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Ovulation Calculator
Estimate when you are likely to ovulate and the days you are most likely to conceive. Enter the first day of your last period and your average cycle length; the calculator gives this cycle’s dates and the next six. It is a calendar estimate, not a measurement.
- Fertile window opens
- Tue, Oct 20, 2026
- Fertile window closes
- Sun, Oct 25, 2026
- Next period expected
- Sun, Nov 8, 2026
- Due date if you conceive this cycle
- Sun, Jul 18, 2027
Next six cycles
| Period starts | Fertile window opens | Ovulation |
|---|
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Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the first day of your most recent period.
- Enter your average cycle length, counted from the first day of one period to the day before the next.
- Read your estimated ovulation day and fertile window, and check the table for the coming cycles.
Worked examples
Last period began January 1, 2026 on a 28-day cycle
- Estimated ovulation day
- Thu, Jan 15, 2026
- Fertile window opens
- Sat, Jan 10, 2026
- Fertile window closes
- Thu, Jan 15, 2026
- Next period expected
- Thu, Jan 29, 2026
- Due date if you conceive this cycle
- Thu, Oct 8, 2026
Last period began March 5, 2026 on a 32-day cycle
- Estimated ovulation day
- Mon, Mar 23, 2026
- Fertile window opens
- Wed, Mar 18, 2026
- Next period expected
- Mon, Apr 6, 2026
- Due date if you conceive this cycle
- Mon, Dec 14, 2026
How the estimate works
The time from ovulation to the next period — the luteal phase — is fairly constant at around 14 days, while the first half of the cycle varies. So ovulation is estimated by counting back 14 days from the next expected period: day 14 of a 28-day cycle, day 18 of a 32-day cycle.
Sperm can survive for up to about five days and the egg for about a day, which gives a six-day fertile window ending on the day of ovulation. The two or three days before ovulation carry the highest chance of conception.
Limits of a calendar method
Cycles vary from month to month and ovulation can move by several days even in regular cycles, so the dates here are approximate. If your cycles are irregular the estimate is much less reliable. Ovulation test kits, basal body temperature and cervical mucus changes track what your body is actually doing.
A calendar estimate should not be relied on to prevent pregnancy. This is an estimate for general information, not medical advice. Talk to a doctor or other qualified clinician before making health decisions based on it.
Questions people ask
When do I ovulate on a 28-day cycle?
Around day 14, counting the first day of your period as day 1. If your period began on January 1, estimated ovulation is January 15 and the fertile window is January 10 to 15.
When do I ovulate on a 32-day cycle?
Around day 18, about 14 days before your next period. With a period starting March 5, that is March 23.
How long is the fertile window?
About six days: the five days leading up to ovulation and the day itself.