We use “K” for thousand because it comes from “kilo,” a Greek-derived prefix that literally means one thousand. The Greek word khilioi, meaning “thousand,” was shortened to “kilo” and formally adopted as a metric prefix in France in 1795. From there it spread into science, computing, finance, and everyday language, giving us the shorthand we now use in everything from job listings ($50K) to fitness apps (10K runs).
The Greek Root Behind “Kilo”
The word traces back to khilioi, the ancient Greek word for “thousand,” which itself descends from a Proto-Indo-European root (gheslo-) shared by words for “thousand” in Sanskrit and Avestan. When the French government officially adopted the metric system in 1795, it needed prefixes to denote multiples of base units. “Kilo” was chosen for the thousand multiplier, and it was paired with base units like the gram and the meter to create the kilogram (1,000 grams) and the kilometer (1,000 meters).
The prefix was slightly irregular, a shortened form of the original Greek rather than a direct transliteration, but it stuck. By 1960, when the International System of Units (SI) was formally adopted at a global conference, “kilo” and its symbol “k” were locked in as the universal standard prefix for 10³.
Why Not “M” for Thousand?
This is where things get confusing. The Roman numeral for one thousand is M, from the Latin word mille. For decades, especially in banking and finance, M was the standard abbreviation for thousands. A figure written as $150M meant $150,000, and $150MM meant $150 million (literally “a thousand thousands”). Some older financial professionals trained in the mid-1980s and earlier learned this as the default convention.
The problem is obvious: if M means thousand in one context and million in another, readers can easily misinterpret a number by a factor of a thousand. The K convention avoids this entirely. When K stands for thousands, M can slide up to mean million (from “mega,” the SI prefix for one million), and the whole system stays internally consistent: K for thousands, M for millions, B (or G, from “giga”) for billions.
Financial conventions still vary by country and even by institution. The Chicago Manual of Style recommends defining whichever system you’re using up front so readers aren’t left guessing whether $5M means five thousand or five million.
How “K” Became Everyday Shorthand
The jump from scientific prefix to casual shorthand happened gradually. The metric system made “kilo” globally recognizable, and as the prefix appeared on food packaging (kg), road signs (km), and school textbooks, the association between K and “a thousand” became second nature for most people. Job postings, social media, and texting cemented it further. Writing “$75K” is faster and universally understood in a way that “$75M” (in the old financial sense) never was outside of banking circles.
Lowercase “k” vs. Uppercase “K”
In the official SI system, the symbol for kilo is a lowercase “k.” This is actually an exception to the general rule: all SI prefixes for large multipliers (mega, giga, tera, and above) use uppercase letters, while prefixes for small fractions use lowercase. Kilo, along with hecto (h) and deka (da), breaks the pattern by staying lowercase despite representing a multiplier.
In casual usage, though, most people write an uppercase K when abbreviating thousands in money or other contexts ($50K, 10K followers). This informal convention is so widespread that it rarely causes confusion, but technically the SI-correct form is a lowercase k.
The Special Case in Computing
Computing added its own twist. Because computer memory is organized in powers of two, and 2¹⁰ (1,024) is very close to 1,000 (less than 2.5% difference), early engineers borrowed “kilo” as a convenient approximation. A “kilobyte” originally meant 1,024 bytes, not exactly 1,000. This double meaning persisted for decades.
In 1998, the International Electrotechnical Commission tried to clean up the ambiguity by creating new prefixes: “kibi” (symbol KiB) for 1,024 bytes, reserving “kilo” (kB) strictly for 1,000 bytes. In practice, both conventions still coexist. You’ll often see KB with an uppercase K used for the binary meaning (1,024 bytes) and kB with a lowercase k for the strict decimal meaning (1,000 bytes), though many manufacturers and operating systems aren’t consistent about this.
Why “K” Won Out
The reason K dominates today comes down to clarity and global reach. The metric system is used in virtually every country, which means “kilo = thousand” is one of the most widely understood numerical conventions on the planet. Latin-based alternatives like M for mille are limited to specific industries and carry genuine ambiguity. K doesn’t. When someone writes 5K, there’s no second interpretation. That simplicity is why a Greek word from antiquity ended up as the universal shorthand on your paycheck, your running app, and your social media follower count.