Step One: How many days are between the first day of bleeding of one cycle until the first day of bleeding for your next cycle?
The average is 28 days. Use 28 if you use a traditional monthly cycle (21/7) birth control pill/ring/patch.
ALERT: If you are not on a birth control pill and you are bleeding more than every 3 weeks or less than every 5 weeks then it is very likely you are not ovulating and it will be difficult to predict your cycle . You may actually need to see a doctor because frequent or infrequent bleeding is not normal unless you are controlling it with hormones.