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    To assess bicycle tire sizes, the International Organization Bicycle Tire Sizefor Standardization (ETRTO) has come up with a measurement system. It consists of the tire width (in millimeters) and the diameter of the rim’s bead seat (mm). The following are the ISO measurements and how they compare with other units of measurement.

    ISO and Traditional Size Equivalents (Bead Seat Diameter)

    The measurement of 28 x 1 1/2 in is equal to 635 mm, while 700 C is equal to 622 mm in the ISO system. 26 x 1.25, x 1.375 is equal to 599 mm; 26 x 1 1/4, 26 x 1 3/8 (S-6) is the same as 597 mm, while 26 x 1 3/8 (E.A.3), 650 A is equal to 590 mm.

    700 D is the same as 587 mm. The bicycle tire sizes 650B, 26 x 1 ½ are the same as 584 mm and measurements 26 x 1, 26 x 1 1/2, 26 x 1 3/4, 650 C are equal to 571 mm.

    26 x 1.00- x 2.125 is equal to 559 mm. 24 x 1, 24 x 1 1/8 in are equal to 520 mm and the 20 x 1 1/8; x 1 1/4; x 1 3/8 are similar to 451 mm. 17 x 1 ¼ is 369 mm in the ISO system and 16 x 1 3/8, 337 mm. 16 x 1 ¾ are equal to 317 mm. 16 x 1.75- x 2.125 is equivalent to 305 mm; 10 x 2 is the same as 152 mm and 8 x 1 ¼ is equal to 137 mm.

    ISO and Decimal Bicycle Tire Sizes

    Measurements from 26 x 1.00 through 2.3 are equal to 559 mm (used in a lot of mountain bikes). 26 x 1.375 is equal to 599 mm. This is used in some old bikes. For juvenile bikes, the decimal measurement is 24 x 1.5-24 x 2.125 which have an ISO equivalent of 507 mm.
     
    20 x 1.5-20 x 2.125 (or 406 mm) is used for BMX and some other bikes. 18 x 1.5 is equal to 355 mm (measurement used by Birdy folding bikes). Other bikes use 18 x 1.75-18 x 2.125 decimal measurements. This is equal to 355 mm. 16 x 1.75-16 x 2.125 is the same as 305 mm in the ISO system.

    ISO and Fractional Bicycle Tire Sizes

    28 x 1 ½ is equal to 635 mm (employed by road brake roadsters). Small road bikes use 26 x 1 (650C) tires; that’s 571 mm in the ISO system. Some British bikes use 26 x 1 ¼ tires, which is equal to 597 mm.

    Other common fractional measurements are 26 x 1 3/8 (S-6) (597 mm), 26 x 1 3/8 (E.A.3) (590 mm), 26 x 1 1/2 (650B) (584 mm), 24 x 1 (520 mm) and 24 x 1 ¼ (547 mm). 20 x 1 ¾ is equal to 419 mm, while 16 x 1 3/8 is the same as 349 mm.

    The bicycle tire sizes can become more complex than this. Some companies may use other measurements, so it’s best to check the status of your bike before getting new tires.

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