How the fertile window works
You can only conceive around ovulation — when an ovary releases an egg. The egg lives for about 12–24 hours, but sperm can survive in the body for up to five days. That gives you a fertile window of roughly six days: the five days before ovulation plus ovulation day itself. The two days before ovulation are the most fertile.
Finding your ovulation day
Ovulation happens about 14 days before your next period — that gap is called the luteal phase, and it's fairly constant even when cycle length varies. So we take the first day of your last period (LMP), add your cycle length to find your next period, then count back your luteal phase to land on ovulation.
Your estimated due date
If you conceive this cycle, your due date is estimated with Naegele's rule: 280 days (40 weeks) from the first day of your last period. Measured from ovulation instead, it's about 266 days. Only about 1 in 20 babies actually arrives on the exact date — most come within the two weeks around it.
If your cycles are irregular
This calculator assumes a reasonably regular cycle. If yours varies by more than a few days, your fertile window shifts too. Tracking ovulation signs — basal body temperature, cervical mucus, or ovulation predictor kits — gives a more precise read, and a doctor can help if you're trying to conceive without success.
Frequently asked questions
When am I most likely to get pregnant?
In the two days before ovulation and on ovulation day. The full fertile window is the five days before ovulation plus ovulation day, because sperm can survive up to five days. This calculator highlights those days for you.
Can I use this to avoid pregnancy?
No — please don't. Ovulation timing varies and the fertile window can shift, so calendar estimates aren't reliable contraception. Use a proven birth-control method and talk to your doctor.
Is my information private?
Yes. Everything is calculated in your browser. Nothing you enter is uploaded, stored on a server, or shared with anyone.