Track your menstrual cycle and predict your most fertile window and next period date.
Typically 21–35 days
14 days if unsure
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions or starting any diet or exercise programme.
For the best accuracy, confirm the prediction with ovulation predictor kits (LH tests) or basal body temperature tracking. This calculator is not a method of contraception.
Next period = LMP + cycle length
Ovulation ≈ next period − luteal phase (usually 14 days)
Fertile window = 5 days before ovulation through ovulation day
Earliest reliable pregnancy test ≈ the day your period is due
Ovulation usually happens about 14 days before your next period starts, because the luteal phase (ovulation to period) is fairly constant while the first half of the cycle varies. So the calculator adds your cycle length to the first day of your last period to predict your next period, then counts back 14 days (or your own luteal phase length) to estimate ovulation.
The fertile window is the six days ending on ovulation day: the five days before ovulation plus the day itself. Sperm can survive up to about five days in the reproductive tract, but the egg survives only 12–24 hours after release. The two days before ovulation and the day of ovulation carry the highest chance of conception.
Only if you have a 28-day cycle with a 14-day luteal phase. With a 32-day cycle, ovulation is around day 18; with a 25-day cycle, around day 11. Even in regular cycles, ovulation can move by several days from month to month.
It gives a reasonable estimate for people with regular cycles, but calendar predictions alone miss the actual ovulation day in many cycles. Ovulation predictor kits that detect the LH surge, basal body temperature charting and cervical mucus observation are more precise. If your cycles are irregular, rely on those signs rather than the calendar.
No. Calendar-based predictions are not reliable contraception, because ovulation can shift unexpectedly and sperm can survive for several days. Speak to a healthcare professional about effective contraception.
Most home pregnancy tests are reliable from the first day of your missed period, about 14 days after ovulation. Testing earlier can give a false negative because hCG levels may still be too low to detect.
The highest probability of conception occurs with intercourse 1–2 days before ovulation. Having intercourse every 1–2 days during the fertile window maximizes chances. Ovulation predictor kits (OPKs) detect the LH surge 24–36 hours before ovulation, giving you advance notice. Tracking cervical mucus changes and basal body temperature can also help identify your fertile window.
Did you know?
An egg survives only 12–24 hours after ovulation, but sperm can survive in the female reproductive tract for up to 5 days. This means the fertile window is approximately 6 days: 5 days before ovulation plus the day of ovulation itself.
The fertile window was scientifically established in the 1930s by Ogino and Knaus (the "rhythm method"), though their calendar-based approach was imprecise. Modern understanding of ovulation timing improved dramatically with luteinizing hormone (LH) testing in the 1980s and ultrasound follicular tracking.
The average menstrual cycle is 28 days, but normal cycles range from 21 to 35 days in adults.
Ovulation typically occurs 14 days before the next period (not 14 days after the last one), which is important for irregular cycles.
Basal body temperature rises 0.5–1°F after ovulation and remains elevated until the next period, confirming ovulation occurred.
Approximately 1 in 4 couples conceive within the first month of trying when intercourse is timed to the fertile window.