In finance and accounting, MM (or lowercase “mm”) commonly denotes that the units of figures presented are in millions. The Roman numeral M denotes thousands. In this context, MM is the same as writing “M multiplied by M,” which is equal to “1,000 times 1,000,” which equals 1,000,000 (one million).

M is the Roman numeral for thousand and MM is meant to convey one thousand-thousand — or million. To take it further; one billion would be shown as $1MMM or one-thousand million.

MM stands for a million. However, M is used to stand for a million often enough, that there is a lot of confusion when it appears, unless the context makes clear which number is intended.

As of my last knowledge update in January 2022, the thickness of U.S. paper currency is relatively consistent. The standard thickness for most U.S. banknotes is approximately 0.0043 inches or 0.11 millimeters.