Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your next ovulation day and the six-day fertile window when pregnancy is possible.
Estimated ovulation date
Tuesday, September 8, 2026
Fertile window opens
Thursday, September 3, 2026
Fertile window closes
Wednesday, September 9, 2026
Next period expected
Tuesday, September 22, 2026
The six days ending at ovulation form the fertile window - sperm survive up to five days, while the egg lives only 12-24 hours after release.
How to Use the Ovulation Calculator
- Enter the first day of your most recent menstrual period.
- Enter your typical cycle length (21-35 days; 28 is average).
- Review your estimated ovulation day and highlighted fertile days.
Formulas Used
Next ovulation ≈ first day of next period − 14 days (luteal phase)
Fertile window = ovulation − 5 days through ovulation + 1 day
Peak fertility: the 2-3 days ending at ovulation
Frequently Asked Questions
How accurate is a calendar-based ovulation prediction?
It assumes regular cycles and a fixed luteal phase, so it works best for women with consistent cycles. Stress, illness and travel can shift ovulation by days in any given cycle.
How long can sperm survive?
Up to five days in fertile cervical mucus. This is why the fertile window begins several days before ovulation even though the egg lives only 12-24 hours after release.
What if my cycles are irregular?
Calendar predictions become unreliable. Ovulation predictor kits (LH strips), basal body temperature tracking or ultrasound monitoring give better signals.
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