Find your fertile window and most likely ovulation date. Enter your last period date and average cycle length for instant results.
Ovulation typically occurs 14 days before your next period — not 14 days after your last one. For a 28-day cycle: ovulation falls on day 14. For a 32-day cycle: ovulation falls on day 18 (32 − 14 = 18). The fertile window spans 5 days before ovulation plus the ovulation day itself, since sperm can survive up to 5 days in the reproductive tract.
How to use this calculator: Enter the first day of your last menstrual period (LMP) and your average cycle length. The calculator will show your estimated ovulation date, your fertile window, and when your next period is due.
Cycle lengths vary from 21 to 40 days and can change month to month due to stress, illness, travel, or hormonal changes. For the most reliable ovulation tracking, combine calendar calculation with basal body temperature (BBT) charting and ovulation predictor kits (OPKs), which detect the LH surge 24–36 hours before ovulation.
Ovulation typically occurs about 14 days before the next period begins — not 14 days after the last one. Calendar-based calculators subtract the luteal phase (assumed 14 days) from your average cycle length to estimate ovulation day, with the fertile window spanning roughly the five days before ovulation plus ovulation day itself.
With a regular 30-day cycle starting 1 July, the next period is due 31 July, ovulation is estimated around 17 July, and the fertile window runs from about 12 to 17 July.
How accurate are calendar estimates? Only approximate — cycle length varies, and ovulation timing shifts with stress and illness. Ovulation predictor kits and basal body temperature tracking are more precise.
Can you conceive outside the window? Sperm survive up to five days, so unprotected sex days before ovulation can result in pregnancy. This tool is informational, not contraception or medical advice.