Health
Ovulation Calculator
Enter the first day of your last menstrual period, your average cycle length, and luteal phase length. The calculator returns estimated ovulation date, fertile window, and next period date.
The first day of bleeding.
Typical 21 to 45. Default 28. · e.g. 28
Time from ovulation to next period. Typical 10 to 17. Default 14. · e.g. 14
Estimates only. Cycles vary, and this is not reliable for preventing pregnancy on its own.
Estimated ovulation
Wed, May 20, 2026
Cycle 28 d · luteal 14 d
Ovulation = next period date minus luteal phase. The fertile window covers the 5 days before ovulation through ovulation day. Cycle length and ovulation timing vary; consider tracking (basal body temperature, LH surge, cervical mucus) for more accurate windows.
Examples
LMP 7 days ago, cycle 28, luteal 14
ovulation ≈ day 14 · fertile window 5 days before to ovulation
Cycle 32, luteal 14
ovulation ≈ day 18
Cycle 26, luteal 12
ovulation ≈ day 14
How it works
Ovulation usually happens a fixed number of days before the next period. The calculator projects the next period from cycle length, then counts back by the luteal phase to estimate ovulation. The fertile window covers the 5 days before ovulation through ovulation day.
Next period · LMP + cycle length
Ovulation · next period − luteal phase
Fertile window · ovulation − 5 days through ovulation day
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Health note. Calendar-based ovulation estimates are not reliable as the only method to prevent pregnancy. People with irregular cycles, or who are trying to conceive after a year of trying (6 months if 35 or older), should speak with a clinician.
Frequently asked questions
The calculator finds the next expected period (LMP plus cycle length), then counts back by the luteal phase length (typically 14 days). Ovulation usually happens around that date. For a 28-day cycle with a 14-day luteal phase, ovulation lands near day 14 of the cycle.
The fertile window covers the 5 days before ovulation through ovulation day. Sperm can live in the reproductive tract for several days, and the egg is viable for about 12 to 24 hours, so the fertile window is about 6 days total. Conception likelihood is highest in the 2 to 3 days leading up to ovulation.
The luteal phase is the time between ovulation and the next period. It is more consistent than the follicular phase (before ovulation), typically lasting 12 to 14 days. Adjusting this in the calculator helps if your tracked luteal length differs from the default 14.
Estimates work best for regular cycles. With irregular cycles, ovulation timing varies and calendar-only estimates may be off by several days. Combine with other tracking: basal body temperature (a sustained rise after ovulation), ovulation predictor kits (LH surge), and cervical mucus changes.
No, not on its own. Calendar-based methods are not reliable as the only form of birth control. If you want to avoid pregnancy, use a method designed for that purpose, ideally chosen with a clinician.
It is a planning estimate built on average cycle and luteal lengths. Real-world ovulation can shift due to stress, illness, travel, weight changes, sleep, and hormones. Combine with body-signal tracking or a clinician's input for higher confidence.
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