Ovulation Calculator: When Are Your Fertile Days?

What is ovulation, and when does it happen?

Ovulation is the release of a mature egg from the ovary, and it's the most important moment of the cycle for pregnancy. It usually occurs about 14 days before the next period starts. So in a 28-day cycle that's around day 14, and in a 30-day cycle around day 16. When cycle length changes, most of the difference occurs in the days before ovulation — which is why women with longer cycles ovulate later.

Our tool adds "cycle length minus 14" days to your last period date to estimate your ovulation day.

Examples by cycle length

Your estimated ovulation day depends on your cycle length. Counting the first day of your cycle (the day your period starts) as day 1, here are a few examples; the fertile window runs from about 5 days before this day to 1 day after:

• 26-day cycle: around day 12
• 28-day cycle: around day 14
• 30-day cycle: around day 16
• 32-day cycle: around day 18
• 35-day cycle: around day 21

How are fertile days calculated?

The most fertile window starts about 5 days before ovulation and ends 1 day after it. This is because sperm can survive for several days in the female reproductive tract, while the egg can only be fertilized for about 12–24 hours after ovulation. The 2–3 days closest to ovulation are when the chance of pregnancy is highest.

Ovulation signs and tracking methods

You can support the calculated date with signs your body gives you:

• Change in cervical mucus: clear, stretchy, egg-white-like discharge
• A slight rise of about 0.2–0.5°C in basal body temperature (measured each morning before getting up)
• Over-the-counter LH (ovulation) tests: detect the hormone surge before ovulation
• In some women, mild one-sided lower abdominal pain

Limitations of the result, and when to see a doctor

The dates here are estimates. In irregular cycles, the actual ovulation day can shift noticeably from the calculation; stress, illness, travel and intense exercise can also delay ovulation. This tool should not be used as a birth control method — "safe day" calculations do not reliably protect against unintended pregnancy.

If you haven't conceived after a year of regular intercourse under age 35, or after 6 months at age 35 or older, or if your cycle is markedly irregular, it's recommended you see an obstetrician.