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What is a Rule of Thumb?

Tricia Christensen
Updated May 23, 2024
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A rule of thumb is defined as a general guidepost for determining behavior, or a rule that allows a person to make a quick mathematical calculation or remember a formula. It exists to recall something else, whether general or scientific in nature. In standard usage one might write: A rule of thumb in fashion is to never wear white after Labor Day. One might also substitute “in general” for the phrase.

The proposed origins of the phrase rule of thumb are quite creative and sometimes a bit shocking. One theory, posited by Sharon Fenick in a 1996 newsgroup article, suggests that it applied to the thickness of the stick one could use to beat one’s wife, servant or child. The evidence for this theory has been disputed, but it is true that English men could beat their wives legally, though the practice was declining as noted by William Blackstone in his 1765 work Commentaries on the Laws of England.

Others suggest that the expression may have derived from typical measurements made in the absence of rulers. For example, the foot inspired the English and American foot, and a thumb might have been used as a form of measure. In Jonathan Swift’s 1726 novel, Gulliver’s Travels, Swift records a rule of thumb for tailors “twice around the thumb is once around the wrist.” Thus the rule may have originated from a simple form of measurement for tailors when determining the width of shirt cuffs. However, in practice, this measurement is not always accurate.

Some have also suggested that the rule of thumb was a way for brewers to test whether a batch of beer was sufficiently warm enough. This too is a little unlikely. The thumb is far less sensitive to heat than a wrist or an elbow and wouldn’t be a very adequate measure of temperature. The thumb may be used to measure the direction of the wind, which might be a possible source. One puts one’s thumb in the mouth, holds it up in the air, and the side that dries fastest gives one wind direction.

Most likely however, the phrase is based on some measuring application that has been lost to antiquity. As a rule of thumb, it is unusual to find new sources that will definitively solve the question as to the origins of the phrase, but it is certainly not unusual to find many new theories as to its original use.

Language & Humanities is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a Language & Humanities contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By surfNturf — On Feb 01, 2011

Latte31 -I had never thought of that. I know that I use rule of thumb measurement when telling my daughter how much conditioner to use.

She uses way too much and it leaves her hair very limp. I tell her as a rule of thumb to use a dime size amount.

This way she could visualize the appropriate amount that she is supposed to use without actually having to measure it.

It is just like when a person is required to use a pinch of salt in a recipe. We don’t have an exact measurement for the salt but this general phrase allows us to assume that it is about a half of a teaspoon.

By latte31 — On Jan 29, 2011

BambooForest - I know what you mean because we have different expressions for different things even though we speak the same language.

I wanted to add that as a general rule of thumb I use a lot of different things in order to determine the portion size of many of my meals.

As a general rule of thumb I consider one portion of pasta to be the size of the palm of my hand. It seems like a small portion but in actuality the portion sizes in restaurants are generally three times that size.

Using this rule of measurement helps to give you a general guideline of the ideal portion size of your meal.

Generalities like this are helpful because you don’t always have measurement tools around to measure the size of the foods, but you will always have your hand that you can use as a guideline.

By BambooForest — On Jan 29, 2011

While a lot of cultures have something similar to the rule of thumb saying, many Europeans will be totally confused the first time they hear about something that is done "by rule of thumb".

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a Language & Humanities contributor,...
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