BMR = Basal Metalbolic Rate (similar to RMR = Resting Metabolic Rate)
Your BMR is represents the number of calories your body burns at rest. The IIFYM BMR calcualtor will calculate the precise amount of calories your individual body requires to keep you alive. The BRM Calculator is not a TDEE Calculator so you should not base any dietary decision on the number our BMR calculator spits out.
Many people make the mistake using the IIFYM BMR calculator to tell them how many calories to eat. Doing this can lead to too drastic a calorie deficit leading to muslce loss, and stalled fat loss. It can also cause a drastic rebound effect, something many YO YO dieters have experienced. Remember, BRM & TDEE are not the same thing!
The IIFYM BMR Calculator should only be used to figure out the exact amount of calories your body needs to stay alive. This does not include getting out of bed, exercising, or even opening your eyes.
If you are trying to figure out how many calories you need to eat in order to lose weight and burn fat, please do not use the BMR calculator. Instead please use the IIFYM TDEE Calculator which will calculate your BMR along with all the other calories your burn during the day.