Ovulation calculator
See your estimated ovulation date and the 6 to 7 day fertile window around it. Calendar-based, free, runs in your browser, with the caveats spelled out so you know what the model can and cannot tell you.
Tell us about your cycle
30 seconds. We do not store anything on a server.
How ovulation is calculated
The standard model assumes a luteal phase of about 14 days, meaning ovulation happens roughly 14 days before your next period. This is the convention used in most clinical fertility guidance. For a 28-day cycle, ovulation is around day 14. For a 30-day cycle, around day 16. For a 32-day cycle, around day 18.
The model holds for most women with regular cycles. It is less reliable in the situations covered in the FAQ below.
Why the fertile window is 6 to 7 days, not 1
Eggs live about 24 hours after release. Sperm can survive in the reproductive tract for up to 5 days, sometimes longer. That means intercourse 4 or 5 days before ovulation can still result in conception. The fertile window is the span where viable sperm and a viable egg overlap.
- Most fertile: the 2 days before ovulation and the day of ovulation itself.
- Fertile: 3 to 5 days before ovulation; sperm is waiting when the egg releases.
- Edge of fertile: the day after ovulation. The egg is still viable for up to 24 hours.
When calendar prediction is not enough
Calendar-based ovulation prediction is best used as a planning tool, not as primary contraception or precision conception timing. For those use cases, pair it with one of:
- Basal body temperature (BBT). Tracks the progesterone-driven temperature rise after ovulation. Confirms ovulation retrospectively.
- LH testing strips. Detects the luteinizing hormone surge that triggers ovulation 24 to 36 hours later. The most direct prospective signal.
- Cervical mucus tracking. Fertile mucus is clear and stretchy in the days leading up to ovulation. Part of the sympto-thermal method.
- Fertility monitors. Combine LH and sometimes estrogen detection. Higher accuracy than calendar alone.
For irregular cycles, especially those linked to PCOS or PMOS (see the May 2026 PMOS rename), calendar prediction is not reliable. Use direct hormone detection.
Frequently asked questions
How does the ovulation calculator work?
It uses the standard luteal-phase model. Ovulation is placed 14 days before your next expected period. So for a 28-day cycle, ovulation is around day 14. For a 32-day cycle, around day 18. The model assumes a consistent luteal phase, which holds for most women within a few days.
What is the fertile window?
The fertile window is the span when conception is biologically possible. The egg lives about 24 hours after ovulation, but sperm can survive in the reproductive tract up to 5 days. So the window is roughly 5 days before ovulation through 1 day after, a 6 to 7 day span centered on the day of ovulation.
How accurate is calendar-based ovulation prediction?
Calendar prediction is reliable within 1 to 3 days for women with regular cycles. It is less reliable if cycles vary by more than 5 days, on hormonal birth control, in perimenopause, or with PCOS (now called PMOS). For trying to conceive or for natural family planning, pair calendar tracking with basal body temperature or LH testing.
Can I use this calculator for natural family planning?
Not as the sole method. Calendar-based prediction has a typical-use failure rate too high for reliable contraception. For contraception, combine calendar tracking with basal body temperature and cervical mucus monitoring (sympto-thermal method), or use a certified fertility monitor with LH testing.
What if I have irregular cycles?
Calendar prediction does not work well for irregular cycles. Track a minimum of 3 cycles to find your personal range. If your cycles vary by more than 7 days, the calendar method is not suitable. Use ovulation prediction kits (LH strips) or fertility monitors for direct hormone-based detection.