Q. What is the convention for abbreviating thousands, millions, and billions in monetary amounts? I have seen K, M, and B, but I’ve also seen millions represented by MM and thousands represented by M. Thanks!

A. You’ve identified two commonly used conventions in finance, one derived from Greek and the other from Latin, but neither one is standard.

Starting with the second convention, M is used for amounts in the thousands and MM for amounts in the millions (usually without a space between the number and the abbreviation—e.g., $150M for $150,000 and $150MM for $150 million). This convention overlaps with the conventions for writing roman numerals, according to which a thousand is represented by M (from mille, the Latin word for “thousand”). Any similarity with roman numerals ends there, however, because MM in roman numerals means two thousand, not a thousand thousands, or one million, as in financial contexts (the year 2020 in roman numerals is MMXX). Likewise, MMM in roman numerals means three thousand, not a thousand times a thousand times a thousand, or one billion. (For an overview of roman numerals, see CMOS 9.65–67.)

According to the other convention, K is used instead of M for thousands (as in $150K for $150,000), because K stands for kilo, the Greek-derived term often used as a prefix to mean “thousand.” This meaning is standard in the sciences, where the kilogram (abbreviated “kg” and equal to 1,000 grams) is the base unit of mass (see CMOS 10.54). K is also used in computing to mean “kilobyte,” but mostly in commercial contexts as a shortening of KB (see CMOS 10.49). To further muddy the waters, K can also mean “kelvin,” which is the base unit of temperature.

Nonetheless, if K is used for thousands, then according to the same convention M (mega) would be used for millions, and billions would be represented by G (giga). If B is typically used instead of G, the reason is obvious: even if you don’t know anything, you might guess that B means “billion.” G, on the other hand, is a slangy shorthand for “grand,” as in a thousand dollars, which might disqualify it as an abbreviation for billions in financial contexts. (Nor is the meaning of “billion” itself an entirely settled matter; see CMOS 9.8.)

So we’re back to square one. Unlike the conventions in science, which are universal (assuming you adhere to the international system of units, or SI), the conventions in finance vary, not only by country but also among institutions, even within the same country. For that reason, in financial contexts it’s best to define up front the convention you are using—whether it’s M and MM and MMM or K and M and B (or G) or something else—to make sure your readers are on the same page. See also CMOS 9.24.