How to use the ovulation calculator
- Enter the first day of your last period.
- Enter your average cycle length: the number of days from the first day of one period to the first day of the next. 28 is the textbook average, but anything from 21 to 35 is common.
- Optionally adjust the luteal phase (the days from ovulation to your next period), usually about 14.
- See your estimated ovulation day, fertile window and next period, a color-coded calendar and the dates for the next six cycles.
How ovulation is estimated
Ovulation usually happens about 14 days before the next period, not 14 days after the last one. The calculator therefore estimates ovulation as: first day of last period + cycle length − luteal phase. With a 28-day cycle that is day 14; with a 32-day cycle, day 18. The fertile window is the five days before ovulation plus the day of ovulation itself, because sperm can survive up to five days, while the egg lives for about 12 to 24 hours. The days just before ovulation carry the highest chance of conception.
Real cycles vary, even in people with regular periods, and stress, illness, travel and breastfeeding can shift ovulation. For better timing, combine this estimate with ovulation predictor kits (which detect the LH surge 24–36 hours before ovulation), cervical mucus changes or basal body temperature charts. Calendar methods are not reliable contraception. If your cycles are irregular or you have been trying to conceive for a year (six months if over 35), talk to a doctor. Your dates are stored only in this browser.
When am I most fertile?
In the two to three days before ovulation and on the day of ovulation. The calendar highlights the full six-day window.
Can I get pregnant outside the fertile window?
It is less likely but possible, because ovulation can come earlier or later than estimated. Do not rely on this calculator for contraception.
What if my cycles are irregular?
Use your average cycle length, but expect the estimate to be less accurate. Ovulation tests are much more useful for irregular cycles.