ISO, the International Organization for Standardization has developed a universal tire sizing system that eliminates this confusion. (This system was formerly known as the "E.T.R.T.O." system, developed by the European Tyre and Rim Technical Organisation.)
The ISO system uses two numbers; the first is the width of the tire or rim in millimeters (The actual tire width will vary a bit depending on the width of the rim. The rim width is the inner width measured between the flanges as shown in the diagram.)
The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm ("B.S.D."). Generally, if this number matches, the tire involved will fit onto the rim; if it doesn’t match, the tire won’t fit.
For example, a 700 x 20 C road tire would be a 20-622; a 700 x 38 hybrid tire would be a 38-622. The width difference between these sizes would make them less-than ideal replacements for one another, but any rim that could fit one of them would work after a fashion with the other.
A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width.
If you flatten out a tire and measure the total width from bead to bead, it should be approximately 2.5 x the ISO width.
If your tire is too narrow for the rim there’s an increased risk of tire/rim damage from road hazards.
If its too wide for the rim, there’s an increase risk of sidewall wear from brake shoes, and a greater risk of loss of control in the event of a sudden flat.
The following is a partial listing of traditional tire sizes that are sometimes seen in the U.S., with their ISO bead seat equivalents.
|36 inch||787 mm||Unicycles, some novelty bicycles|
|32 inch||686 mm||Unicycles, some novelty bicycles|
|29 inch||622 mm||This is a marketing term for wide 622 mm ("700C") tires.|
|28 x 1 1/2||635 mm||English, Dutch, Chinese, Indian Rod-brake roadsters
(Also marked F10, F25, 700 B)
|622 mm||(F.13)Rare Canadian designation for the (F.13)|
|28 x 1 5/8 x
|Northern European designation for the 622 mm (700 C) size|
|27 x anything except "27 five"||630 mm||Older road bikes|
|26 x 1 (650 C)||571 mm||Triathlon, time trial, small road bikes|
|26 x 1 1/4||597 mm||Older British sport & club bikes|
|26 x 1 3/8
|26 x 1 3/8 (E.A.3)||590 mm||Most English 3-speeds, department-store or juvenile 10 speeds|
|26 x 1 1/2 (650B)||584 mm||French utility, tandem and loaded-touring bikes,
a very few Raleigh (U.S.) & Schwinn mountain bikes.
|26 x 1 3/4
|571 mm||Schwinn cruisers|
|26 x 1, 1 1/8||High performance wheels for smaller riders, common on Cannondale bicycles|
|24 x 1||520 mm||High performance wheels for smaller riders; Terry front|
|24 x 1 1/8||520 mm or
|24 x 1 1/4||547 mm||British or Schwinn Juvenile|
|24 x 1 3/8
|Schwinn Juvenile lightweights|
|24 x 1 3/8
|540 mm||British Juvenile, most wheelchairs; common on women’s utility bicycles in Japan.|
|20 x 1 1/8
20 x 1 1/4
20 x 1 3/8
|451 mm||Juvenile lightweights, BMX for light riders, some recumbents|
|20 x 1 3/4||419 mm||Schwinn juvenile|
|17 x 1 1/4||369 mm||Alex Moulton AM series|
|16 x 1 3/8||349 mm||Older Moulton; Brompton & other folders, recumbent front, juvenile|
|16 x 1 3/8||337 mm||Mystery tire|
|16 x 1 3/8||335 mm||Polish juvenile|
|16 x 1 3/4||317 mm||Schwinn Juvenile|
|12 1/2 x anything||203 mm||Juvenile, scooters|
|10 x 2||152 mm||Wheelchair caster|
|8 x 1 1/4||137 mm||Wheelchair caster|
Traditionally, fractional sizes are made for straight-sided rims.
High-performance sizes (520 mm, 571, 622 mm etc.) are preferably used with hook-edge rims, which can hold higher pressure and center the tires more reliably.
|29 inch||622 mm||This is a marketing term for wide 622 mm ("700c") tires.|
|28 x decimal||Some German tire companies use this non-standard designation for 622 mm ("700c") tires.|
|"27 five" (meaning 27.5)||584 mm||Marketing term for wide, knobby 584 mm tires. Some Mountain bikes|
|26 x 1.00 through 5.0||559 mm||Most Mountain bikes, cruisers, fatbikes etc.|
|26 x 1.25 (rare)||599 mm||Very old U.S. lightweights|
|26 x 1.375||Very old U.S. lightweights|
|24 x 1.5-24 x 2.125||507 mm||Juvenile mountain bikes, cruisers|
|22 x 1.75, 22 x 2.125||457 mm||Juvenile|
|20 x 1.5-20 x 2.125||406 mm||Most BMX, juvenile, folders, trailers, some recumbents|
|18 x 1.5||355 mm||Birdy folding bikes|
|18 x 1.75-18 x 2.125||Juvenile|
|16 x 1.75-16 x 2.125||305 mm||Juvenile, folders, trailers, some recumbents|
In the French system, the first number is the nominal diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual diameters as small as 660 mm.
|700 A||642 mm||Obsolete|
|700 B||635 mm||Rod-brake roadsters.|
|700 C||622 mm||Road bikes, hybrids, "29 inch" MTBs.
(28 x 1 1/2 F.13 Canada)
|700 D||587 mm||Oddball size formerly used on some GT models.|
|650 A||590 mm||French version of 26 x 1 3/8; Italian high-performance bikes for smaller riders|
|650 B||584 mm||French utility bikes, tandems, and loaded-touring bikes; some older Raleigh and Schwinn mountain bikes|
|650 C||571 mm||Triathlon, time trial, high performance road bikes for smaller riders|
|600 A||540 mm||European juvenile road bikes, most wheelchairs|
|550 A||490 mm||European juvenile road bikes|
|500 A||440 mm||European juvenile, folding|
|450 A||390 mm||European juvenile|
|400 A||340 mm||European juvenile|
ISO 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|
|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 five"|
|571 mm||26 x 1, 26 x 1 3/4, 650 C|
|559 mm||26 x 1.00- x 2.125, also fatbike tires up to 5 inches wide|
|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|
|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|
|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|
|203 mm||12 1/2 X anything.|
|152 mm||10 x 2|
|137 mm||8 x 1 1/4|
Most of this information was compiled by John Allen for Sutherland’s Handbook For Bicycle Mechanics, the bible of bicycle technology. Sutherland’s, 6th edition has a more detailed, more thorough version of this chart.
Got an unmarked rim but no tire? Click Here for how to measure Rim Size.
Although you can use practically any tire/rim combination that shares the same bead seat diameter, it is unwise to use widely disparate sizes.
If you use a very narrow tire on a wide rim, you risk pinch flats and rim damage from road hazards.
If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated…but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls.
As tofitting inner tubes, have a look at the section on dimensions in our inner-tube article.
Georg Boeger has kindly provided a chart showing recommended width combinations:
|Which tire fits safely on which rim?
[all dimensions in millimeters]
Note: This chart may err a bit on the side of caution. Many cyclists exceed the recommended widths with no problem.
From the WTB Website:
GMS Global Measuring System The current industry standard for specifying the actual inflated size of a bicycle tire does not account for subtle variation in tread and casing size. To address this problem and provide you with more information for comparing tires, WTB has introduced the Global Measuring System (GMS) for tire measurement.
The GMS uses a two-number system: the first number is the width of the casing, and the second number is the width of the tread, both in millimeters. These measurements are taken on a rim which is 20 mm wide at the bead-capturing point, with a tire inflated to 60psi and maintained for 24 hours.
In addition to being able to accurately size a tire, knowing the actual casing size and tread width provides an indication of air volume, tread characteristics and tread contact area; all of which provide you with a more concise idea of what ride characteristics to expect from each of WTB’s tires.
Tubular tires are mainly used for racing. A tubular tire has no beads; instead, the two edges of the carcass are sewn together (hence the term "sew-up") with the inner tube inside. Tubulars fit only on special rims, where they are held on by cement.
Unless special cement which does not allow on-road replacement of a tire is used, tubulars "squirm" against the rims and are slower than the best wired-on tires, even though lighter — see details from Jobst Brandt.
Tubulars existed in several different sizes, but only 700c and 26-inch tubulars are readily available these days. Beware: sizes of 26" and 24" tubulars are not well-standardized. Take the rim with you when buying a tire, and vice versa. Size variations of tubulars are covered in Sutherland’s Handbook for Bicycle Mechanics, 7th Edition, available from Sutherland’s, also to be found on the mechanic’s bookshelf at better bike shops.
- Full-sized tubulars fit rims of the same diameter as 622 mm (700c) clinchers. This size is sometimes referred to as "28 inch" or "700". It is also, confusingly, sometimes referred to as "27 inch." The "27 inch" designation is inaccurate and obsolete, but you’ll sometimes run into it in older printed material. In clincher tires, there is a real difference between "700c" and "27 inch" sizes, but for tubulars this is a false distinction. Whenever you see mention of "27 inch tubulars" the writer is actually referring to standard full-sized tubulars, as used on racing bikes.
- "26 inch" or "650" tubulars are smaller, mainly used on time-trial or motorpacing track bikes. There are two common "26 inch" sizes, corresponding to the 584 mm and 571 mm ISO sizes. There are a couple of oddball sizes as well.
- "24 inch" (two slightly different sizes), "22 inch" "20 inch" and "18 inch" tubulars are sizes formerly used for children’s racing bikes, but pretty much extinct these days. The "18 inch" size corresponds to the "17 inch" (369 mm) size used on Moulton bicycles.
Tubulars are also sometimes called "sew-ups" or "tubs" (British usage.)
If you want to sound like an ignorant yahoo, call them "tubies" or "tubeless tires."