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.
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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

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
References in periodicals archive ?
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.
Everyone's financial situation is different, requiring a more thorough assessment than a rule of thumb can deliver in order to ensure financial protection for loved ones.