Free Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation date, your fertile window, and your next period.
This free ovulation calculator estimates ovulation as 14 days before your next expected period: enter the first day of your last period and your average cycle length, and it returns the ovulation date, the six-day fertile window ending on ovulation day, and the next period date. For a 28-day cycle starting October 1, ovulation falls around October 15 with a fertile window of October 10 through 15.
Estimates only, based on average cycle timing. Real ovulation varies cycle to cycle. This tool is not medical advice and should not be used as contraception. Talk to your clinician about fertility planning.
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How ovulation timing works
Ovulation is the release of an egg from the ovary, and it typically happens about 14 days before the next period starts, not 14 days after the last one began. That distinction matters: in a 28-day cycle the two landmarks coincide at day 14, but in a 35-day cycle ovulation lands around day 21. Counting backward from the expected next period is the more reliable anchor.
The fertile window spans about six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in fertile cervical mucus, while the egg survives only 12 to 24 hours after release. That is why the days leading up to ovulation matter more than the day after.
Cycle length varies more than most people expect. While 28 days is the textbook average, normal cycles run 21 to 35 days, and the follicular phase (period to ovulation) is the part that stretches or shrinks. The luteal phase (ovulation to next period) is steadier at roughly 12 to 14 days, which is why the calculator counts backward from the next expected period.
Calendar estimates are a starting point, not a measurement. Stress, illness, travel, and perimenopause can shift ovulation by days. People tracking precisely usually layer in observable signs: basal body temperature rise after ovulation, cervical mucus changes before it, or LH surge test strips that detect the hormone spike 24 to 36 hours ahead.
One caution this tool states plainly: calendar-based estimates must not be used as contraception. The fertile window can shift, sperm survival varies, and natural family planning methods that rely on calendars alone have high typical-use failure rates. Use reliable contraception if pregnancy is not the goal.
Ovulation questions
When do I ovulate?
Usually about 14 days before your next period starts. In a 28-day cycle that lands around day 14 of the cycle; in a 35-day cycle, around day 21. The reliable way to estimate is to count backward 14 days from your expected next period, since the luteal phase is steadier than the follicular phase.
How long does ovulation last?
The released egg survives only 12 to 24 hours, but the fertile window spans about six days: the five days before ovulation plus ovulation day. Sperm can survive up to five days in fertile cervical mucus, so intercourse in the days leading up to ovulation can still result in conception.
What are the signs of ovulation?
Common signs include stretchy, egg-white cervical mucus in the days before ovulation, a small rise in basal body temperature after ovulation, mild one-sided pelvic cramping (mittelschmerz), and a positive LH surge on an ovulation predictor test about 24 to 36 hours beforehand. No single sign is definitive on its own.
Can I get pregnant outside the fertile window?
Conception can only happen when sperm meets the egg during its 12- to 24-hour lifespan, which falls inside the fertile window. The catch is that the window itself can shift from cycle to cycle with stress, illness, or hormonal changes, so calendar estimates must never be used as contraception.