rule of thumb

• rule-of-thumb (in attributive usage)

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.[1]

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rule of thumb (plural rules of thumb)

1. A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.

   The usual rule of thumb says that to calculate when an investment will double, divide 70 by the interest rate.

   • c. 1935, Ogden Nash, “Reflection on Ingenuity”, in Verses from 1929 On, published 1959:

     Here's a good rule of thumb:
     Too clever is dumb.

   • 2016, James Lambert, “Ornithonymy and Lexicographical Selection Criteria”, in International Journal of Lexicography, volume 30, number 1, pages 39–62:

     Since it is impossible to second-guess what a user will want to look up, there is a general lexicographical rule of thumb to err on the side of inclusion.

2. (attributive, usually hyphenated) Approximated, guesstimated.

   I made a quick, rule-of-thumb estimate of the manhours required for the job.

• See also Thesaurus:saying

• → German: Daumenregel (calque)