Ovulation & Fertile Window Calculator
Your likely ovulation day and six-day fertile window, for the next three cycles.
Ovulation happens a fairly fixed number of days before your next period — usually 14 — rather than a fixed number of days after the last one. That is why cycle length matters so much here, and why the fertile window sits earlier in short cycles than in long ones.
How this is worked out
Ovulation is estimated as cycle length minus luteal phase length, counted from day one of your period. The fertile window runs from five days before ovulation to the day itself, because sperm survive up to five days in the reproductive tract while an egg lives for around 12 to 24 hours. The two days before ovulation and the day of ovulation carry the highest chance of conception.
Calendar prediction is an estimate, not a test. Cycles vary month to month even in regular cyclers, and stress, illness and travel all move ovulation. Ovulation predictor kits, basal body temperature charting and cervical mucus changes give you a real-time signal that a calendar cannot.
This calculator is for planning or avoiding conception by understanding your cycle. Calendar-based methods are among the least reliable forms of contraception, with typical-use failure rates of roughly one in four over a year — do not rely on this to prevent pregnancy.
This calculator runs entirely in your browser. Nothing you enter is transmitted to us, saved on our servers, or shared with anyone.
Common questions
When am I most fertile?
The two days before ovulation and ovulation day itself. Intercourse every one to two days across the fertile window is the usual advice.
Can I ovulate on a different day each month?
Yes, and most people do to some degree. Studies tracking hormones find the fertile window is far more variable than calendar methods assume, particularly in cycles shorter than 26 or longer than 32 days.