Start at 3 mm — about an eighth of an inch — for a short beard, or 5 mm for a fuller one, then set the trimmer one step longer than the length you actually want.
Taking more off takes seconds; growing it back takes days.
The three lengths worth knowing, in the units American trimmers use and the ones their guards are stamped with. A three-day stubble is 1 to 2 mm (0.04 to 0.08 in), under a sixteenth of an inch. A short beard is 3 to 5 mm (0.12 to 0.20 in), an eighth to three-sixteenths. A full beard starts around 10 mm (0.39 in), three-eighths and up, which is past the longest guard on most beard trimmers.
Why one step longer. A beard grows about 0.3 to 0.5 mm (0.01 to 0.02 in) a day, so a setting taken off by mistake is two or three days of waiting — and that is per step. Going one longer than you think you want costs you one pass with the trimmer if you were wrong, and nothing if you were right.
This is the single most useful habit in the whole category and it applies to beards and haircuts equally.
The practical version: pick the setting you think you want, go one higher, do the whole beard, then look. If it is too long, drop one and repeat. Two passes at the right length beat one pass at the wrong one.
It matters most on a first use of an unfamiliar tool, because guard numbering is conventional and two trimmers marked “3” do not necessarily leave the same length.
Once you have found the setting that works, note it. It is the number you will use for the next two years, and it is not printed anywhere you will remember.
Last reviewed August 22, 2026