Bicycle tyre sizing and dimension standards

Bicycle tyre sizing and dimension standards

Updated: 16/11/2018.

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.

 

1. Expressing (noting) bicycle tyre dimensions

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:

Bigger wheel with a narrower tyre has the same outer diameter as a smaller wheel with a wider tyre.
Bigger wheel with a narrower tyre has the same outer diameter as a smaller wheel with a wider tyre.

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.

When bead seat diameter (BSD) is known, a tyre of appropriate dimensions in mm is required. It is as simple as that.

Bead seat diameter and inner rim width - all that matters.
Bead seat diameter and inner rim width – all that matters.

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.

Tyre with dimensions noted in ISO, English and French standard.
Tyre with dimensions noted in ISO, English and French standard.

In case a tyre has no ISO marks, here is a cross reference:

ISO Bead Seat DiameterTraditional Designations
787 mm36 inch
686 mm32 inch
635 mm28 x 1 1/2, 700 B
630 mm27 x anything except “27 five”
622 mm700 C, 28 x (two fractions), 29 inch, 28 x 1 1/2 F.13 Canada   Road 28″ wheels and MTB 29″.
599 mm26 x 1.25, x 1.375
597 mm26 x 1 1/4, 26 x 1 3/8 (S-6)
590 mm26 x 1 3/8 (E.A.3), 650 A
587 mm700 D
584 mm650B, 26 x 1 1/2, 27.5″  MTB 27,5″ wheels
571 mm26 x 1, 26 x 1 3/4, 650 C
559 mm26 x 1.00- x 2.125, also fatbike tyres up to 5 inches wide MTB 26″ wheels.
547 mm24 x 1 1/4, 24 x 1 3/8 (S-5)
540 mm24 x 1 1/8, 24 x 1 3/8 (E.5), 600 A
520 mm24 x 1, 24 x 1 1/8
507 mm24 x 1.5- x 2.125 – Big kids bikes with 24″ wheels.
490 mm550 A
457 mm22 x 1.75; x 2.125
451 mm20 x 1 1/8; x 1 1/4; x 1 3/8
440 mm500 A
419 mm20 x 1 3/4
406 mm20 x 1.5- x 2.125  – Kids small 20″ wheels.
390 mm450 A
369 mm17 x 1 1/4
355 mm18 x 1.5- x 2.125
349 mm16 x 1 3/8
340 mm400 A
337 mm16 x 1 3/8
317 mm16 x 1 3/4
305 mm16 x 1.75- x 2.125  – Kids 16″ wheels.
203 mm12 1/2 X anything. Small kids 12″ wheels.
152 mm10 x 2
137 mm8 x 1 1/4

 

2. Tyre widths

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.

Rim cross section, with inner and outer diameter. This one has 19 mm inner rim diameter.
Rim cross section, with inner and outer diameter. This one has 19 mm inner rim diameter.

The following table tells the range of acceptable tyre widths depending on inner rim width:

Tire section width (mm)
Rim width
18
20
23
25
28
32
35
37
40
44
47
50
54
57
60
62
13
x
x
x
x
15
x
x
x
x
17
x
x
x
x
x
x
x
x
x
19
x
x
x
x
x
x
x
x
x
x
21
x
x
x
x
x
x
x
x
x
x
23
x
x
x
x
x
x
x
x
x
25
x
x
x
x
x
x
x
27
x
x
x
x
x
x
29
x
x
x
x

If a tyre that is too narrow is mounted on a wide rim, rim can easily be damaged on a bump. If too wide a tyre is mounted, there is a risk of tyre coming off a rim (with a loud explosion of the tube).

 

3. Frame fit room (maximum width)

If there’s limited space for fitting tyres in a frame, note that rim width also influences tyre width and height when mounted:

Difference of outer dimensions when mounted on various width rims.
Difference of outer dimensions when mounted on various width rims.

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, 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:

Related post – To what pressure should I inflate my bicycle tyres:

To what pressure should I inflate my bicycle tyres?
To what pressure should I inflate my bicycle tyres?

3 thoughts on “Bicycle tyre sizing and dimension standards”

  1. I have a road bike and hybrid bike with front tire sizes 700×23 and 700×40. I want to get one wheel that can fit on both bikes (the wheel has an electric motor). I believe the road bike rim inner width is 15mm. Is it possible to fit a wheel with 17mm inner width on the road bike? Can the road brakes be adjusted to fit wider rim? Can i get brakes that will fit wider rim?

    • Short answer: yes, no problems.

      Brakes on a rim with a 2 mm more outer width need to go out by just 1 mm at each side.

      There is a tendency for road bike rims (and tyres) to go wider. For both comfort and aerodynamics (wider tyres being more aero on wider rims, than narrower tyres).

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