|Tire Sizing Charts:|
|Inch Based Systems:||Metric Based Sizing Systems:|
Traditional Sizing Systems | Width Compatibility | WTB's Global Measuring System
Bicycle tires come in a bewildering variety of sizes. To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers!
The traditional sizing systems are based on a measurement of the outside diameter of a tire. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.).
Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter!
A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)
Note that the inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tire sizing, but I would confidently state the following:
Brown's Law Of Tire Sizing:Note that even some tire manufacturers get confused about this. In particular, some Continental models are mis-marked, using a fractional designation where they should be using a decimal.
If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable.
Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. This would put them at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.
This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements.
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 Organization.)
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 measured between the flanges.)
The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm. 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.
The following is a partial listing of traditional tire sizes that are sometimes seen in the U.S., with their ISO bead seat equivalents.
|28 x 1 1/2||635 mm||Rod-brake roadsters|
|27 x anything||630 mm||Older road bikes|
|26 x 1 (650C)||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 (S-6)||597 mm||Schwinn "lightweights"|
|26 x 1 3/8 (E.A.3)||590 mm||Most 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 (S-4) mountain bikes.
|26 x 1 (650C)||571 mm||Triathlon, time trial, small road bikes|
|26 x 1 1/2 (F.12)||Older Canadian bikes|
|26 x 1 3/4 (S-7)|
|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 (S-5)||547 mm||Schwinn Juvenile lightweights|
|24 x 1 3/8 (E-5)||540 mm||British Juvenile, most wheelchairs|
|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|
|16 x 1 3/8||349 mm||Older Moulton, 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|
|8 x 1 1/4||137 mm||Wheelchair|
Traditionally, fractional sizes are made for straight-sided rims.
High-performance sizes (571 mm
/26 x 1 & 630 mm /27") have evolved toward hook-edged rims.
Decimal ISO Applications 26 x 1.00 through 2.3 559 mm Most Mountain bikes, cruisers, etc. except: 26 x 1.25 (rare) 599 mm Very old U.S. lightweights 26 x 1.375 599 mm 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 2.125 355 mm 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.
French Size ISO Applications 700 C 622 mm Road bikes, hybrids. 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 635 mm 28 x 1 1/2 630 mm 27 x anything 622 mm 700 C 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 1/2 571 mm 26 x 1, 26 x 1 1/2, 26 x 1 3/4, 650 C 559 mm 26 x 1.00- x 2.125 547 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 has a more detailed, more thorough version of this chart.
Width ConsiderationsAlthough 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.
Georg Boeger has kindly provided a chart showing recommended width combinations:
Which tire fits safely on which rim?
[all dimensions in millimeters]
Tire width Rim width
18 20 23 25 28 32 35 37 40 44 47 50 54 57 13 X X X X 15 X X X X 17 X X X X X 19 X X X X X X 21 X X X X X X 23 X X X X 25 X X X X X
Wilderness Trail Bikes' Global Measuring System
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 20mm 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.
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