The idiom rule of thumb refers to a tried-and-true way of doing something based on experience and not theory. A “rule of thumb” is a guideline or helpful hint on how to do something.
“Rule of thumb” originated in the 1600s, as the width of a thumb was used as a rough measurement in many trades, particularly textiles. Because using your thumb was an easy way of measuring, the “rule of thumb” grew into a way of saying that’s how something is done.
How to use rule of thumb
Use “rule of thumb” when you are explaining how to do something or when you are recounting a time someone told you how to do something (e.g., “The rule of thumb says to only water it twice a month”).
This does not refer to a step-by-step process but rather an overarching rule that is helpful to know when completing a certain task, such as when to walk a dog or how much to save for retirement.
The phrase “rule of thumb” is usually, though not always, preceded by the definite article “the” or the indefinite article “a” or “a”/“an” + an adjective.
Rule of thumb origin
The first time “rule of thumb” appeared in print was in 1685, in a posthumous collection of sermons by preacher James Durham. He writes, “Many profest 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.”
A collection of Scottish proverbs from 1721 states, “No Rule so good as Rule of Thumb, if it hit.”
As the “rule of thumb” originally referred to people using the width of their thumb as a measuring tool, “rule” can refer to either a ruler/measuring stick or a guideline.
Frequently asked questions about rule of thumb
What is a synonym for rule of thumb?
Some synonyms and near synonyms for the idiom “rule of thumb” include:
- Golden rule
- Unwritten rule
Where does rule of thumb come from?
In the seventeenth century, many trades used the width of a thumb as a rough measurement, as it equaled about an inch across on average. As this was a quick rule for measurement, any loose guideline like this became known as a “rule of thumb.”