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Tyre Sizing Systems
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by Sheldon "ISO/E.T.R.T.O." Brown
Tyre Sizing Charts:
Inch Based Systems: Metric Based Sizing Systems:
Decimal Fractional French ISO/E.T.R.T.O.
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This article is also available in American English!

For more general tyre information, see my tyres article.

Which Size Tyre Fits Which Size Rim?

Bicycle tyres 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. The same size tyire would be known by different numbers in different countries. Even worse, different-sized yres that were not interchangeable with one another were often marked with the same numbers!

This page covers sizes in common use as of its writing, and a number of older sizes. Sutherland's Handbook for Bicycle Mechanics, 3rd through 6th edition, covers dozens of additional, antique sizes. The 6th edition is available on CD ROM from Sutherland's. Better bike shops have a copy.

Which Size Tyre Fits Which Bicycle?

To determine which tyre size will fit, perform measurements on the bicycle's fork and frame.

If the bicycle has rim brakes, the rim must line up with the brake shoes and so only one rim size or a small range of sizes will work. So, first measure the distance from where the center of the hub axle would be in a dropout to the center of a brake shoe. Then look up the rim radius in the ISO table on this page. A different rim size may be possible with different brakes, though longer brake reach generally results in poorer brake performance.

The front tyre must not reach the fork crown; the rear tyre must not reach the seatstay bridge or chainstay bridge. Take measurements from the axle position to the fork crown, chainstay bridge and seatstay bridge. Tyre outside radius is nominally the rim radius plus the tyre width, and as much as 1 cm greater for a tyre with a deep tread. A tyre must have typically 1 cm clearance, 2 cm if a fender will be installed, but do not use a wheel that is much smaller, or a pedal is too likely to strike the ground in cornering. On a bicycle with horizontal dropouts, additional clearance to the chainstay bridge is desirable so the wheel can be removed without deflating the tyre.

The space between the fork blades or stays must be wide enough to clear the tyre, with a few mm extra on each side in case the wheel goes slightly out of true. Measure at the radius of the widest part of the tyre, usually the rim radius plus half the tyre width --except that the widest part may be at the tread of an off-road tyre. Nominal tyre width is marked on the tyre; actual width can be measured if the tyre is installed on a rim.

If the bicycle has hub brakes (drum, disk, coaster), different rim sizes are possible as long as the tyre is fits the frame. A larger rim goes with a skinnier tyre, and vice versa..

Traditional Sizing Systems

The traditional sizing systems are based on a measurement of the outside diameter of a tyre. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.).

Unfortunately, evolution of tyres 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 tyre" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tyre is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tyres, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tyres, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tyres 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 tyres as narrow as 25 mm to fit these rims, so you wind up with a "26 inch" tyre that is more like 24 7/8" in actual diameter!

A second number or letter code would indicate the width of the tyre. (26 x 1.75, 27 x 1 1/4...650B, 700C...)

Does Point Seven Five Equal Three Quarters?

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 tyres, which are NOT interchangeable. It is dangerous to generalize when talking about tyre sizing, but I would confidently state the following:

Brown's Law Of Tyre Sizing:

If two tyres are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tyres will not be interchangeable.

Dishonesty in Sizing

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 tyre; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tyre Company and the Coke Tyre Company had tyres of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tyre 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.

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B.S.D.

rim diagram The ISO (E.T.R.T.O.) System:

ISO, the International Organization for Standardization, has developed a universal tyre 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 width in millimeters. For the rim, this is the inner width between the flanges, as shown in the diagram; for the tyre, it is the inflated width. This will vary a bit depending on the width of the rim.

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 tyre involved will fit onto the rim; if it doesn't match, the tyre won't fit.

For example, a 700 x 20 C road tyre would be a 20-622; a 700 x 38 hybrid tyre 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 tyre width should be between 1.45/2.0 x the inner rim width.

If you flatten out a tyre and measure the total width from bead to bead, it should be approximately 2.5 x the ISO width.

If your tyre is too narrow for the rim there's an increased risk of tyre/rim damage from road hazards.

If its too wide for the rim, there's an increase risk of sidewall wear, and a greater risk of loss of control in the event of a sudden flat.

The tables below give a partial listing of traditional tyre sizes, with their ISO bead-seat equivalents. The ISO comparison list at the bottom of this page covers all sizes which we know to be in production as of 2016. The fractional, decimal and French lists cover common sizes.

Fractional sizes:

Fractional ISO Applications
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") tyres.
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
28 x 1 5/8 x
1 1/2
Northern European designation for the 622 mm (700 C) size
635 mm Old Swedish designation
27 x anything except "27 five" and 609 mm Danish 630 mm Older road bikes.
27 x 1 1/2 609 mm Rare Danish size
26 x 1 (650 C)
571 mm Triathlon, time trial, small road bikes. Old Schwinn S-4
26 x 1 1/4 597 mm Older British sport & club bikes
26 x 1 3/8
(S-6)
Schwinn "lightweights"
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
(S-7)
571 mm Schwinn "middleweight" 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
540 mm!
Caveat emptor. 540mm is common on wheelchairs.
24 x 1 1/4 547 mm British or Schwinn Juvenile
24 x 1 3/8
(S-5)
Schwinn Juvenile lightweights

24 x 1 3/4
(S7)

520 mm Schwinn "Middleweights"
24 x 1 3/8
(E-5)
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, some folding bicycles
20 x 1 3/4 419 mm Schwinn juvenile
18 x 1 3/8 400 mm British 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 tyre
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

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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.

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Decimal sizes:

Decimal ISO Applications
29 inch 622 mm This is a marketing term for wide 622 mm ("700C") tyres.
28 x decimal Some German tyre companies use this non-standard designation for 622 mm ("700C") tyres -- violates Brown's law!
"27 five" (meaning 27.5) 584 mm Marketing term for wide, knobby 584 mm tyres. Some Mountain bikes
26 x 1.00 through 5.0 559 mm Most Mountain bikes, cruisers, fatbikes etc. Old Schwinn designation was S-2
26 x 1.25 (rare) 599 mm Very old U.S. lightweights
26 x 1.375 (rare) 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
14 x 1.75-14 x 2.125 254 mm Juvenile
12 1/2 x anything 203 mm Juvenile, scooters

French sizes:

In the French system, the first number is the nominal outside 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 tyre width, since narrow tyres are often made for rim sizes that originally took wide tyres; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual outside diameters as small as 660 mm.

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French Size ISO Applications
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)
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. Also called 27 five. See We have a page about this size.
700 D 583 mm Oddball size formerly used on some GT models. 650B tyre (584 mm) is close enough, maybe with wide rim tape.
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

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ISO Cross Reference:

Bead Seat Diameter, mm Bead Seat Radius, mm Traditional Designations
787 393.5 36 inch
686 343 32 inch
635 317.5 28 x 1 1/2, 700 B, 28 x 1 5/8 x 1 1/2 (old, Sweden)
630 315 27 x anything except "27  five" and 609 mm
622 311 700 C, 28 x 1 5/8 x 1 1/2 and other pairs of numbers, (but also see 635), 29 inch, 28 x 1 1/2 F.13 Canada
609 304.5 Rare Danish size, 27 x 1 1/2
599 299.5 26 x 1.25, x 1.375 -- old US size
597 298.5 26 x 1 1/4, 26 x 1 3/8 (S-6)
590 295 26 x 1 3/8 (E.A.3), 650 A
584 292 650B, 26 x 1 1/2, "27 five"
583 291.5 700 D -- oddball size made by GT
571 285.5 26 x 1, 26 x 1 3/4, 650 C
559 279.5 26 x 1.00- x 2.125, also fatbike tyres up to 5 inches wide
547 273.5 24 x 1 1/4, 24 x 1 3/8 (S-5)
541 270.5 600 A
540 270 24 x 1 1/8, 24 x 1 3/8 (E.5),
520 260 24 x 1, 24 x 1 1/8, 24 x 1 3/4
507 253.5 24 x 1.5- x 2.125
501 250.5 British, 22 x 1 3/8, 22 x 1.00
490 245 550 A
489 244.5 Dutch juvenile 22 x 1 1/8 NL, 22 x 1 3/8 NL
484 242 550 B
457 228.5 22 x 1.75; x 2.125
451 225.5 20 x 1 1/8; x 1 1/4; x 1 3/8
440 220 500 A
438 219 Dutch juvenile, 20 x 1 3/8 NL
428 214 Swedish, 20 x 2
419 209.5 20 x 1 3/4
406 203 20 x 1.5- x 2.125
390 195 450 A
369 184.5 17 x 1 1/4
355 177.5 18 x 1.5- x 2.125
349 174.5 16 x 1 3/8
340 170 400 A
337 168.5 16 x 1 3/8
317 158.5 16 x 1 3/4
305 152.5 16 x 1.75- x 2.125
288 144 350 A
254 127 14 x 1.75
203 101.5 12 1/2 X anything.
152 76 10 x 2
137 68.5 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.

Got an unmarked rim but no tyre? Click Here for how to measure Rim Size.

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Width Considerations

Although you can use practically any tyre/rim combination that shares the same bead seat diameter, it is unwise to use widely disparate sizes.

If you use a very narrow tyre on a wide rim, you risk pinch flats and rim damage from road hazards.

If you use a very wide tyre 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 tyres, unless the tyres are overinflated...but that defeats the purpose of wide tyres, and puts undue stress on the rim sidewalls.

Georg Boeger has kindly provided a chart showing recommended width combinations:

Which tyre fits safely on which rim?
[all dimensions in millimeters]
Tyre width
Rim width
(interior)
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
Note: This chart may err a bit on the side of caution. Many cyclists exceed the recommended widths with no problem.

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Wilderness Trail Bikes' Global Measuring System

From the WTB Website:

imageGMS Global Measuring System The current industry standard for specifying the actual inflated size of a bicycle tyre does not account for subtle variation in tread and casing size. To address this problem and provide you with more information for comparing tyres, WTB has introduced the Global Measuring System (GMS) for tyre 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 tyre inflated to 60psi and maintained for 24 hours.

In addition to being able to accurately size a tyre, 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 tyres.

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Tubular Tyres ("Sew-ups")

Tubular tyres are mainly used for racing. A tubular tyre 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.

Tubulars existed in 6 different sizes, but only two of them are readily available these days.

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 tyres."

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