Tip: calculate first — your result is included when you share.
How to Use the Ovulation Calculator
Enter the first day of your last menstrual period and your average cycle length (28 days is the default; most cycles fall between 21 and 35 days), then press Calculate Fertile Window. You’ll see your estimated ovulation day, the full fertile window, and when your next period is expected.
The part most people get wrong: the fertile days come before ovulation, not after. The egg must already be waiting when sperm arrive — and because sperm can survive up to 5 days, timing intercourse a few days before ovulation matters more than the day itself.
A reality check: this is an estimate built on averages. Irregular cycles (for example with PCOS, while breastfeeding, or around perimenopause) make it much less reliable — and it is never a contraception method. If you’re actively trying to conceive or have concerns, a doctor is the right next step.
If you do conceive, our Pregnancy Due Date Calculator estimates your due date from that same last-period date. For a general wellness check, try our BMI Calculator.
Frequently Asked Questions
What is ovulation?
Ovulation is when a mature egg is released from the ovary, roughly once per menstrual cycle. The egg survives about 12–24 hours, but sperm can live up to 5 days — which is why the fertile window is wider than the day itself.
How is my ovulation day calculated?
We take the first day of your last period and add (cycle length − 14) days. With the standard 28-day cycle that lands on day 14. This assumes ovulation happens about 14 days before your next period starts.
What is the fertile window?
The fertile window is the roughly 6-day span ending on ovulation day — about 5 days before it plus the day itself. These are the days when intercourse is most likely to lead to pregnancy.
How accurate is this calculator?
It is an estimate based on averages, not a medical diagnosis. Cycles vary month to month and between people, and ovulation can shift with stress, illness or travel. Ovulation predictor kits or a doctor can confirm actual timing.
Can I get pregnant outside the fertile window?
The chance is very low outside the window, but not zero — cycles can be irregular and sperm timing varies. This calculator should not be used as a contraception method; talk to a doctor about family planning.
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