a rule of thumb

rule of thumb

An approximation; a suggested method or guideline. A good rule of thumb is to plant your seedlings around the end of May.
See also: of, rule, thumb
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

a rule of thumb

COMMON A rule of thumb is a general rule about something which is right in most cases. As a rule of thumb, drink a glass of water or pure fruit juice every hour you are travelling. A good rule of thumb for any type of studio photography is to use no more light sources than are strictly necessary. Note: This expression probably dates back to the use of the first joint of the thumb as a unit of measurement.
See also: of, rule, thumb
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

a rule of ˈthumb

a quick, practical, but not exact, way of measuring or calculating something: As a rule of thumb you need a litre of paint to every 12 square metres of wall.This phrase may come from the fact that people often used their thumbs to estimate measurements.
See also: of, rule, thumb
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
In a pattern that emerges repeatedly from Inland's data, it's the biggest papers -- those over 100,000 circulation -- that are closest to a rule of thumb. But even those newsrooms are slightly "overstaffed," at 1.3 FTEs per thousand.
Again, as a rule of thumb, the best time to prune trees is during their dormant season.
As a rule of thumb, the best time to transplant any tree is during its dormant season, the winter months.
It's a rule of thumb that if it grows locally, it's going to be good for you and it's going to be appropriate because it will grow at the right season.
If a rule of thumb is utilized to develop a range of value for a small closely held business, one should be careful to understand what the rule of thumb proports to value.