement, while wider are better suited to rough terrain, or heavily loaded bicycles (heavy rider, carrying children or heavy baggage on a bicycle etc.). Depending on rim width, a range of tyre widths can be used. The important measure here is inner rim width. Outer rim width is irrelevant (for this purpose), just the inner rim diameter.
The following table tells the range of acceptable tyre widths depending on inner rim width:
Fat-bicycle tyre sizing chart:
If a tyre that is too narrow is mounted on a wide rim, the rim can easily be damaged on a bump. If too wide a tyre is mounted, there is a risk of the tyre coming off a rim (with a loud explosion of the tube).
If there’s limited space for fitting tyres in a frame, note that rim width also influences tyre width and height when mounted:
It should be also noted that one nominal tyre size (e.g. 622-28), in various models and manufacturers, is not of the same actual width when mounted and inflated. Some models are known to be of a smaller “real” size, some are known to be larger. If, for example, the old tyre has a nominal width of 23 mm, and one considers replacing it with a wider one of 25 mm (nominal) width, and there is only about two mm extra room, it should be confirmed what the actual tyre widths when mounted are. If the old 23 mm tyre is among the “narrower 23s”, and the considered 25 mm one is among the “wider 25s”, it is very likely that the wheel will not fit into the frame with the new tyre.
Pictures below are taken on a frame with lots of extra room for wider tyres (and mudguards). They are just a demonstration of where the “tight” spots usually are, and where attention should be paid (and measuring taken) before going for wider tyres. Depending on frame geometry and brake type, some other spots may be the tight ones, it should always be checked and measured at the tightest spot, with the least tyre clearance – and that is usually in (one of) these places: