Period Calculator: Next Period & Ovulation Dates

This calculator predicts your next period, ovulation day, and fertile window from the start date of your last period, your average cycle length, and how many days your period usually lasts. It also builds a six-month calendar so you can see several cycles ahead at once. ### How the Dates Are Calculated Your next period is the start date of your last period plus your cycle length. Ovulation is counted backward from that date rather than forward from your last period, because the luteal phase (the days between ovulation and the next period) stays close to 14 days for most people, while the days before ovulation vary more from cycle to cycle. The fertile window covers the five days before ovulation through the day after it, based on how long sperm can survive in the reproductive tract and the roughly 24-hour lifespan of the released egg. ### Worked Example Say your last period started on March 1 and your average cycle length is 28 days. Your next period is predicted for March 29, since March 1 plus 28 days lands there. Ovulation falls on March 15, 14 days before that. The fertile window runs from March 10 through March 16. If today were March 20, the calculator would show cycle day 20 of 28. For irregular cycles, enter your last three cycle lengths, for example 26, 29, and 27 days, and the tool averages them to 27 and uses that number instead. ### Typical Cycle Length | Cycle length | What it means | |---|---| | Under 21 days | Shorter than the typical range | | 21 to 35 days | Typical range | | Over 35 days | Longer than the typical range | These figures match the ranges cited by ACOG and Mayo Clinic; a period lasting 2 to 7 days also falls within what's considered typical. This calculator produces estimates for planning purposes only. It doesn't diagnose anything, and it isn't a substitute for medical advice, so talk to a doctor about missed, highly irregular, or unusually painful periods.