Health / 04
Ovulation Calculator
Estimate your fertile window and ovulation day for the next six cycles from the first day of your last period and your usual cycle length, when you are trying to get pregnant.
Ovulation Calculator: The page places ovulation 14 days before your next expected period, the average length of the second half of the cycle, and marks the six days ending on ovulation day as the fertile window: the days on which, in a study of 221 women trying to conceive, intercourse led to pregnancy. Runs 100% locally in your browser with zero server file uploads.
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- Health & sport tools
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- In your browser
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- Free · no sign-up
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- Ready to use
Runs entirely in your browser
Not a method of birth control. Ovulation moves from cycle to cycle, even in regular cycles, so a date estimated from a calendar cannot tell you when you are safe from pregnancy.
Pick the first day of your last period to see your estimated fertile days.
The research behind the fertile window
A study by Wilcox, Weinberg and Baird in the New England Journal of Medicine in 1995 followed 221 women trying to conceive and measured ovulation with hormone tests. Conception followed intercourse only in the six days ending on the day of ovulation, with the chance highest in the last two or three of them. That six-day span is the fertile window this page shows.
A follow-up study by the same team in the BMJ in 2000, using the same women's cycles, found that only about 30% of women had their fertile window entirely within days 10 to 17, the days often given as the fertile ones, and that on every day from day 6 to day 21 at least one woman in ten was in her fertile window, regular cycles included. A calendar estimate is a starting point for timing, not a certainty.
Making the estimate more useful
Enter the average of your last few cycles rather than the textbook 28 days, counting from the first day of one period to the first day of the next. Ovulation tests sold in pharmacies detect the hormone surge a day or two before ovulation, and a basal body temperature chart shows a rise just after it; either tells you more about your own cycle than an average. Once your period is late, the due date calculator counts from the same date.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length, from the first day of one period to the first day of the next.
- Read your estimated fertile window and ovulation day for this cycle and the next five.
Privacy & limitations
Everything is calculated in your browser. The dates you enter are never sent anywhere or stored.
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Frequently asked questions
Can I use this to avoid getting pregnant?
No. Ovulation varies from cycle to cycle, even for people whose periods are regular, so a date worked out from a calendar cannot tell you which days are safe. Talk to a doctor, nurse, or pharmacist about contraception.
How accurate is the ovulation date?
It is an average-based estimate. In one study only about 30% of women had their fertile window entirely within days 10 to 17 of the cycle. Ovulation tests, which detect the hormone surge before ovulation, and tracking your temperature show when it actually happens.
Why is the fertile window six days?
Sperm can survive in the body for several days, while the egg lives for about a day. In a study published in the New England Journal of Medicine in 1995, conception only followed intercourse in the six days ending on the day of ovulation.
What if my cycles are irregular?
The estimate is only as good as the cycle length you enter, and it gets less useful the more your cycles vary. If they vary by more than a few days, ovulation tests are a better guide, and a doctor can check for a cause.
Free tool · runs in your browser · no account required