Enter your last period date to see your fertile window and ovulation calendar.
How Ovulation and Fertile Window Work
Ovulation usually happens about 14 days before your next period starts, so in a 28-day cycle it falls about 14 days after the first day of your last period. Your fertile window is the 5 days before ovulation plus ovulation day. The calculator above finds those dates for your cycle length and luteal phase and maps the next three months.
Ovulation occurs approximately 14 days before your next period, not 14 days after your last period, unless your cycle is exactly 28 days. For a 28-day cycle, ovulation falls around day 14. For a 32-day cycle, it falls around day 18.
The fertile window spans the 5 days before ovulation plus the day of ovulation itself: 6 days total. Sperm can survive 3 to 5 days in the reproductive tract, while an egg lives only 12 to 24 hours after release. The highest probability of conception occurs in the 2 days before ovulation and on ovulation day itself.
Signs of Ovulation
Basal Body Temperature (BBT)
Irregular Cycles
Luteal Phase
Ovulation Day by Cycle Length
With the average 14-day luteal phase, ovulation comes cycle length minus 14 days after your period starts. The dates below are for a period that started on September 1, 2026.
| Cycle length | Ovulation, days after period starts | Fertile window | Ovulation | Next period |
|---|---|---|---|---|
| 21 days | 7 | Sep 3 to Sep 8 | Sep 8 | Sep 22 |
| 24 days | 10 | Sep 6 to Sep 11 | Sep 11 | Sep 25 |
| 26 days | 12 | Sep 8 to Sep 13 | Sep 13 | Sep 27 |
| 28 days | 14 | Sep 10 to Sep 15 | Sep 15 | Sep 29 |
| 30 days | 16 | Sep 12 to Sep 17 | Sep 17 | Oct 1 |
| 32 days | 18 | Sep 14 to Sep 19 | Sep 19 | Oct 3 |
| 35 days | 21 | Sep 17 to Sep 22 | Sep 22 | Oct 6 |
If you know your luteal phase is shorter or longer than 14 days (from temperature charting, for example), set it in the calculator. A 12-day luteal phase in a 28-day cycle moves ovulation 2 days later, to 16 days after your period starts.
Chance of Pregnancy by Day of the Fertile Window
In the landmark study of 221 women trying to conceive (Wilcox and colleagues, New England Journal of Medicine, 1995), pregnancy only followed sex in the 6 days ending on ovulation day. The chance per act of intercourse rose toward ovulation:
| Day of sex | Chance of conception |
|---|---|
| 5 days before ovulation | 10% |
| 4 days before | 16% |
| 3 days before | 14% |
| 2 days before | 27% |
| 1 day before | 31% |
| Ovulation day | 33% |
The practical takeaway: you do not need to hit ovulation day exactly. Sex every 1 to 2 days from the start of the window covers the best days even if ovulation comes a little earlier or later than predicted.
Pinning Down Ovulation More Precisely
LH tests
Ovulation predictor kits pick up the LH surge 24 to 36 hours before ovulation. A positive test means the next day or two are your best days. Start testing a few days before the window in the table above.
Basal body temperature
Your waking temperature rises by about 0.4°F to 0.9°F (0.2°C to 0.5°C) after ovulation. It confirms ovulation after the fact, so its main use is learning your pattern over several cycles and setting the luteal phase in the calculator.
Cervical mucus
Clear, slippery, stretchy mucus like raw egg white shows up in the days before ovulation and is a good sign you are in the fertile window.
To predict your next periods over a whole year use the period calculator, and once you are pregnant the due date calculator takes over.