How many mm in a Mm?

Answer 1

A mm (millimetre) is one thousandth of a metre, i.e. 0.001 m.

A Mm (megametre, a valid unit but very seldom used) is one million metre, i.e. 1,000,000 m (or 1,000 km).

So, there are (1 Mm) / (1 mm) = (1,000,000 m) / (0.001 m) = 1,000,000 / 0.001 = 1,000,000,000 mm (one billion mm) in a Mm.