🌸 Ovulation Calculator
Estimate your ovulation date, fertile window, and next period for the next three cycles.
How the Ovulation Calculator Works
Ovulation usually happens about 14 days before the next period starts — the phase after ovulation (the luteal phase) is fairly constant, while the phase before it varies from person to person. The calculator therefore takes the first day of your last period, adds your average cycle length, and counts back by your luteal phase length (14 days unless you know yours) to estimate ovulation.
The fertile window is the day of ovulation plus the five days before it, because sperm can survive for up to about five days. These are estimates based on averages. Cycles vary from month to month, and illness, stress, travel, breastfeeding, and conditions such as PCOS can shift ovulation. Ovulation tests, basal body temperature, and cervical mucus tracking give more personal information, and a doctor can help if you are trying to conceive.
Formula
Next period = first day of last period + cycle length Ovulation (est.) = next period − luteal phase length (default 14 days) Fertile window = ovulation − 5 days to ovulation day
How to Use It
- Enter the first day of your last menstrual period.
- Enter your average cycle length (from the first day of one period to the first day of the next).
- Change the luteal phase only if you know yours (14 days is typical).
- Read the estimated fertile window and ovulation date for the next three cycles.
Frequently Asked Questions
When am I most fertile?
The fertile window is typically the five days before ovulation plus the day of ovulation, with the highest chance of conception in the two to three days leading up to and including ovulation.
Can I use an ovulation calculator as birth control?
No. Calendar-based estimates are not a reliable method of contraception because ovulation can shift unpredictably. Talk to a doctor or family planning service about effective contraception.
Does this work with irregular periods?
Not reliably. If your cycle length changes by more than a few days from month to month, date-based predictions can be well off. Ovulation predictor kits or advice from a doctor are better options.
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Formula and content last reviewed: September 2026. All calculations run in your browser — nothing you enter is sent to or stored on our servers.
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See all →✔ Reviewed by GelirLab Editöryal Ekibi · Last reviewed: September 2026 · How we check our calculators