K? M? or MM? Business seems to vary on the use of K to equal $1000. I thought this was the standard convention, but I’ve seen businesses apply M = $1000 and thus MM = $1,000,000. Yet, is this alternate convention understood? I fear my audience may think my figures are grossly inflated.
Thus, what symbol is understood to mean millions (and won’t be misconstrued as billions)? - Jinx 
K, to indicate 1000s, has been around for a while and, I believe comes to us from the computer industry.
M, to indicate 1000s, derives from the Latin ‘mille’ meaning ‘thousand’.
MM, to indicate 1,000,000s, derives from Latin as above, and means ‘thousand thousands’ or ‘millions’.
I’ve never encountered any issues, in the business world, with using ‘MM’ to indicate millions. I’d think that would get you by.
Dear Dr. Chance,
Thanks for the reply. Yet, I must ask…if using the K convention, then M would stand for million, correct? And, doesn’t MM then mean billion?
…It’s all a grand illusion! 
Six zeros or “$1 million” or such tends to be pretty clear, I’ll just note.
MM, in that case, would be a million millions, which is significantly more than a billion.
I rather doubt that, since we in the computer industry insist that K means 1024, and get very annoyed when marketing people insist that it means 1000 when they sell us storage drives of various sorts. 
(Yes, we’re petty.)
K for 1000 (or properly a lower-case k,) derives from the ancient greek, and came into widespread use when Revolutionary France dreamed up the metric system of measurements, yes?
That’s actually a subject of debate, and the cause of a number of class-action lawsuits.
Whether k = 210 or 103 depends on the context. In communications engineering, k = 103 and M = 106, and the base unit is the bit, just like SI. I could argue that disk drives should use the same conventions, since at the lowest level, they are recording serial streams of bits.
See:
We brought it on ourselves when we started using the term “kilobyte” to mean 1024 of them, when “kilo” had a long established meaning of 1000 in other contexts. It’s useful to discuss things which are accessed using binary addresses in blocks of some power of two, but a slight amount of confusion would have been removed from the world if a new term had been coined. Unfortunately, we are now stuck with a context-sensitive term, which even people who use it to mean 1024 in computer contexts use informally to mean 1000. When you say “That car costs 60K”, you mean $60,000, or thereabouts, not $61,440.