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Period Predictor Calculator

Enter your last period start date and average cycle length to predict upcoming periods.

How it is calculated

The tool counts forward from the first day of your last period. The cycle length is rounded to a whole number of days, and each upcoming period is start + cycle × i for i = 1, 2 and 3. Estimated ovulation is start + (cycle − 14), which assumes a luteal phase (ovulation to next period) of 14 days. If you leave the cycle empty or enter a value below 20 or above 45, the engine quietly uses 28 days instead. Dates are always printed in the form Apr 9, 2026. The result is a calendar projection from averages, not a medical prediction, and a cycle that varies from month to month will drift away from it.

Worked example

The last period started on 2026-03-10 and the average cycle is 30 days. The next period is 10 March + 30 days = Apr 9, 2026. The following two are 60 and 90 days after the start: May 9, 2026 and Jun 8, 2026. Ovulation is estimated at 30 − 14 = 16 days after the start, which is Mar 26, 2026. If the real cycle is 28 days, the same start date would give Apr 7, 2026 instead, so a two-day error in the average moves the first prediction by two days and the third by six.

Try this example →

Common mistakes

  • Entering the last day of the period instead of the first. The cycle is counted from the first day of bleeding, so every predicted date will be too late by the length of the period.
  • Typing a date like 10/03/2026. It is read as the American month-first form, so it becomes 3 October, not 10 March. Use the year-first form 2026-03-10.
  • Entering a cycle outside 20 to 45 days and trusting the output. The tool silently replaces it with 28 days, so a 50-day cycle will not be reflected in the dates.
  • Entering the length of the bleeding (for example 5 days) in the cycle field. The cycle runs from one start date to the next start date, usually near 28 days.
  • Treating the ovulation date as exact. It is always the next-period date minus 14 days, so it is wrong for anyone whose luteal phase is shorter or longer, and it should not be relied on for contraception.

Sources

About this calculator

Menstrual cycle prediction works from two inputs: the start date of your last period and your average cycle length (the days from the start of one period to the start of the next, typically 21–35 days). Adding the cycle length repeatedly to the last start date projects the next several period start dates, while ovulation is generally estimated to occur about 14 days before the next period begins, in the luteal-phase model used by most cycle-tracking tools.

This calculator takes your last period start date and average cycle length and projects your next three period dates along with an estimated ovulation date. People tracking their menstrual cycle for general awareness, fertility-awareness planning, or simply to anticipate PMS and pack accordingly use it, keeping in mind that cycles vary naturally and this is a projection based on averages, not a guarantee — especially for anyone with irregular cycles.

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Comments (1)

  • Editorial team

    The dates are plain arithmetic from one start date and one average, so an irregular cycle makes the second and third dates drift further than the first. If you track a few cycles, enter their average rather than the last one. Ovulation is placed 14 days before the predicted period, which is an assumption, not a measurement, and the result is not suitable as contraception or fertility advice. If your cycles vary a lot, would a range of dates help you?

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