I had my resting metabolism measured recently by my friend who is a diet counsellor. Very interesting thing to have done - you have to starve for 4 hours (not difficult if your appointment is just before lunch!) then sit quietly for 10 mins with an uncomfortable peg on your nose and a tube in your mouth that you breathe into normally. This gives the amount of calories you would use if you sat in a chair all day, which varies widely with different sizes/ages/physical types of person and from which the testing person can extrapolate your actual calorie usage. They have to rely on a questionnaire of what you say you do all week so you have to not lie about how much exercise you do.
Mine came up as about 1900 per day, taking into account my cycling and general activity. Actually quite high for a 50-year-old woman - the "2000 per day" is no good for you when you're a, ahem, mature person....