Why Do We Say “Rule of Thumb”? (And No, It’s Not About Violence)

You’ve probably heard someone say, “As a rule of thumb…” — maybe in a recipe, a meeting, or while giving advice.
But have you ever stopped to ask: what’s your thumb got to do with it?

Let’s break it down.

What Does “Rule of Thumb” Mean?

Today, “rule of thumb” simply means a general guideline — something that usually works but isn’t exact or scientific.

It’s the kind of advice based on experience, not a formula.

Examples:

• “As a rule of thumb, restart your computer before calling tech support.”
• “A good rule of thumb is to leave a 20% tip at restaurants in the U.S.”

It’s not a law. It’s a shortcut.

Where Did It Come From?

The phrase dates back to the 1600s, when tradespeople didn’t always have precise measuring tools.
Instead, they used their own hands — and their thumbs — as a quick guide.

A thumb’s width is roughly one inch, so it became a handy reference point. Over time, “rule of thumb” came to describe any rough-and-ready method based on practical experience rather than theory.

So when someone says “rule of thumb,” they’re echoing that old habit of using the human body as a built-in measuring tool.

The Myth: A Law About Beating Wives?

There’s a long-standing myth that the phrase came from an old law allowing men to hit their wives with a stick no thicker than their thumb.

Let’s be clear: that’s false.
No such law has ever been found in English or American legal history.

The story seems to have grown later — partly from satirical reports about a British judge in the 1700s and later from social commentary in the 20th century.
It caught on because it illustrated a real historical issue — gender inequality — but it isn’t the source of the phrase itself.

In short: the phrase does not come from any law about violence. It’s a myth that stuck around because it sounded shocking.

Why We Still Use It

Because it works.
Humans love shortcuts — simple principles that make life easier.
Even in an age of calculators, charts, and AI, a “rule of thumb” still means something that’s practical, proven, and easy to remember.

In Easy Words

People once used their thumbs to measure things when they didn’t have rulers.
That habit turned into a saying — “rule of thumb” — meaning a rough rule that usually works.
And no, it has absolutely nothing to do with beating anyone.