How much to save $1M in 20 years

Interestingly I came accross an article that gave me the challenge of working out, how much do I have to save / mth if I want $1M in savings in 20 years time.

The answer is, at say an average compounding interest return of 4% (any excel program has the formulae embedded as a feature), roughly $2750 / mth gives you in 20 years = $1,000,000  in savings