Rule of Thumb Meaning | Definition, Origin & Examples
Rule of thumb is an idiom that describes a common-sense but approximate way of deciding on a course of action. Tradespeople will often know the span of their hands and the length of their thumbs and use them as a rough-and-ready way of measuring. So rule of thumb became a way of describing a rough guideline.
If you want to turn square feet into square meters, a good rule of thumb is to divide by ten.
When it comes to providing the customer with an estimate, you need to measure accurately, as rule of thumb measurements can be way off.
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How to use rule of thumb
You can use rule of thumb in circumstances where you mean “rough guide” or “general rule.” It implies an informal alternative to a strict set of rules or procedures.
In American English, rule of thumb is sometimes used on its own without an article, but it is usually preceded by:
- The definite article “the”
- The indefinite article “a”
- The indefinite article and an adjective
In British English, it would be very unusual to find it used on its own.
My sister-in-law and I could not be more different in the kitchen; she measures things out precisely, and I’m a rule of thumb kind of guy.
“One yard equals one meter” is fine as a rule of thumb, but that three-inch difference mounts up with long distances, especially if you’re trying to be precise.
The origin of rule of thumb
The idiom rule of thumb dates back at least 400 years to a time when it was common to use parts of the body to provide measurements. These include:
- Foot: a measurement based on, well, the length of the human foot
- Cubit: a measurement found in Egyptian hieroglyphs and equal to the length of the forearm
- Yard or meter: both these measurements equate roughly to the length of a person’s stride
- Span: the distance from thumb to little finger
- Hand: a measurement based on the span of a human hand and still used in the equestrian world (standardized to 4 inches)
A rule of thumb will give you a rough idea, or “guesstimate,” of a measurement or course of action.
