Ovulation Calculator
Fertile window and likely ovulation day.
Calendar prediction is only moderately reliable — ovulation shifts month to month even in regular cycles. It is not adequate as contraception.
Likely ovulation
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How the Ovulation Calculator works
Ovulation is usually about 14 days before the next period starts, not 14 days after the last one began — a distinction that matters for anyone whose cycle is not 28 days. The fertile window spans roughly five days before ovulation and the day itself, because sperm survive far longer than the egg.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
When do I ovulate?
Roughly 14 days before your next period. On a 28-day cycle that is day 14, but on a 35-day cycle it is day 21. The luteal phase after ovulation is relatively fixed at 12-16 days, while the follicular phase before it varies.
How long is the fertile window?
About six days: the five days before ovulation plus ovulation day. Sperm survive up to five days in fertile cervical mucus, while the egg is viable for only 12-24 hours, which is why the window sits mostly before ovulation rather than after.
How accurate are calendar-based predictions?
Only moderately. Ovulation timing varies month to month even in regular cycles, and one study found the predicted fertile window is accurate for a minority of cycles. Ovulation predictor kits, basal body temperature or cervical mucus tracking are considerably more reliable.
Can I get pregnant outside the fertile window?
It is much less likely but not impossible, because ovulation can occur earlier or later than predicted. Calendar-based methods are not reliable contraception on their own.