The calendar method, step by step
Calculating ovulation from your cycle takes three simple steps. The calculator above does each step with your own dates, but here is the method so you can check it yourself:
- Find your next period. Add your average cycle length to the first day of your last period.
- Count back 14 days. The time from ovulation to the next period — the luteal phase — is usually about 14 days. Subtracting it gives your estimated ovulation day.
- Mark the fertile window. The five days before ovulation and the day of ovulation are when pregnancy is possible, with the highest chances in the last three of those days.
Worked examples
| Last period started | Cycle | Next period | Ovulation (−14 days) | Fertile window |
|---|---|---|---|---|
| March 1 | 28 days | March 29 | March 15 (day 15) | March 10–16 |
| March 1 | 32 days | April 2 | March 19 (day 19) | March 14–20 |
| March 1 | 25 days | March 26 | March 12 (day 12) | March 7–13 |
Notice that the day of ovulation moves with the cycle length while the time from ovulation to the next period stays the same. That is why counting back from the next period works better than counting forward from the last one.
A note on counting: cycle day 1 is the first day of bleeding, so the date 14 days after March 1 is cycle day 15. Different books number days slightly differently — what matters is that the method is consistent.
Other ways to calculate ovulation
Basal body temperature (BBT)
Take your temperature with a sensitive thermometer every morning before getting up. After ovulation, progesterone raises it by about 0.2–0.5 °C (0.3–0.9 °F) for the rest of the cycle. The rise confirms ovulation happened, and after a few months of charting you can see which cycle day you usually ovulate.
Cervical mucus
In the days before ovulation, cervical mucus typically becomes clear, wet and stretchy, like raw egg white. The last day of this fertile-quality mucus is often close to ovulation.
Ovulation tests
LH tests detect the hormone surge that comes a day or so before ovulation. See Ovulation Predictor Kits for when to start.
When the calculation is less reliable
If your cycles vary by more than about a week, are shorter than 21 days or longer than 35, or your periods are absent, a calendar calculation may not reflect when you ovulate. Talk to a doctor or nurse, especially if you are trying to conceive. For your ovulation dates months ahead, use the Ovulation Date Calculator.