Ovulation and fertile days calculator

Ovulation happens, on average, about 14 days before your next period, not 14 days after the last one: that difference is the key when your cycles are not 28 days long. And the fertile window is six days: the five before ovulation plus ovulation day itself.

Enter the first day of your last period and your cycle length and you get your estimated ovulation day, the fertile window, your next period and a calendar of the next six cycles. You can adjust the luteal phase and, if your cycles are irregular, enter the shortest and longest for a wider window. Everything runs in your browser.

Ovulation is found by subtracting the luteal phase (14 days by default) from the expected next period. The five days before count as fertile because sperm can survive that long in the female reproductive tract, while the egg lives very briefly: the day after ovulation there is no fertility left.

With irregular cycles, the classic calendar method (Ogino-Knaus) subtracts 18 days from the shortest cycle for the first fertile day and 11 from the longest for the last, which gives a wider window.

When you ovulate

The egg is released once per cycle, usually about 14 days before the next period. The second half of the cycle (the luteal phase) is fairly constant; the first half is what varies and makes some cycles long and others short. That is why we count backwards from the next period.

The fertile window

Wilcox and colleagues (1995) followed cycles with hormonal measurement of ovulation: pregnancies only resulted from intercourse in the six days ending on ovulation day. Probabilities rise as ovulation approaches, and intercourse the day after did not lead to pregnancies.

Why it is only an estimate

The same group showed in 2000 that the fertile window can fall on almost any day of the cycle, even in women with regular cycles. A calendar gives an idea, but actual ovulation is only confirmed by measuring it (ovulation tests, basal temperature, cervical mucus, ultrasound).

Irregular cycles

If your cycles vary, the classic calendar method subtracts 18 days from the shortest cycle and 11 from the longest. The resulting window is wide and of little use for planning; if you are trying to conceive, ovulation tests are more precise.

Common mistakes

• Counting 14 days from the previous period instead of backwards from the next.
• Using the calendar as contraception.
• Treating ovulation as one exact day instead of a window.
• Using the length of a single cycle when they vary.
• Waiting for the window to end: the useful days are the ones before.

Limits of this calculator

• It estimates ovulation from the calendar; it does not detect it.
• The default luteal phase is 14 days; yours may differ.
• The irregular-cycle window uses the classic Ogino-Knaus rule, which is imprecise.
• It is not a contraceptive method nor a fertility diagnosis.
• It accepts cycles of 20 to 45 days and a luteal phase of 10 to 16.

Notice

This calculator is for information and is not a contraceptive method. If you have been trying to conceive for a year (six months if you are over 35), see a professional.