Where Does the Phrase “Rule of Thumb” Come From?

The phrase “rule of thumb” is variously used to mean a general principle, a widely held standard or guideline, or a simple common-sense method or procedure. But where did such a curious expression come from? 

The Possible, Yet Dubious, Legal Origins of “Rule of Thumb”

Popular history will have you believe that this phrase has a very curious (and somewhat distasteful) story attached to it. According to the tale, the original “rule of thumb” was an English law that supposedly permitted a husband to legally beat his wife, so long as the stick he chose to do so with was no broader or thicker than his thumb. 

Some versions of this story even attach it to a specific ruling, introduced by a specific justice: Sir Francis Buller, a notoriously no-nonsense judge in 18th-century England, who was known for his often harsh and overtly prejudiced judgments and his swaggering, blustering arrogance.

The popular theory here claims it was he who introduced this grim ruling in 1782—the same year that he appeared in a satirical cartoon in the English press, which labeled him “Judge Thumb.” 

As oft-repeated a story as this might be among popular etymologists and folk historians, however, there’s little evidence to back it up. Yes, it’s certainly true that Sir Francis Buller was a notorious English judge, and yes, he was indeed satirized as “Judge Thumb” in a notorious 1782 newspaper cartoon.

But no English law has ever made any kind of overt reference to a “rule of thumb” along the lines of Buller’s supposed ruling—and in the 19th century, the acclaimed English biographer and legal expert Edward Foss even went so far as to write that, following extensive research, he had found “no substantial evidence” that Sir Francis had ever even “expressed so ungallant an opinion.” 

Providing even further evidence to suggest that Sir Francis is not to blame here, the earliest record of a “rule of thumb” in print dates from around 1658, according to the Oxford English Dictionary—which is a full 88 years before the judge was even born. So if not a rule for beating one’s spouse, what really is a “rule of thumb”? Well, perhaps the actual answer here lies in the wording of the earliest known record of it.

The Likely Actual Origins of “Rule of Thumb”

In 1658, a speech given by the Scottish Christian preacher James Durham was put in print in which Durham had explained that “many profest [i.e. professed] Christians are like to foolish builders,” who “build by guess, and by rule of thumb (as we use to speak), and not by square and rule.” 

Durham’s words here seem to imply that the original “rule of thumb” was a way of making rough measurements or assessments of size—a bit like a painter holding their thumb up to their subject to make an approximation of its dimensions. 

As the Oxford English Dictionary itself explains, in fact, this phrase indeed seems to have been coined “on account of the thumb being used as a reference for approximate measurements of various kinds.” 

So the thumb in a “rule of thumb” is indeed the digit of the hand. But the idea that it’s somehow related to wife-beating or domestic chastisement—or to Sir Francis Buller, for that matter—seems to be nothing more than old etymological folklore. 

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