rule of thumb

American  

noun

1. a general or approximate principle, procedure, or rule based on experience or practice, as opposed to a specific, scientific calculation or estimate.

2. a rough, practical method of procedure.

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rule of thumb British  

noun

1. 1. a rough and practical approach, based on experience, rather than a scientific or precise one based on theory

   2. ( as modifier )

      a rule-of-thumb decision

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rule of thumb Cultural  

1. A practical principle that comes from the wisdom of experience and is usually but not always valid: “When playing baseball, a good rule of thumb is to put your best hitter fourth in the batting order.”

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rule of thumb Idioms  

1. A rough and useful principle or method, based on experience rather than precisely accurate measures. For example, His work with the youth group is largely by rule of thumb. This expression alludes to making rough estimates of measurements by using one's thumb. [Second half of 1600s]

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Etymology

Origin of rule of thumb

First recorded in 1685–95

Example Sentences

A common rule of thumb says to buy builders at one-times book value and sell at two-times.

From Barron's • Jun. 1, 2026

My rule of thumb is to add one cup of cottage cheese for every cup of sauce.

From Salon • May 24, 2026

“The rule of thumb I use with clients is six months liquid, minimum, and that number goes up if you’re carrying financial responsibility for adult children or aging parents.”

From MarketWatch • May 14, 2026

A space rule of thumb holds that costs drop 90% when robots replace people.

From The Wall Street Journal • Apr. 24, 2026

“Over the years military planners have arrived at a rule of thumb which dictates that functional fighting units cannot be substantially larger than 200 men,” Dunbar writes.

From "The Tipping Point" by Malcolm Gladwell

Related Words

• criterion