Bicycle Tire Size Table

Because the two numbers measure different eras with different tires. The French 700 referred to the approximate outer diameter in millimeters when tires were narrow tubes of around 20 to 25 millimeters; the bead underneath that marking settled at 622 mm. The English 27 x 1 1/4 was sized for a heavier tire on a 630 mm bead. When wide tires arrived, makers kept the 622 mm rim and simply gained the outer diameter in rubber, so a modern 700 x 35C rides taller than an old 700 x 20 - while the 27-inch rim stayed a specialist size. The result is the counterintuitive table row: 622 (700C) is 8 millimeters smaller at the bead than 630 (27 inch), and the two do not interchange despite the inches saying otherwise. It is the same lesson as everywhere else on this chart: the nickname is history, the bead number is physics.

The 650 family shares an outer-diameter nickname but splits into three different beads, which is why the letter suffix matters: 650A rides a 590 mm bead (the old 26 x 1 3/8 English size, now mostly wheelchairs and utility bikes), 650B rides a 584 mm bead, and 650C rides a 571 mm bead favored by some triathlon and track wheels. The 584 mm bead is the one the mountain-bike industry rebranded as 27.5 - literally the same circle, marketed fresh. So if your tire says 650B or 27.5 x 2.2, you are shopping for a 584 bead; if it says 650A you need 590, and they will not swap despite sharing the 650. The bead column in the chart is the referee: same bead number, same fit, whatever the letter on the box.

Usually yes within limits, and the limit is the rim width. The bead number guarantees the tire seats; the width is a compatibility window, not a free choice. Too narrow a tire on a wide rim folds the sidewalls and bruises the tubes; too wide a tire on a narrow rim floats the tread and wanders in corners, and at pressure can unseat. The rim-width match table gives the pairing bands: a 25 mm road tire wants a rim roughly 13 to 22 mm across the inside, a 47 mm gravel tire wants 17 to 30 mm. Two more honest checks beyond the table: frame clearance (a 40 mm tire may fit the rim but rub the fork crown or brake bridge) and brake type - rim brakes fix the outer diameter, disc brakes do not, which is why disc-frame bikes accept far wider rubber on the same bead.

There are two standard 20-inch beads because the nickname came from outer diameter: 406 mm is the fat-bead BMX and folding size (tires from about 1.5 to 2.5 inches wide), while 451 mm is the skinny high-pressure size (20 x 1 1/8, once marked 500A) used on racing folders and some recumbents. The beads differ by 45 millimeters - completely incompatible, and both are sold as 20 inch, which is why a folding-bike owner ordering a spare online keeps meeting the wrong one. The same nickname-splitting repeats all over the chart: 16 inch means both 305 (BMX, kids) and 349 (Brompton-type folders, marked 16 x 1 3/8), and 24 inch means 507 (mountain), 547 (English juvenile) and 540 (wheelchair and 24-inch road). Match the ISO code on the sidewall, or the old marking through the chart, and the wrong-size problem disappears.