Merriam-Webster Logo
SINCE 1828
Menu
dictionary thesaurus

rule of three

Definition of rule of three 

: a rule in mathematics: the product of the means in a proportion equals the product of the extremes —used for finding the fourth term of a proportion where three are given

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

  • More than 250,000 words that aren't in our free dictionary
  • Expanded definitions, etymologies, and usage notes
  • Advanced search features
  • Ad free!

Learn More about rule of three

Share rule of three

Statistics for rule of three

Look-up Popularity

Comments on rule of three

What made you want to look up rule of three? Please tell us where you read or heard it (including the quote, if possible).

Show Comments  Hide Comments 

WORD OF THE DAY

perforce

by force of circumstances

Get Word of the Day daily email!

Test Your Vocabulary

Difficult Vocab Quiz

  • the-education-of-achilles-eugne-delacroix
  • Which is a synonym of discomfit?
Name That Thing

Test your visual vocabulary with our 10-question challenge!

TAKE THE QUIZ
Bee Cubed

Test Your Knowledge - and learn some interesting things along the way.

TAKE THE QUIZ

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

Words at Play

Ask the Editors

Word Games