CC, if you are looking for simple keystrokes using every-day units of aviation, the following will give an accurate answer:
TAS=(643 + (1.25 x DegC)) x Mn
Mn =TAS / (643 + (1.25 x DegC))
Where: TAS=Knots, Mn=Mach No, DegC=Degrees C.
You can run that on a pocket calculator, or feed it into a simple computer program on your PC or Organiser.
If you really want Kms (Russia perhaps?),
Km = Nm x 1.8518518 (Nm = Nautical miles)
Our milkman is called Kelvin. He says it's perishing cold whenever the OAT drops below 6 Deg C.
Hope this helps.