A Rule of Thumb Can Save You Tons of Time

You probably use at least rule of thumb per day without even noticing. BartCo / Getty Images

Let's talk about an expression you've probably heard before: "rule of thumb." This phrase is a handy way of saying, "Here's a general guideline that works in most cases."

A rule of thumb isn't about precise measurements or hard-and-fast laws; instead, it's a practical, go-to method for figuring things out quickly. The phrase shows up all the time when people need a rough estimate or a basic principle to follow. From cooking to science to everyday life, these "thumb rules" help us simplify complex subjects.

But what does it really mean, and where did this phrase come from?

'Rule of Thumb' Meaning

A "rule of thumb" is a general principle or guideline that's intended to offer practical, often quick advice rather than precise instructions. It's a way of saying, "Here's a good rule to follow most of the time."

This heuristic doesn't necessarily have an exact science behind it; it's more about what has worked well in practice. In other words, it's a useful rule for most situations but not a strict rule or law.

The rule of thumb gives a general sense of direction, especially when exact measurements or detailed procedures aren't necessary. For example, in cooking, a good rule of thumb is to add salt to taste. There's no strict amount that applies to every dish, but the guideline helps you use your best judgment.

Origins of the Phrase

While the exact origins of "rule of thumb" are a bit of a mystery, the phrase likely originates from the old practice of using the thumb as a rough measuring tool, especially before rulers or standardized measurements were common.

Craftspeople and laborers would use the width of their thumbs as a quick way to measure distances or approximate sizes. This "thumb rule" made practical sense because a thumb was always "on hand," ready to help out.

There's a persistent myth that the phrase comes from an outdated legal "rule" allowing people to use their thumb to measure a rod's thickness for physical discipline. However, there's no historical evidence to support this, and it's widely regarded as an incorrect theory.

Instead, the phrase likely developed from the idea of using a rough-and-ready approach, with no exact measurements needed — just common sense and practical wisdom.

3 Examples of 'Rule of Thumb' in Everyday Conversation

The phrase "rule of thumb" appears in many situations where people need a quick guideline or estimate. Here are some common examples.

Cooking: "A good rule of thumb for cooking pasta is to use one quart of water for every four ounces of pasta."
Science or engineering: "A rule of thumb for electric power is that every 750 watts roughly equals one horsepower." In this case, the conversion isn't exact, but it offers an easy and "close enough" way to estimate power needs.
Everyday budgeting: "A rule of thumb for saving money is to set aside 20 percent of your income for savings and investments." This guideline helps people practice financial responsibility without needing to follow strict financial procedures.

In each example, the rule of thumb provides useful guidance without requiring precise definitions or a great deal of advance knowledge about a subject. So next time you hear someone mention a "rule of thumb," remember that it's all about a general principle that works well for most purposes, even if it's not intended as a formal rule or law.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.