A rule of thumb is a simple, practical guide based on experience rather than exact calculation. It usually works, but there are exceptions. The expression is neutral, so you can use it both in conversation and in an essay.
When would you say a rule of thumb?
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as a rule of thumb, + clause; a good/useful rule of thumb is to + verb; a rule of thumb for + noun
- As a rule of thumb, I get to the airport three hours before an international flight.
- A useful rule of thumb for exams is to spend five minutes planning each essay.
- My grandmother's rule of thumb was simple: never go food shopping when you're hungry.
What changed
First versionAs a thumb rule, drink a glass of water with every cup of coffee.
RevisionAs a rule of thumb, drink a glass of water with every cup of coffee.
It is tempting to build the phrase like an ordinary compound noun and say thumb rule. But the fixed form is different: rule of thumb, and at the start of a sentence as a rule of thumb. Thumb rule sounds unusual in both British and American English.
Don't confuse it with as a rule, which means usually: As a rule, I walk to work. As a rule of thumb introduces rough advice: As a rule of thumb, save a tenth of your income. In strict academic writing, you can write a general guideline.
Your turn
Spending no more than a third of your salary on rent is only a rule of thumb, so treat it as a ___ guide.
Show the answer
rough — Only a rule of thumb stresses that this is an approximate guide, so rough guide fits. Strict would mean a firm rule with no exceptions, and the sentence says the opposite.
Share a practical rule you follow in your studies, at work or in everyday life. Start the sentence with as a rule of thumb and explain how the rule helps you.
As a rule of thumb, I learn ten new words a day, because a small daily goal stops me from giving up.
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