Rule of thumb
Meaning
A rough practical principle or method based on experience rather than scientific calculation.
Origin
The phrase "rule of thumb" is widely believed to have originated from a time when the human thumb served as a common, albeit imprecise, measuring tool. Before standardized units, artisans and craftsmen would gauge dimensions, such as the width of a brick or the length of fabric, by referring to the approximate width of a man's thumb—roughly an inch. This quick, practical method provided a useful estimate for everyday tasks, establishing a 'rule' directly from the 'thumb.' The inherent imprecision of such a measurement gave the phrase its lasting meaning: a practical principle based on experience, not exact science, but good enough for general guidance.
Examples
- As a rule of thumb, always arrive at the airport two hours before your international flight.
- My grandmother always told me, as a rule of thumb, to never put cold milk directly into hot tea, or it would curdle.