rule of thumb

Rule of thumb refers to a broadly accurate guide or principle based on practice and experience rather than theory. It implies a method of procedure or analysis that is based on common sense and approximation rather than precise scientific calculation. The phrase dates back to the 17th century and most likely originates from the varied use of body parts for measurement in trade and agriculture—specifically, the width of a thumb being roughly one inch.

The term is contested due to a persistent folk etymology suggesting it derives from an old English common law allowing a man to beat his wife, provided the stick used was no thicker than his thumb. While legal historians and etymologists have widely debunked this specific origin—noting that no such law existed and the phrase was used in other contexts prior to this association—the connection to domestic violence remains a point of sensitivity. Some speakers avoid the term to ensure they do not trivialize abuse, while others argue that its innocuous origins in carpentry and trade make it acceptable for continued use.

Example:
"As a general rule of thumb, you should save at least twenty percent of your income."

Example:
"Gardeners often use the rule of thumb that you should plant bulbs three times as deep as their height."