On a wheel they are millimeters of cutting length. On comb attachments they are eighths of an inch — #1 is 1/8 inch, #4 is a half, #8 is one inch — and the millimeter figure beside them is the same series rounded.
Trimmers with a zoom or precision wheel state the length directly. The Remington MB5000 covers 0.4 to 18 mm (0.02 to 0.71 in) across seventeen positions; the Braun BT9545 covers 52 positions at 0.25 mm (0.01 in) each. There is no number to decode: the figure on the wheel is the length.
Comb attachments are where the numbering comes in, and it is not arbitrary. The clipper trade counts in eighths of an inch, which is why the numbers stop at eight. The Wahl set on this site publishes #1 to #8 at 3, 6, 10, 13, 16, 19, 22 and 25 mm (0.98 in) — and each of those is the eighth-inch figure rounded to the nearest millimeter, within half a millimeter throughout.
Two practical consequences. First, every step in that series is one eighth of an inch, so eight guards give you one size of step repeated eight times — comparing two sets on their step size tells you more than comparing them on how many combs are in the box. Second, a comb whose printed number has worn off is a comb you stop trusting, which is why the sets worth buying mark the size as well as the number.
Where the two systems disagree, the published millimeter figure is the specification. A set that follows the convention loosely still prints what it actually cuts, and that is the figure to compare. Which guard leaves what length sets out the full table.
Last reviewed October 10, 2026