Rule of thumb

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A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. Compare this to heuristic, a similar concept used in mathematical discourse, psychology and computer science, particularly in algorithm design.


Origin of the phrase

The exact origin of the phrase is uncertain: either it is derived from the use of the thumb as a measurement device ("rule"), or it is derived from use of the thumb in a number of apocryphal "rules" (law, principle, regulation, or maxim). The earliest citation comes from Sir William Hope’s The Compleat Fencing-Master, second edition, 1692, page 157: "What he doth, he doth by rule of thumb, and not by art."[1][2] The phrase also exists in other languages, for example Swedish tumregel, Norwegian tommelfingerregel, sometimes in the variant "rule of fist", for example Finnish nyrkkisääntö, German Faustregel or Dutch vuistregel. This suggests that it has some antiquity, and does not originate in specifically English-language culture.

Thumb as measurement device

The term is thought to originate with wood workers who used the width of their thumbs (i.e. inches) rather than rulers for measuring things, cementing its modern use as an imprecise yet reliable and convenient standard.[3] This sense of thumb as a unit of measure also appears in Dutch, in which the word for thumb, duim, also means inch.[4] The use of a single word or cognate for "inch" and "thumb" is common in many other Indo-European languages, for example, French: pouce inch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb; Sanskrit: angulam inch, anguli finger; Slovak: palec, Slovene: palec inch/thumb.

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