Rule of thumb

Etymology

The phrase, attested from the 17th century, likely refers to using the thumb as a rough measuring tool — the first joint is about an inch. A popular claim that it derives from a law allowing a man to beat his wife with a stick no thicker than his thumb is a persistent myth with no basis in actual law.

noun phrase · /rool uhv thum/
A rough, practical guideline based on experience rather than precise measurement or theory.
What it names
The good-enough shortcut that works in most cases without demanding exact calculation.
Example
As a rule of thumb, save three to six months of expenses for emergencies.
Related idea
A rule of thumb is a practical, informal shortcut; a heuristic is the same idea stated more formally in psychology, and an axiom is its opposite — a precise, foundational principle taken as certainly true.