Ovulation Calculator
Estimate your next ovulation day and fertile window based on your last period start date and average cycle length.
Your cycle
Your estimated fertile period
The 2–3 days leading up to and including ovulation are when conception is most likely. Sperm can survive up to five days, which is why the window starts well before ovulation.
Frequently asked questions
How is ovulation day calculated?
Ovulation typically happens about 14 days before your next period starts. This calculator counts forward from your last period date based on your average cycle length, so shorter or longer cycles shift the ovulation estimate accordingly.
What is the fertile window?
The fertile window is the roughly six days when pregnancy is most likely: the five days before ovulation plus the day of ovulation itself, because sperm can survive in the body for several days.
When in the cycle am I most fertile?
You are most fertile in the 2–3 days leading up to and including ovulation. Pinpointing the exact day is tricky because cycle length varies, so the estimate is a guide, not a guarantee.
Why do I need my average cycle length?
Cycle length determines when ovulation lands. The classic 28-day assumption is not right for everyone — using your own average gives a more accurate estimate.
How accurate is an ovulation calculator?
It is a calendar estimate only. It does not measure hormones or ovulation directly — for accurate tracking, many people combine it with an ovulation predictor kit or basal temperature tracking. Not for birth control.