According to the Astrophysical Journal, our universe is home to 200 trillion galaxies—that’s ten times more than the 200 billion galaxies previously believed to be in existence.1 I can’t envision 200 billion galaxies, let alone 200 trillion. However, I can envision writing large numbers according to the recommendations provided by our primary style guides. So, whether you’re writing about the number of galaxies in the universe, cells in the human brain, or hairs on your cat, let’s count down the details of how to write large numbers in the millions, billions, trillions, and even quintillions.
Are you wondering which style guide you should follow? Here is a quick summary of three of our primary style guides’ target audiences:
- The Chicago Manual of Style (Chicago style) is ideal if you want to adhere to traditional publishing styles or if you’re writing general business content.
- The Associated Press Stylebook (AP style) is best if you want to follow traditional journalism styles.
- The MLA Handbook (MLA style) from the Modern Language Association is primarily for undergraduate humanities students. MLA style, in general, is the least versatile guide and not one typically recommended for writers outside of academia.
Our fourth primary style guide, the Publication Manual of the American Psychological Association (APA style), isn’t included in this post because it doesn’t provide specific recommendations for writing big numbers. Visit “Which Style Guide Is Best for You?” for the pros and cons of all four guides.
The Chicago Manual of Style (Chicago style), recommends spelling out whole numbers through one hundred with words such as million, billion, and trillion.2
The human brain is made up of approximately one hundred billion cells.
Use digits with words such as million, billion, and trillion for non-whole numbers and numbers above one hundred.3
The 6.5 million residents of Indiana are known as Hoosiers.
The Associated Press Stylebook (AP style) prefers digits with words such as million, billion, and trillion.4
Domesticated cats have an average of 40 million hairs on their bodies.
Approximately 78 million dogs live in the United States.
The Modern Language Association’s MLA Handbook (MLA style), suggests spelling out large numbers that can be expressed in two words.5
The Smithsonian reports that about ten quintillion insects are alive on the planet right now.
Combine digits with words such as million, billion, and trillion for numbers that need to be expressed in more than two words.6
Over 323 million people live in the United States.
Has the time spent reading these numerical recommendations multiplied your interest in writing about numbers—particularly time-related numbers? If so, check out these related posts:
How to Write Decades as Words and Numerals
How to Write Centuries as Words and Numerals
When to Hyphenate Numbers with Units of Time
- “Hubble Reveals Observable Universe Contains 10 Times More Galaxies than Previously Thought,” NASA, last modified August 4, 2017.
- The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 9.8.
- The Chicago Manual of Style, 9.8.
- The Associated Press Stylebook 2020–2022 (New York: Associated Press, 2020), 196.
- MLA Handbook, 8th ed. (New York: Modern Language Association, 2016), 1.4.1.
- MLA Handbook, 1.4.1.