rule of thumb

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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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rule of thumb

a general principle developed through experiential rather than scientific means. As a rule of thumb, I move my houseplants outside in May. Going by a rule of thumb, we stop for gas every 200 miles when we are traveling.
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rule of thumb

A rough and useful principle or method, based on experience rather than precisely accurate measures. For example, His work with the youth group is largely by rule of thumb. This expression alludes to making rough estimates of measurements by using one's thumb. [Second half of 1600s]
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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.
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rule of thumb

a broadly accurate guide or principle, based on experience or practice rather than theory.
1998 New Scientist The best forecast of tomorrow's weather in any one place often comes not from a supercomputer, but from the rule of thumb that says: tomorrow it will be similar to today.
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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.
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rule of thumb

A rough measure or method, without precise mathematical or scientific basis. This term, which probably alluded to using one’s thumb as an approximate measuring device, has been around since the seventeenth century and made it into James Kelly’s collection of Scottish proverbs (1721): “No rule so good as rule of thumb, if it hit. But it seldome hits!” Some individuals have pointed to the “rule” proposed in 1782 by an English judge, Francis Buller, who proclaimed that men had the right to beat their wives provided that the stick used was no thicker than the husband’s thumb. Misinterpretation linked it to the cliché, which is about a century older and today is never used in this context.
See also: of, rule, thumb