A rule of thumb is a rough practical guide that works most of the time. As a rule of thumb, a useful rule of thumb, the old rule of thumb.
It is approximate by design, which is the whole value of it, since a rule of thumb can be used in your head. The origin is probably the thumb used as a rough measure, and the widely repeated legal story is false.
1phraserule of thumbA rough practical guide that works most of the time.
“As a rule of thumb, one cup of dry rice needs two cups of water.”
rule of thumb is a guide.
Attested from 1658, probably from using the thumb as a rough measure; the widely repeated legal explanation is unfounded.
Attested since 1658. Of uncertain origin, but probably based on the use of the thumb as reference for various measurements. For instance, one theory notes that the inch originated as the distance between the base of the thumbnail and the first joint, another notes the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air. A third theory notes that English royal banquet plate setters used the distance of their thumbs to equally space each plate from the table edge. The erroneous claim that the term referred to the maximum thickness of a stick with which it was permissible for a man to beat his wife has appeared in The Washington Post and Time; it may originate from Del Martin's 1976 book Battered Wives.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms