The Cycle Calculation Formula

To calculate your cycle length:

Cycle Length = (Date of Next Period Start) - (Date of Current Period Start)

Example: If your previous period started on March 1st and your new period begins on March 29th, your cycle length is 28 days.

Predicting Next Period and Ovulation

Because the luteal phase (time between ovulation and period) remains remarkably constant at approximately 14 days, you can estimate ovulation day by subtracting 14 from your projected period start date.