Ovulation and fertile window calculator
Enter the first day of your last period and your cycle length — the calculator estimates your likely ovulation day, fertile window (the days with the highest chance of conception) and your next period date. It’s a calendar-method guide, not an exact day.
Calculate ovulation day and fertile window
Enter the date of your last period — the result appears instantly.
How ovulation is estimated
Calendar method: ovulation ≈ 14 days before the next period; the fertile window is 5 days before and 1 day after.
| Event | When |
|---|---|
| Fertile window | 5 days before ovulation |
| Ovulation | ≈ mid-cycle (14 days before period) |
| After ovulation | ~1 more fertile day |
How the ovulation day is calculated
Ovulation — the release of an egg — happens about 14 days before the next period starts, regardless of cycle length (the luteal phase is relatively constant). So with a 28-day cycle ovulation is around day 14, with a 32-day cycle around day 18. The calculator counts back from your expected next period, not from a fixed “mid-cycle”.
If you know your luteal phase length (from tests or tracking), enter it — the estimate becomes more accurate than the standard 14 days.
The fertile window
The fertile window is when conception is most likely: about 5 days before ovulation and 1 day after. That range exists because sperm can survive in the body for up to 5 days and the egg for about a day. The highest chance is on the day of ovulation and the 1–2 days before it.
Why it’s a guide, not an exact day
The calendar method assumes a regular cycle and stable ovulation, but in practice the ovulation day shifts with stress, illness and hormonal changes. It’s pinned down more accurately with LH tests, basal body temperature or ultrasound monitoring. For contraception the calendar method is unreliable.
Check the hormones that drive your cycle
Upload your tests (estradiol, LH, FSH, progesterone) — AI explains the values and what affects ovulation.
This calculator is for reference and information only and gives a calendar-method guide. It’s unreliable for contraception; the exact ovulation day is determined by tests, ultrasound and a doctor.