Menstrual Cycle Tracker Calculator

Average Cycle Length, Period Duration, Last Period Start (day of month), and Cycle Variation feed this estimate of ovulation timing, fertile window, and cycle regularity. Estimated Ovulation is calculated as Cycle Length minus a fixed 14-day luteal phase, so it -- along with the Fertile Window that surrounds it (five days before ovulation through one day after) and Follicular Phase Length, which equals the ovulation day itself -- responds only to Average Cycle Length; Period Duration, Last Period Start, and Cycle Variation have no effect on where ovulation or the fertile window fall within the cycle. Luteal Phase Length works differently: real luteal phases vary far less between cycles than follicular phases do, so this calculator holds it at a fixed 14 days regardless of Cycle Length or any other input, rather than deriving it.

Next Period (est. day of month) instead uses real calendar-date arithmetic, adding Cycle Length in days to your Last Period Start date to project the day of the following month your next period should begin. Cycle Regularity Score converts Cycle Variation into a 0-100 score using the formula (1 - variation/10) x 100, so a Cycle Variation of 0 days scores 100 (perfectly regular) while a variation of 10 days or more scores 0. Estimated Lower Fertility Window reports the days between the end of your period and the start of the fertile window -- described as "estimated" and "not a guarantee of infertility" because ovulation timing genuinely varies and no day in a natural cycle is entirely risk-free.