In order to use this tool you will need to keep track of the dates of your recent menstrual periods. Your cycle length is the number of days between the first day of bleeding of one period, and the first day of bleeding of your next period. This tends to vary from 23 to 35 days with the average being a 28 day cycle. If you have irregular menstrual cycles, or your cycle length varies from month to month, it may be difficult to calculate your ovulation date. In this case, use your shortest period date. It is important to start earlier to ensure your ovulation date is not missed. Ovulation urine tests may be useful, and you should consider seeking further advice from your GP or fertility specialist.