How ovulation prediction works
Ovulation is the release of an egg from an ovary, usually once per menstrual cycle. The days around it matter because sperm can survive in the reproductive tract for up to about five days, while an egg remains viable for roughly 12 to 24 hours after release. That biology creates a fertile window of about six days: the five days before ovulation and the day of ovulation itself, with this calculator also showing the day after as the end of its displayed window, since exact timing within a day is unknowable from dates alone.
Calendar prediction works backwards from the next period rather than forwards from the last one. The second half of the cycle, the luteal phase between ovulation and the next period, is fairly consistent for many people at around 14 days, while the first half varies more. So estimated ovulation is your expected next period minus the luteal phase, which is the same as last period plus cycle length minus luteal phase. This page lets you change the luteal length if you know yours differs. All of this is an estimate for general information only; it is not medical advice, and it is not a contraception method.
The calculation used here
With the defaults, that is the first day of the last period plus (28 − 14) = 14 days. The fertile window runs from five days before ovulation through one day after it. The next period is estimated as the last period plus one full cycle length, and the following ovulation repeats the same offset from that date. Dates are calculated in UTC to avoid timezone drift.
Why real cycles move around
Even people with usually regular cycles ovulate earlier or later sometimes. Stress, illness, travel across time zones, significant weight change, intense training and simply natural variation can all shift a cycle by several days, and some cycles have no ovulation at all. A calendar estimate cannot see any of that. Signs such as changes in cervical mucus, basal body temperature patterns and ovulation test kits respond to your actual body in the current cycle and are used alongside, or instead of, calendar math by people trying to conceive or to understand their cycles.
Worked example
Last period starts September 15, 2026, with a 28-day cycle and a 14-day luteal phase. Ovulation = Sep 15 + (28 − 14) = September 29, 2026. The fertile window runs from five days before to one day after: September 24 to September 30, 2026. The next period is expected on Sep 15 + 28 days = October 13, 2026, and the following ovulation estimate is October 27, 2026.
How to use this calculator
- Enter the first day of your last period.
- Enter your average cycle length in days, counting from the first day of one period to the first day of the next.
- Adjust the luteal phase length if you know yours is not 14 days.
- Read your estimated ovulation day and fertile window.
- Check the day-by-day fertile window table below, and remember the estimate can shift by several days.
Important limits of calendar prediction
This method should never be relied on to prevent pregnancy. Sperm survival, cycle variability and the difference between an estimated and an actual ovulation day make calendar-only prediction too unreliable for contraception, and no statement on this page changes that. Use a recognised contraception method and professional guidance for that purpose.
Talk to a qualified health professional if your cycles are very irregular, frequently shorter than 21 or longer than 35 days, absent, or very painful, or if you have been trying to conceive for a year without success (six months if you are over 35), or sooner if you have any concern. Those are situations where personal assessment matters far more than a date estimate. This calculator provides general cycle information only and is not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.