Enter your last period date and cycle length to find your fertile window
Ovulation typically occurs 14 days before the start of your next period, not 14 days after the start of your last period. This distinction matters for anyone with a cycle length different from 28 days. In a 28-day cycle, ovulation occurs around day 14. In a 35-day cycle, ovulation is around day 21. The formula is: Ovulation day = Cycle length − 14.
The fertile window extends from about 5 days before ovulation through 1 day after. Sperm can survive in the reproductive tract for up to 5 days, while an egg is viable for only 12–24 hours after release. This creates a roughly 6-day window each cycle when pregnancy is possible.
| Day relative to ovulation | Pregnancy probability |
|---|---|
| 5 days before | ~4% |
| 4 days before | ~16% |
| 3 days before | ~14% |
| 2 days before | ~27% |
| 1 day before | ~31% |
| Ovulation day | ~33% |
| 1 day after | ~8% |
Physical signs can help confirm ovulation: cervical mucus becomes clear, stretchy, and egg-white in consistency in the days leading up to ovulation. Basal body temperature rises by about 0.2–0.5°C after ovulation and remains elevated until the next period. Some people experience a dull ache on one side of the lower abdomen (mittelschmerz) during ovulation. Ovulation predictor kits detect the LH surge that occurs 24–36 hours before ovulation.
This calculator assumes a regular cycle. If your cycle length varies by more than 7 days from month to month, the predicted ovulation date becomes less reliable. Factors that can affect cycle regularity include stress, significant weight changes, intense exercise, illness, and certain medications. Tracking your cycle for several months gives a better average cycle length for more accurate predictions.
For conception, timing intercourse during the fertile window maximises chances — the two days before ovulation and ovulation day itself have the highest probability. For natural family planning (avoiding pregnancy), abstaining during the fertile window plus a safety margin is required, but this method has a typical-use failure rate of about 12–24% per year. It is not considered a reliable contraceptive method without proper training and consistent cycle tracking.
The two days before ovulation and the day of ovulation itself have the highest probability of conception — roughly 27–33% per cycle. The fertile window extends from 5 days before to 1 day after ovulation.
Multiple eggs can be released within a 24-hour window (which is how fraternal twins occur), but you cannot ovulate again days later in the same cycle. Once ovulation occurs, progesterone rises and prevents further egg release until the next cycle.
Ovulation itself (the release of the egg) happens within minutes. The egg remains viable for 12–24 hours after release. However, the fertile window is about 6 days because sperm can survive up to 5 days in the reproductive tract.
Cycle lengths between 21 and 35 days are considered normal and do not inherently affect fertility. Very short cycles (under 21 days) or very long cycles (over 35 days) may indicate ovulatory disorders and should be discussed with a healthcare provider.
Yes. Physical or emotional stress can delay or suppress ovulation, making your cycle longer than usual. The luteal phase (after ovulation) tends to stay consistent at about 14 days, so a delayed period usually means delayed ovulation rather than a longer luteal phase.