Ovulation Calculator
Estimate your ovulation date and fertile window from the first day of your last period and your usual cycle length.
An ovulation calculator counts back 14 days from the date your next period is due, since the stretch between ovulation and a period is the steadiest part of the cycle. Your fertile window is that day plus the five days before it. If your last period began on September 14 and your cycle runs 28 days, ovulation is estimated for September 28, the window runs September 23 to 28, and the next period is due October 12.
This calculator gives a calendar estimate for general education only. It is not medical advice, it cannot confirm that you ovulated, and it is not a method of birth control or a way to prevent pregnancy. Ovulation can happen earlier or later than any estimate. For help trying to conceive, preventing pregnancy, or with irregular, missed or very painful periods, talk to a doctor, nurse or other qualified health professional.
Estimated ovulation
Mon, Sep 28, 2026
- Estimated fertile window
- Wed, Sep 23, 2026 to Mon, Sep 28, 2026
- Next period expected
- Mon, Oct 12, 2026
The two cycles after that
- Period Mon, Oct 12, 2026, fertile window Wed, Oct 21, 2026 to Mon, Oct 26, 2026
- Period Mon, Nov 9, 2026, fertile window Wed, Nov 18, 2026 to Mon, Nov 23, 2026
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About the Ovulation Calculator
The calendar method here rests on one steady feature of the menstrual cycle. The first half, from your period to ovulation, shifts from month to month and from person to person. The second half, the luteal phase from ovulation to the next period, tends to hold close to 14 days. So the calculator finds the date your next period is due, counts back 14 days to estimate ovulation, and marks the five days before it as the rest of your fertile window. Sperm can survive in the body for several days while an egg lasts about a day, which is why sex before ovulation can still lead to conception. A 35-day cycle pushes ovulation a week later than a 28-day cycle, and a 21-day cycle pulls it a week earlier. Use an average of your last several cycles rather than a single month. Stress, illness, travel and breastfeeding can all move ovulation, so in any month it may land days away from the estimate. Ovulation test strips and basal temperature charts track what your body is doing that month instead of predicting it.
Frequently asked questions
How accurate is an ovulation calculator?+
It gives a reasonable starting guess for people with regular cycles and a poor one for everyone else. Even in regular cycles, ovulation often lands a few days away from the calendar estimate. Home ovulation tests, which detect the hormone surge a day or so before an egg is released, are more precise for a given month.
Can I use this to avoid getting pregnant?+
No. A calendar estimate can be off by several days, and sperm survive long enough that a miss of a day or two matters. This page is not birth control and should not be used to decide when sex is safe from pregnancy. A doctor, nurse or family planning clinic can go over methods that are designed for that.
What if my cycles are irregular?+
The calculator still runs, but the dates mean less. If your cycle length changes by more than about a week from month to month, or regularly falls outside 21 to 35 days, date-based estimates are unreliable and it is worth mentioning to a clinician. Tracking signs such as temperature or ovulation test results will tell you more.
When is the best time to have sex if we are trying to conceive?+
Within the fertile window, and especially in the two or three days leading up to and including ovulation. Because the estimate can drift, many people aim for every day or two across the whole window rather than a single day.
What is the luteal phase and should I change it?+
It is the time from ovulation to the start of your next period. For most people it sits near 14 days, which is why that is the default. If you have charted your temperature or used ovulation tests for a few months and know yours is, say, 12 days, enter that for a closer estimate.
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