The fat burn zone calculator calculates the heart rate needed to enter the fat burn zone. The fat burn zone refers to the intensity of aerobic exercise needed to burn fat without burning glycogen (the body's store of carbohydrate). The fat burn zone occurs when one exercise below approximately 65-70% VO2MAX.
Calculate your fat burn zone:
Fat burning zone = <65% Vo2MAX = < 65% HRmax = < 0.65(220-age)
So, if you're 49 years old, to burn fat you should exercise below 0.65(220-49)