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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. 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.
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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."
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."
Thumb as measurement device
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.
According to phrases.org.uk, "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."
Reference to spousal abuse
It is often claimed that the term rule of thumb is derived from a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife. English common law before the reign of Charles II permitted a man to give his wife "moderate correction", but no "rule of thumb" (whether called by this name or not) has ever been the law in England.
Belief in the existence of a "rule of thumb" to excuse spousal abuse can be traced as far back as 1782, the year that James Gillray published his satirical cartoon Judge Thumb. The cartoon lambastes English judge Sir Francis Buller for allegedly ruling that a man may legally beat his wife provided that he used a stick no thicker than his thumb, although there is no other written record of Buller making such a pronouncement.
In the United States, where the "rule of thumb" was mentioned in case law, it was usually to reject it as a legal standard. Legal decisions in Mississippi (1824) and North Carolina (1868 and 1874) make reference to—and reject—an unnamed "old doctrine" or "ancient law" by which a man was allowed to beat his wife with a stick no wider than his thumb. In 1976, women's rights activist Del Martin used the phrase "rule of thumb" as a metaphorical reference to describe such a doctrine. She was interpreted by many as claiming the doctrine as a direct origin of the phrase, and the connection gained currency in 1982, when the U.S. Commission on Civil Rights issued a report on wife abuse titled "Under the Rule of Thumb".