If you're interested in knowing how to calculate your BMR rather than let a web-based calculator do it for you, then this is a good place to start. If you read the article, you will see that BMR can be calculated in a number of ways. The most accurate, it says, is to use lean body mass. That's the weight of your body minus the fat! You need to know your body fat percentage to do this, which makes it a little less accessible for those without access to the technology to do this. However, it's not that difficult to get it measured. Be aware that when you measure it can affect the reading.
The basic calculation is this:
BMR= 370+(21.6 x LBM)
LBM is Lean Body Mass in Kg.
It's not gender specific, so the same equation works for male and female.
The BMR figure I played with yesterday for myself was based on what I think would be an ideal personal weight for me as an active individual. When I used this formula for me at my current weight and BF% the BMR came out very similar (1882 vs 1811). So it's not a big difference, although 70 calories a day over a year would add up!
On the other hand, there's no allowance for exercise or activity in these numbers, so as a general guide I'm not sure either would be a problem.