Rule-of-thumb

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. It is based not on theory but on practical experience. Compare this to heuristic, a similar concept used in mathematical discourse, psychology, and computer science, particularly in algorithm design.

The exact origin of the phrase is uncertain. The earliest known citation comes from J. Durham’s Heaven upon Earth, 1685, ii. 217: "Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb." The phrase also exists in other languages, for example Italian "Regola del pollice", Swedish tumregel, Norwegian and Danish tommelfingerregel, sometimes in the variant "rule of fist", for example Finnish nyrkkisääntö, Estonian rusikareegel, German Faustregel and Pi mal Daumen, Hungarian ökölszabály or Dutch vuistregel, as well as in Turkish parmak hesabı, and in Hebrew "כלל אצבע" (rule of finger) and in Persian "قاعده سرانگشتی," which is translated as finger tip's rule. This suggests that it has some antiquity, and does not originate in specifically Germanic language culture.

Sir William Hope wrote in his The Compleat Fencing Master, 1692: "What he doth, he doth by rule of Thumb, and not by Art." James Kelly's The Complete Collection of Scottish Proverbs, 1721, pg 257, includes: "No Rule so good as Rule of Thumb, if it hit."

The term is thought to originate with carpenters who used the length of the tip of their thumbs (i.e., inches) rather than rulers for measuring things, cementing its modern use as an imprecise yet reliable and convenient standard. This sense of thumb as a unit of measure also appears in Dutch, in which the word for thumb, duim, also means inch.

Another possible origin of the phrase comes from measurement, in particular in agricultural fields. The plants need a fairly precise depth to seed properly, whether planted from seed or being replanted, but the depth can sometimes be estimated using the thumb. That is, a "rule (measurement) of thumb". According to Gary Martin, "The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things—judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down."

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