Ovulation Calculator
Find your likely ovulation day and six-day fertile window from your last period and cycle length, for up to 12 cycles, with a calendar and .ics export.
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About this toolWhat is the Ovulation Calculator?
Give the calculator the first day of your last period and your usual cycle length, and it marks your estimated ovulation day, the six-day fertile window that ends on it, and the two days just before ovulation when the chance of conceiving is highest. It projects the same pattern forward for 3 to 12 cycles on a colour-coded calendar, lists each window in a table, and can download all of them as a calendar file.
Ovulation is placed by counting back from the next expected period rather than forward from the last one. The second half of the cycle, the luteal phase, is usually close to 14 days, while the first half is what stretches or shrinks in long and short cycles. If you have tracked your luteal phase with temperature charts or ovulation tests, you can enter your own value.
These are calendar estimates. They are useful for timing ovulation tests or sex when trying to conceive, but they are not a form of birth control.
- Ovulation counted back from the next period with an adjustable luteal phase
- Six-day fertile window with the three peak days highlighted
- Colour-coded calendar for up to 12 cycles
- .ics download of fertile windows and ovulation days
- Runs locally; dates are never uploaded
How to use the Ovulation Calculator
- Enter the first day of your most recent period.
- Enter your average cycle length, counted from the first day of one period to the first day of the next.
- Leave the luteal phase at 14 unless you have measured yours.
- Choose how many cycles to show. The summary cards describe the current cycle; the calendar and table cover the rest.
- Use Add to calendar (.ics) to put the fertile windows in Google Calendar, Apple Calendar or Outlook, or Copy dates to paste them elsewhere.
Worked examples
Regular 28-day cycle
Input
Last period 1 October 2026, cycle 28, luteal 14
Output
Fertile 10 to 15 October, ovulation Thursday 15 October, next period 29 October
Next period minus 14 days lands on cycle day 15. The most fertile days are 13, 14 and 15 October.
Long cycle, short luteal phase
Input
Last period 1 October 2026, cycle 35, luteal 12
Output
Ovulation 24 October (cycle day 24), fertile 19 to 24 October
A generic "day 14" rule would have put the fertile window ten days too early here.
Planning ahead
Input
Same 28-day cycle, 6 cycles
Output
Windows 10 to 15 Oct, 7 to 12 Nov, 5 to 10 Dec, 2 to 7 Jan, 30 Jan to 4 Feb, 27 Feb to 4 Mar
Each window moves exactly one cycle length later. If a period arrives early or late, re-enter the new start date.
Background and insight
The six-day window comes from a well-known study by Allen Wilcox, Clarice Weinberg and Donna Baird (New England Journal of Medicine, 1995). They followed 221 women trying to conceive, pinned down ovulation with daily urine hormone measurements, and found that every pregnancy came from sex in the five days before ovulation or on the day itself. The chance per act rose to about a third in the two days before ovulation and on ovulation day. Sperm can survive in fertile cervical mucus for up to about five days, while an egg is viable for roughly 12 to 24 hours, which explains the shape.
Counting back 14 days from the next period works well on average, but real cycles wander. A follow-up analysis of the same hormone data (Wilcox, Dunson and Baird, BMJ, 2000) found that only about 30% of women had their whole fertile window inside days 10 to 17 of the cycle, and that even women with regular cycles could not be sure which days were fertile in a given month. That is why this page labels every date as an estimate and why a positive ovulation test (an LH surge, usually 24 to 36 hours before ovulation) beats the calendar when the two disagree.
Common use cases
Trying to conceive
Know which days to prioritise and when to start using ovulation predictor kits.
Timing a pregnancy test
See the expected next period, the earliest day a home test is reasonably reliable.
Understanding your cycle
Visualise how a long or short cycle shifts ovulation away from day 14.
Preparing for a fertility appointment
Bring a calendar of estimated windows alongside your own tracking data.
Tips and limitations
- Start ovulation tests two or three days before the fertile window opens, especially if your cycle length varies.
- If your cycles differ by more than about a week from month to month, the calendar estimate is weak; temperature charting or LH tests will tell you more.
- Sex every one to two days through the fertile window covers it without needing to hit the exact day.
- Do not use these dates to avoid pregnancy. Calendar methods alone have high failure rates in typical use.
- Seek advice after 12 months of trying, or after 6 months if you are 35 or older, or sooner if cycles are very irregular.
Frequently asked questions
How many days after my period do I ovulate?
In a 28-day cycle, about 14 days after your period starts (cycle day 15 counting the first day of bleeding as day 1). In a 32-day cycle it is closer to day 19, because ovulation is usually about 14 days before the next period rather than a fixed number of days after the last one.
How long is the fertile window?
About six days: the five days before ovulation plus ovulation day. Sperm can live up to about five days, but the egg survives only 12 to 24 hours after release, so the window closes soon after ovulation.
Which days give the best chance of getting pregnant?
The two days before ovulation and ovulation day itself. In the Wilcox study the chance per act peaked at roughly a third on those days. The calculator shades them darker green.
Can I get pregnant right after my period ends?
It is possible with short cycles. In a 21-day cycle ovulation can be around day 7, so the fertile window may start while bleeding is ending, and sperm from sex on day 3 or 4 could still be alive.
Is an ovulation calculator accurate for irregular periods?
Not very. It assumes each cycle repeats the length you enter. With irregular cycles, ovulation predictor tests, basal body temperature or cervical mucus tracking show what is happening in each cycle, and a clinician can check for causes such as polycystic ovary syndrome or thyroid problems.
What is the luteal phase and should I change it?
It is the time from ovulation to the next period, usually 12 to 14 days and fairly stable for one person. Change it only if you have measured it, for example from a temperature rise or a positive LH test to the next period.
Can I use this as birth control?
No. Calendar-based estimates cannot detect a cycle where ovulation comes early or late, and that is exactly when unplanned pregnancies happen. Talk to a clinician about reliable contraception.
Is there an in-depth guide to the Ovulation Calculator?
Yes. “Your Menstrual Cycle in Numbers: Periods, Ovulation and the Fertile Window” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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