A handy hint for calculating Reynolds number quickly.

In the atmosphere table, I have added one more quantity that I find useful, although I have never seen it in another atmosphere table. This is the ratio of speed of sound to the kinematic viscosity. This quantity has units of 1/length. Multiplying this number by a reference length and then by the Mach number yields the Reynolds number.

For example, if I am cruising at M=0.8 at 34000 ft. in a jet
transport with a chord of 16 feet, then from the table at 34000 ft,
this ratio is 2.48 ft^{-1}, and so the Reynolds number is
2.48*16*0.8=31.7 million (the million is built in). I realize that
this only saves one division, but I seem to do it so much faster,
probably because I don't have to deal with powers of 10. In the
SI table this ratio is given in units of 1/m while it is 1/ft
in the US table.

If you work in US customary units and you can remember that this ratio is about 7 at sea level and about 2-3 in the lower stratosphere and about 0.5 for SST cruise, then you can estimate Reynolds number in your head. Astound your colleagues!