Ovulation calculator
Your fertile window, ovulation day and next period.
About this calculator
This ovulation calculator estimates when you're most likely to ovulate and your most fertile days. Enter the first day of your last period and your usual cycle length.
You'll see your fertile window, your next period and a calendar for the next six cycles, which can help if you're trying to get pregnant.
How it works
Ovulation usually comes about 14 days before the next period (the luteal phase). The fertile window is the 5 days before ovulation plus the day after.
Last reviewed:
Common questions
When am I most fertile?
In the two or three days before ovulation and on the day itself. Sperm can survive for up to five days, which is why the fertile window starts before ovulation.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough for contraception, because ovulation can move from cycle to cycle. Speak to a doctor or nurse about birth control.
What if my cycle is irregular?
The dates will be less reliable. Ovulation test kits, tracking your temperature or a doctor's advice give a better picture when cycles vary.
Sources and further reading
The story behind it
Where the idea came from: In the 1920s, Kyusaku Ogino in Japan and Hermann Knaus in Austria each worked out that ovulation comes about two weeks before the next period.
Before their work, many doctors thought ovulation happened at varying times. Ogino and Knaus showed that the second half of the cycle is fairly constant, about 14 days, which made it possible to estimate ovulation from period dates. A large 1995 study then showed that the fertile window is the six days ending on ovulation day.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, not medical advice.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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