Despite the thumb being the shortest digit, there’s a long list of thumb-based idioms found in the English language. Someone may have a “green thumb,” be “under your thumb,” or even “stick out like a sore thumb.” There’s also the “rule of thumb,” a phrase that some mistakenly attribute to an 18th-century English law dictating the bounds of spousal abuse. While it is true that many popular terms and slang phrases have uncomfortable origins, we’re happy to be able to refute this theory. In reality, the origins of this idiom date back roughly a hundred years earlier than the purported law.
Even before the “rule of thumb” phrasing came about, the thumb was used for a unit of measure as early as the 15th century. This unit would have been roughly equivalent to the inch we use today, and at that time, it was a convenient tool everyone had handy (bad pun intended). A hand was also a unit of measure, but “foot” seems to be the only measurement term from this time period we still use.
The idiom “rule of thumb” essentially refers to any method derived from personal experience rather than one based in precise science or calculation. The Oxford English Dictionary notes an early reference appears circa 1685 in Heaven Upon Earth, a book written by Scottish preacher James Durham: “Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb.”
As Durham implied, performing duties by rule of thumb may rely on guesswork, but it can also involve finely honed instinct or common sense. One example could be a carpenter who has been building houses for so long, they eyeball things instead of precisely measuring. Or perhaps a chef who doesn’t use a meat thermometer, and estimates when a steak is perfectly medium-rare by touch. Both of these situations forgo scientific tools that are readily available — instead, the person in question uses some sort of “rule of thumb” based on personal experience. The carpenter might base a cabinet installation on their knowledge of how far apart studs are usually placed, and the chef might judge the feel of a steak in the pan.
While these examples demonstrate where a rule of thumb might be used to replace more specific measurements, the phrase is also used for generally accepted knowledge. For example, “As a rule of thumb, plan to stop for gas every few hours; that will add time to your overall road trip.” This isn’t a specific measurement, but it’s giving advice or knowledge based on experience.
All told, consider “rule of thumb” to be a rough approximation more than anything. It may work out in the end, but we’d still recommend using scientific tools to confirm when something important is at stake.
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