Rule of thumb
A rule of thumb is an idiomatic expression referring to a practical principle or method providing a rough, approximate guide based on empirical experience rather than exact theory or scientific precision.[1][2]The phrase emerged in the 17th century, deriving from the longstanding custom in trades such as textiles, brewing, and carpentry of using the average width of an adult thumb—approximately one inch—as a convenient unit for quick estimations when precise tools were unavailable.[1][2][3]
A widely circulated but baseless folk etymology attributes the term to an purported English common law allowance for men to discipline wives using sticks no thicker than a thumb, a claim traced to satirical 18th-century ridicule of judge Sir Francis Buller that lacks any evidentiary support in legal records and was fabricated as a modern misconception in the 1970s amid second-wave feminist rhetoric.[3][4][5]
In contemporary usage, rules of thumb serve as heuristics across disciplines including engineering, aviation, economics, and daily life, offering efficient approximations like estimating fuel consumption or cooking proportions, though they inherently risk inaccuracy without validation against precise data.[1][2]