M is the roman numeral for 1,000. So MM is meant as 1,000 x 1,000, which equals 1,000,000 (one million). OP should have used capital letters, as mm does indeed denote millimeters. Better yet, just use the more common M symbol to mean "million."

Pedantic: MM means 2 tousand if we are really to stick to roman numerals.

Another pedantic remark is that people use $1K instead of $1M to mean $1000.

I think it's fine to use whatever it takes to get the message across.

Which stands for kilo and is correct.

Lowercase k stands for kilo; K stands for Kelvin (as long as we're being pedantic).

It is uppercase in the comment above.

And a letter can stand for multiple things, from temperatures to chemical elements to whatever.

As long as we're being pedantic, that is.

That's why I always think that $1MM = $1M * 1000000

There isn't a way to write one million in Roman numerals on a standard keyboard (it would be M with a bar over it, or "M" 1,000 times). Instead the shortcut "thousand thousand" (MM) gets used.

People from the finance side use MM to mean million, which as you note is really 1,000 X 1,000.

The other side you see it is in CPM which is cost per m (being thousand).

So people with this background tend to adopt the MM = million because that's how its done.

here we go again :) http://www.hnsearch.com/search#request/all&q=mm++m&s...

You've probably gotten all your answers based on the other comments. MM or mm should (if at all) only be used within finance. There is enough confusion on units already, that this can't possibly contribute to the better.

A few arguments as to why this is such a lousy notation:

* m and M already exist in the International System of Units, which is used by pretty much everybody else (http://en.wikipedia.org/wiki/File:Non-Metric_User.svg). 'm' is either milli (10^-3) or meters, depending on the context. And M is Mega, or the number in question: one million.

* If the reason is to use roman numerals. M, for mille, means 1000. However, MM would mean two thousand, the same way XXII is 22.

* "Million" isn't ambiguous, unlike "billion", which either means 10^9 or 10^12.

i was taught to do that back in the early 80s by a couple VCs i worked for at that time. i've done it ever since.

I thought it was million*million as I was used to note million as a kk, e.g. 10M=10kk.

Thanks everyone. My curiosity has been satisified.