Summary: how to find your ovulation date for free and privately
An ovulation calculator estimates the day an egg is released from your last period date and your average cycle length. To use this one, pick the First Day of Last Period, enter your cycle length, and read the result panel. Ovulation is estimated at your cycle length minus 14 days after the period started, and the fertile window is the five days before that day plus ovulation day itself.
For a 28-day cycle that starts on 1 September, ovulation falls on 15 September and the fertile window runs from 10 to 15 September. Nothing is uploaded: the maths runs in your browser, and you can confirm it in the Network tab of your browser DevTools. It is an estimate, not a medical test and not contraception.
Table of use: example inputs and results
Examples assume a period that starts on 1 September and lasts five days.
| Situation | What you enter | What you get |
|---|---|---|
| Trying to conceive, regular cycle | Cycle 28 days | Ovulation 15 Sep (cycle day 15), fertile window 10 to 15 Sep, most fertile 13 to 15 Sep |
| Longer cycle | Cycle 32 days | Ovulation 19 Sep (day 19), fertile window 14 to 19 Sep, next period 3 Oct |
| Shorter cycle | Cycle 24 days | Ovulation 11 Sep (day 11), fertile window 6 to 11 Sep, next period 25 Sep |
| Planning ahead | Any last period date and cycle length | Next period date and likely end, the date to take a pregnancy test, and a table of six cycles |
| Cycle outside 21 to 35 days | For example 40 days | The estimate plus a warning that dates are harder to predict; entries below 20 or above 60 days are rejected |
The formula is: ovulation date = first day of last period + (cycle length - 14) days. The fertile window starts 5 days before that date and ends on it, and the three most fertile days are the two days before ovulation plus ovulation day. With the first day of bleeding as day 1, ovulation lands on cycle day (cycle length - 13).
Features
- Ovulation, fertile window and peak days. The six-day window is split into fertile days and the three most fertile days.
- Next period and test date. It shows the next expected period, its likely end from your Period Length setting, and the date of your missed period, which the tool gives as the earliest day to take a home pregnancy test.
- Six cycles at once. The Your Next 6 Cycles table lists period start, fertile window, most fertile days and ovulation.
- Colour-coded monthly calendar. Period, fertile, most fertile and ovulation days are shaded, today is outlined, and arrows move month by month.
- Cycle day and warnings. It shows the cycle day for today and warns when your last period date is at least one cycle old or your cycle is outside 21 to 35 days.
- Input checks. Cycle lengths of 20 to 60 days and period lengths of 1 to 10 days are accepted; a last period date in the future is rejected.
- Copy button and privacy. One click copies the dates as plain text. Nothing is stored; see our privacy policy for the rest of the site.
How to use the ovulation calculator
- Under Your Cycle Info, choose First Day of Last Period: the first day of full bleeding, not spotting.
- Type your Average Cycle Length (days), from the first day of one period to the day before the next. If unsure, average your last three cycles.
- Check Period Length (days). It only affects the shaded period days and the likely end date.
- Read the Results panel: Estimated Ovulation, Fertile Window (6 days), Next Period Expected and Today.
- Browse the calendar, scroll to the table of six cycles, or press Copy to take the dates with you.
Want the dates on paper? Print a month with the printable calendar generator and mark them by hand, or count down the days to your next period date with the countdown timer.
Who can use it
People trying to conceive can use it to time intercourse in the fertile window. Anyone who wants a heads-up on their next period gets the same dates, and students or health educators can use it to see how cycle length moves ovulation. It suits regular cycles and is not meant for diagnosis or preventing pregnancy.
Your last period date also gives a due date in the pregnancy due date calculator, and the date difference calculator counts the days between any two dates. Weight can affect cycle regularity, so some people also check the BMI calculator or the calorie calculator. After the birth, the age calculator shows how old a baby is in days and weeks. More tools are in the calculators hub and the full tools list.
Frequently asked questions (FAQs)
How do I calculate my ovulation date?
Subtract 14 from your cycle length, then count that many days forward from the first day of your last period. With a 28-day cycle that lands on cycle day 15; with a 32-day cycle, on day 19. The 14 is the luteal phase, the time between ovulation and the next period, which lasts about 14 days for most people. The calculator does this sum and repeats it for six cycles.
How many days after my period do I ovulate?
It depends on your cycle length, not on a fixed number of days after bleeding stops. Ovulation falls about 14 days before your next period starts. In a 28-day cycle with a five-day period, that is roughly 10 days after the period ends; in a 24-day cycle only about 6 days after.
How long is the fertile window?
It is six days: the five days before ovulation plus ovulation day. Sperm can survive in the reproductive tract for up to about five days, while an egg lives roughly 12 to 24 hours. A study of couples trying to conceive (Wilcox et al., New England Journal of Medicine, 1995) found pregnancies only from intercourse in those six days, most often in the last three.
How accurate is an ovulation calculator?
It is an estimate built on averages, so it is most reliable for regular cycles. The post-ovulation phase is about 14 days for most people but can range from roughly 10 to 16, and stress, illness, travel or hormonal changes can shift ovulation. For a firmer answer, add ovulation predictor kits, which detect the LH surge about 24 to 36 hours before ovulation.
Can I use this to avoid pregnancy?
No. Ovulation can move from one cycle to the next without warning, and sperm survive for days, so a calendar estimate is not reliable contraception. If you want to prevent pregnancy, talk to a doctor or nurse about methods that have measured failure rates, including properly taught fertility awareness methods that track more than dates.
Is it safe to enter my period dates here?
Yes. The calculation runs entirely in your browser, so the dates you type are not uploaded, saved or shared, and they are gone when you close or reload the page. You can verify this by opening the Network tab in your browser DevTools and watching that nothing is sent while you change the inputs.
This page gives general information and estimates, not medical advice. See a healthcare provider about fertility concerns or cycles that are unusually short, long or irregular.