Why Does ‘K’ Mean ‘Thousand’?

If someone asked how many steps you averaged each day this week, you could boast that you broke 10K. Or perhaps you have $100,000 sitting in your savings account, in which case you would tell a financial adviser that you have 100K available to invest in the stock market. Whatever the context may be, “K” has become synonymous slang for a thousand, even though the letter isn’t found anywhere in the word “thousand.” So why do we use “K” instead of “T”? It comes from a Greek prefix you’re likely already familiar with.

In ancient Greece, the word χίλιοι (pronounced “chilioi”) meant “1,000.” This word later became written in the Latin alphabet as kilo. Today, “kilo-” primarily serves as a prefix used in metric measurements — for example, one kilometer is equal to 1,000 meters. Or perhaps you’re more familiar with it in the context of computer storage, as a kilobyte (abbreviated “KB”) is equal to 1,000 bytes of memory.

After countries began adopting the metric system in the late 18th and early 19th centuries, academics and scientists alike began developing shorthand for written measurements. “Kilometer” was written as “km,” “kilogram” as “kg,” “kiloliter” as “kl” etc. These shorthand notations were soon adopted by other professions, such as accountants. They could write “100K” instead of “100,000” over and over, thus saving them time and also space on the paper.

By the mid-20th century, “K” had become common slang for 1,000 in a variety of contexts. Today, if someone earns an annual salary of $75,000, people might say they earn 75K. This shorthand is also prevalent in the world of social media. If a person has 15,500 followers on Instagram or X, you might see the total on their profile as “15.5K” instead of the full number.

The U.S., of course, uses imperial measurements without prefixes, but you don’t need to rely on the metric system to understand what the letter “K” means. It’s a simple shorthand of the word “kilo,” and it became popular due to repeated use over the last several centuries.