rule of thumb
Pluralrules of thumb
- 1A general guideline based on experience or common knowledge, used as an approximate measure rather than a strict rule.
Carpenters often rely on a rule of thumb when estimating how much lumber a project needs.
As a rule of thumb, double the recipe if you expect more than eight guests.
Note
A measure turned into a habit of mind: the phrase goes back at least to 1658, and its origin is uncertain but probably bodily. The thumb served as a ready unit of measurement, and one theory traces the inch to the span from the base of the thumbnail to the first joint. In the plural, the rule multiplies and the thumb stays single: rules of thumb.
- 2Arrived at by rough approximation or estimation rather than precise calculation.
The engineer offered a rule-of-thumb figure before running the detailed calculations.
Using a rule-of-thumb approach, they sketched the budget in an afternoon.
Similarballpark
Note
The phrase put to work as a modifier, standing before another noun and usually joined by hyphens: a rule-of-thumb estimate, a quick rule-of-thumb figure. Written open, it is the guideline noun again, as in "as a rule of thumb." In this position the noun after it carries any plural, so the form is rule-of-thumb estimates.
Practice
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Origin
Attested since 1658, the phrase is of uncertain origin but probably comes from using the thumb as a reference for various measurements: for example, the theory that the inch began as the distance between the base of the thumbnail and the first joint, or the practice of wetting the thumb and raising it to judge the wind. The often-repeated claim that it referred to the thickness of a stick a man could use to beat his wife is false and may stem from Del Martin's 1976 book Battered Wives.