Find your fertile window.
Enter the first day of your last menstrual period and your average cycle length to see your estimated ovulation date, your 6-day fertile window, when your next period is due and the earliest day a pregnancy test becomes reliable.
Your cycle
The first day of your most recent menstrual period.
Your average menstrual cycle length in days.
Ovulation typically happens 14 days before your next period, so the calculator adds your cycle length minus 14 days to the first day of your last period. The fertile window covers the 5 days before ovulation plus ovulation day itself. Results update as you type.
What your dates mean
Your cycle on a timeline
Key dates in this cycle
Your next three cycles
Every row is projected from your last period date and cycle length. The highlighted row is the cycle you are in right now, if today falls inside one of them.
| Cycle | Period starts | Fertile window | Ovulation |
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Ovulation and fertility, explained
What is Ovulation?
Ovulation is the release of a mature egg from the ovary, which then travels down the fallopian tube where it can be fertilized. It typically occurs 14 days before the start of your next period. The egg survives 12-24 hours after release, making timing crucial for conception.
Your Fertile Window
The fertile window spans approximately 6 days, the 5 days before ovulation and the day of ovulation itself. Sperm can survive in the female reproductive tract for up to 5 days, so intercourse in the days leading up to ovulation can result in pregnancy. The highest conception rates occur 1-2 days before ovulation.
Signs of Ovulation
Physical signs include changes in cervical mucus (becomes clear and stretchy like egg whites), a slight rise in basal body temperature (0.4-1.0°F), mild pelvic pain or cramping (mittelschmerz), increased libido, and breast tenderness. Ovulation predictor kits detect the LH surge 24-36 hours before ovulation.
Factors Affecting Ovulation
Stress, illness, weight changes, excessive exercise, travel, and hormonal imbalances can affect ovulation timing. Polycystic ovary syndrome (PCOS) is a common cause of irregular ovulation. If you have cycles shorter than 21 days or longer than 35 days, consult a healthcare provider about potential ovulation issues.
Common questions
When do I ovulate?
Most women ovulate 14 days before their next period, not 14 days after their last period. For a 28-day cycle, this is day 14. For a 30-day cycle, it is day 16. For a 26-day cycle, it is day 12. This calculator adjusts for your cycle length.
How long is the fertile window?
The fertile window is approximately 6 days long, the 5 days before ovulation plus the day of ovulation. The highest probability of conception is during the 2 days before and the day of ovulation. Having intercourse every 1-2 days during this window maximizes chances.
Can I ovulate twice in one cycle?
It is possible to release two eggs within 24 hours of each other (which can result in fraternal twins), but you cannot have two separate ovulation events days apart in the same cycle. After ovulation, progesterone levels rise and suppress further egg release.
Does cycle length affect fertility?
Cycle length itself does not determine fertility, but very short (under 21 days) or very long (over 35 days) cycles may indicate ovulation problems. The most important factor is whether ovulation occurs regularly. Tracking cycles and ovulation signs helps identify patterns.
This calculator gives a calendar-based estimate, not a medical diagnosis. Real ovulation timing varies from cycle to cycle, even in women with regular periods, and stress, illness, travel or hormonal changes can shift it by several days. This tool must never be used as a method of contraception. If you are trying to conceive, combine it with ovulation predictor kits or basal body temperature tracking, and if your cycles are shorter than 21 days, longer than 35 days or very irregular, talk to your gynecologist or a fertility specialist.