A simple heart rate is useless for determining whether it is atrial fibrillation or not. Although in most cases, people with afib have a rapid heart rate (>100 beats per minute), many people with afib have a perfectly normal heart rate (60-100 beats per minute). In fact, a more useful indicator of afib is the variation of heart rate. For example, your heart rate changes from 60 to 90 rapidly. That is to say, changes in heart rate is more suggestive of afib than a simple heart rate value.