What does 'rule of thumb' mean?

What it means: A practical general guideline based on experience, not a precise rule. Something that works well enough in most situations without being formally calculated.

"As a rule of thumb, allow one hour per presentation slide."
"The rule of thumb is to save three months of expenses."

Where it comes from: Various theories — carpenters measuring with their thumb, brewers testing temperature — but none are definitively proven. It's been in English since at least the 17th century.

What it implies: The guideline is approximate and practical, not guaranteed. "Rule of thumb" acknowledges that the real world is more complex but this works well enough for most cases.

• "Generally speaking" — introducing a similar approximation
• "As a guideline" — formal version
• "Roughly" — simpler
• "A good ballpark is..." — similar approximate estimate

Register: Neutral. Works in professional and casual contexts.

Tags: idioms, guidelines, practical English, approximation