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. 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.