Typically you might use "mm" for coins. For currency, I would suggest micron units.
The millimeter.
Milimeter
That would be measured in millimetres.
Millimeters.
Millimeters.
Millimeters.
cenemeters
A micrometre.
There is only one metric system. The relevant units, within that system would be millimetres.
What measure of units would you use to measure the thickness of a phone book
one page of a phonebook is measured in mm. unit thickness of a phone book is cm
The commonest convenient unit is the millimetre (mm), and they are around 1.2 mm thick.
i think it is mm