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Ovulation Calculator

Enter the first day of your last period and your cycle length; we estimate your ovulation day, your most fertile window and your next period date.

These results are estimates; they vary with irregular cycles and should not be used as a method of contraception.

How it is calculated

The tool assumes ovulation happens about 14 days before the next period. ovulation = first day of last period + (cycle length − 14); the fertile window runs from 5 days before ovulation to 1 day after it; next period = first day of last period + cycle length. Cycle length is counted from the first day of one period to the first day of the next. Values outside 20–45 days (or a missing value) are replaced by 28. This is a calendar estimate for regular cycles; it does not detect actual ovulation and is not a contraceptive method.

Worked example

First day of the last period is 1 March 2026 and the cycle is 30 days. Ovulation = 1 March + (30 − 14) = 16 days, which gives 17 March 2026. The fertile window is 5 days before to 1 day after: 12 March 2026 – 18 March 2026. The next period is expected 1 March + 30 days = 31 March 2026.

Try this example →

Common mistakes

  • Entering the date the period ended instead of the first day of bleeding.
  • Counting cycle length from ovulation or to the last day of bleeding; it runs start to start.
  • Assuming ovulation is always on day 14; with a 30-day cycle the estimate is day 17.
  • Relying on the estimate with irregular cycles, where the actual date can shift by many days.
  • Using the result as contraception or as proof of infertility; it is only a rough guide.

About this calculator

This calculator estimates the days in a menstrual cycle when conception is most likely, working backward from the typical length of the luteal phase, which tends to be more consistent than the first half of the cycle, and counting forward from the first day of your last menstrual period, with the cycle length you enter adjusting the estimate. The result is a fertile window rather than a single guaranteed day, since ovulation timing can shift from cycle to cycle.

It is used by people trying to conceive who want to time intercourse around the most fertile days, as well as by people tracking their cycle for general awareness or to anticipate symptoms. Because it relies on average patterns, it tends to work best for those with fairly regular cycles.

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