When are you most fertile?
Your fertile window is the roughly 6 days ending on ovulation day — the 5 days before, plus ovulation itself. Sperm can survive up to 5 days, while an egg lives about 24 hours, so the days leading up to ovulation are the most fertile.
Ovulation by cycle length
All figures are days after the first day of your period, assuming a typical 14-day luteal phase — the same method the calculator uses. Calendar estimates can be several days out, especially if your cycle varies, and aren't a reliable way to avoid pregnancy.
| Cycle length | Ovulation | Fertile window | Next period |
|---|---|---|---|
| 21 days | 7 | 2–8 | 21 |
| 22 days | 8 | 3–9 | 22 |
| 23 days | 9 | 4–10 | 23 |
| 24 days | 10 | 5–11 | 24 |
| 25 days | 11 | 6–12 | 25 |
| 26 days | 12 | 7–13 | 26 |
| 27 days | 13 | 8–14 | 27 |
| 28 days | 14 | 9–15 | 28 |
| 29 days | 15 | 10–16 | 29 |
| 30 days | 16 | 11–17 | 30 |
| 31 days | 17 | 12–18 | 31 |
| 32 days | 18 | 13–19 | 32 |
| 33 days | 19 | 14–20 | 33 |
| 34 days | 20 | 15–21 | 34 |
| 35 days | 21 | 16–22 | 35 |
How ovulation is estimated
Ovulation typically happens about 14 days before your next period (the luteal phase), not 14 days after your last one. So: ovulation day = last period date + cycle length − luteal phase. That's why cycle length matters so much.
When do I ovulate on a 28-day cycle?
About 14 days after your period starts, counting from the first day of bleeding. Your most fertile days are the five before ovulation plus the day after — roughly days 9 to 15 after your period starts.
When do I ovulate with a 30-day cycle?
About 16 days after your period starts, with the fertile window from about day 11 to day 17. Longer cycles push ovulation later because the phase after ovulation stays roughly 14 days.
When do I ovulate with a 35-day cycle?
About 21 days after your period starts. With long cycles it's worth confirming with ovulation test strips, since timing tends to vary more from month to month.
Is the fertile window the same every month?
No. If your cycle length varies, so does ovulation. Track a few cycles for a better average, and consider ovulation tests for precision.
Can I use this to avoid pregnancy?
Calendar estimates alone are not a reliable form of contraception. Use a proper method if you're avoiding pregnancy.