Ovulation Calculator
Estimate your ovulation day, fertile window, and next period.
How this works
This uses the calendar method: ovulation day is estimated as your cycle length minus 14 days after your last period started, since the luteal phase (the time from ovulation to your next period) stays close to 14 days for most people even when the rest of the cycle varies. Your fertile window is the 5 days before ovulation plus the day of ovulation itself, since sperm can survive in the body for up to about 5 days waiting for an egg to be released.
Cleveland Clinic puts this 6-day window (5 days before ovulation, plus ovulation day) as the highest-probability stretch for conception. A cycle of 21-35 days is considered typical -- if yours varies by more than about a week from month to month, this calendar-based estimate gets noticeably less reliable, since it assumes a fairly consistent cycle length.
If this cycle does result in a pregnancy, switch over to the Pregnancy Due Date Calculator -- it estimates your due date from the same last-period date this calculator already uses.
Frequently asked questions
How is ovulation day calculated?
Ovulation day is estimated as your cycle length minus 14 days after the first day of your last period. The 14-day figure comes from the luteal phase -- the stretch between ovulation and your next period -- which stays fairly consistent even in people whose overall cycle length varies.
What exactly is the “fertile window”?
The 5 days before ovulation, plus the day of ovulation itself -- 6 days total. Sperm can survive in the body for up to about 5 days, so intercourse any time in that window can lead to conception once an egg is released, not just on the day of ovulation itself.
Can I use this as birth control?
No -- this is a calendar-based estimate meant for people trying to conceive, not a reliable method to avoid pregnancy. Cycles vary from month to month even for people who consider their cycle “regular,” and this estimate can be off by several days in either direction.
What if my cycle is irregular?
This calculator assumes a fairly consistent cycle length, so the estimate gets less reliable the more your cycle actually varies month to month. Tracking methods that respond to your body’s own signals (like basal body temperature or ovulation predictor kits) are more reliable than a calendar estimate for anyone with an irregular cycle.