Ovulation Calculator

First day of your last period

Cycle length (days)

Luteal phase (days)

Not sure about your luteal phase? 14 days is a typical average.

Fertile window

estimated ovulation

next expected period

Next 3 cycles

Estimates assume regular, predictable cycles and won't be precise if your cycles vary. For a clearer picture, pair this with ovulation predictor kits, cervical mucus changes or basal body temperature tracking — and check in with your doctor if you're trying to conceive without success after several months.

How this is calculated

This calculator works backward from your luteal phase — the stretch of time between ovulation and your next period, which is generally the more consistent half of a cycle. It's the phase before ovulation that varies more from person to person. So instead of just counting forward a fixed number of days from your last period, we subtract your luteal phase length from your total cycle length to estimate how many days after your last period ovulation likely happened.

Fourteen days is the commonly used default luteal phase length, but it can genuinely run anywhere from about 10 to 16 days for different people, and even vary slightly cycle to cycle for the same person. If you know your own luteal phase length — often learned through several cycles of basal body temperature charting — entering it here will sharpen the estimate considerably.

Because a formula can only work from averages, direct cycle tracking (ovulation predictor kits, cervical mucus changes, or basal body temperature) will always be more accurate than any calculation, especially if your cycles vary from month to month.

Common edge cases

Irregular cycles are the biggest limitation here — if your cycle length swings widely month to month, a single "average" cycle length won't predict any individual cycle very well. This is common with PCOS, thyroid conditions, high stress, significant weight changes, or intense exercise.

The first several cycles after stopping hormonal birth control are often unpredictable while your natural cycle re-establishes itself, so formula-based estimates tend to be less reliable during that window. Breastfeeding can suppress ovulation unpredictably as well, sometimes for many months postpartum.

Perimenopause tends to bring less predictable cycles too, as ovulation becomes more irregular in the years leading up to menopause.

Sources

The luteal-phase and fertile-window assumptions used here reflect widely cited reproductive health guidance. For more on ovulation, fertility tracking, and cycle health, see the American College of Obstetricians and Gynecologists (ACOG), Mayo Clinic, and NICHD.