Sheldon Brown’s Bicycle Handlebar and Stem Dimension

Handlebar dimensions

Stem Clamp

Size

Grip Area

Size

Application

22.2 mm 7/8″ 22.2 mm 7/8″ Steel bars. Mainly BMX, older Mountain bike bars.
23.8 mm 15/16″ 22.2 mm 7/8″ Obsolete British size for steel handlebars, common on older 3-speeds.
This size was also used for older British steel drop bars.
25 mm   23.5 mm   Obsolete French size.
25.4 mm 1″ 22.2 mm 7/8″ Standard I.S.O. size, used on the vast majority of newer bicycles with upright handlebars.
This size was formerly common for steel drop bars.
25.4 mm 1″ 23.8 mm 15/16″ Standard I.S.O. size, used on most bicycles with drop handlebars.
Also used on older British aluminium upright handlebars.
25.8 mm   23.8 mm 15/16″ Unofficial in-between size used by some Italian handlebar makers for handlebars designed to be usable in either ISO (25.4) or Italian (26.0) size stems.
26.0 mm   23.8 mm 15/16″ Italian standard for drop bars, other bars made to fit Italian stems and some high-end aftermarket drop bars.
This is sometimes incorrectly called “road” size.
26.4 mm   23.8 mm 15/16″ Older Cinelli and Cinelli copies. Cinelli changed over to 26.0 mm in 1998.
27 mm   23.8 mm 15/16″ Titan (obsolete).
31.8 mm 1 1/4″ 23.8 mm 15/16″ Road oversized.

 This table from Sheldon Browns Crib Sheet