Million Abbreviation: M vs. MM
“M” and “MM” are both used for millions in different financial and business conventions, but Chicago explicitly notes that neither convention is universal. “MM” can represent a thousand thousands in one finance convention, while many modern contexts use “M” for million. Define the convention when ambiguity matters.
Do not treat M or MM as universally correct for “million.” Follow the document’s finance/house style and define the convention when readers could misread it.
Key details
Important caveats
Chicago Boundary
Chicago identifies more than one convention and says neither is standard.
Si Boundary
Do not confuse a house-style “M” for million currency amounts with the SI prefix mega in scientific measurement.
Further guidance
Clarity
In financial tables, define the abbreviation in a legend, note, or style guide when audience expectations are uncertain.
Practical Application
Do not assume that M and MM mean “million” in every context. Use the notation established by the document’s finance or house style, define it when readers could interpret it differently, and prefer the full word or full numeral when cross-domain ambiguity matters.
Reader Check
Has the document established what M or MM means, and could the symbol conflict with a technical convention used by the same audience?
Worked Example
A financial table can use “$12M” when its notation key defines M as million; a mixed technical report may be clearer with “$12 million” if M could be read as another symbol.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Abbreviations FAQ — thousands, millions, and billions (opens in a new tab)Chicago Manual of Style · Chicago Q&A explaining M/MM finance conventions and lack of universal standard
- Writing with SI (Metric System) Units (opens in a new tab)National Institute of Standards and Technology · NIST unit-symbol rules, including no plural s on symbols