rule of thumb

WordReference Random House Learner's Dictionary of American English © 2025

ˈrule of ˈthumb, n. [countable]

1. a general rule based on experience, and not taken from scientific reasoning:
   As a rule of thumb I add a spoonful of salt.

WordReference Random House Unabridged Dictionary of American English © 2025

rule′ of thumb′, 

1. a general or approximate principle, procedure, or rule based on experience or practice, as opposed to a specific, scientific calculation or estimate.
2. a rough, practical method of procedure.
• 1685–95

Collins Concise English Dictionary © HarperCollins Publishers::

rule of thumb noun

1. a rough and practical approach, based on experience, rather than a scientific or precise one based on theory
2. (as modifier): a rule-of-thumb decision