Period Calculator (Next Period & Cycle Length)

Predict your next periods and estimated ovulation for up to 12 cycles, and work out your average cycle length from past start dates.

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Results will appear here once you enter your values.

How the Period Calculator Works

A menstrual cycle is counted from the first day of one period (day 1, the first day of real flow, not spotting) to the day before the next period starts. The calculator adds your average cycle length to the first day of your last period to estimate when the next one begins, then repeats that for as many cycles as you choose. With a 28-day cycle and a last period that started on September 1, the next period is due September 29, then October 27, then November 24.

Averaging several cycles. A single cycle is a noisy guide. If you tick the history option and enter earlier start dates, the calculator measures each interval and averages them. Start dates of July 1, July 31 and September 1 give intervals of 30 and 32 days and an average of 31. When the average isn't a whole number, each future date is rounded separately so the projection doesn't drift over a year.

Estimated ovulation. The phase after ovulation (the luteal phase) is usually close to 14 days, while the phase before it varies. So ovulation is estimated at 14 days before each projected period, which is day 14 of a 28-day cycle but day 21 of a 35-day cycle.

What Counts as a Normal Cycle

ACOG describes cycles in adults as typically 21 to 35 days, with bleeding lasting up to about 7 days. In the first years after periods start, cycles of 21 to 45 days are common while the hormonal system matures. This calculator accepts 21 to 45 days for that reason.

  • Variation is normal. Most people's cycles differ by a few days from month to month, and stress, illness, travel, breastfeeding, weight change and some medicines can shift a period.
  • Big swings are worth mentioning. If your shortest and longest recent cycles differ by more than 7 to 9 days, if periods regularly come more often than every 21 days or less often than every 45, or if you bleed longer than 7 days, bring it up with a clinician. The calculator flags cycles that vary by more than 7 days.
  • Tracking over time helps. A written record of start dates is also what a clinician will ask for, and it makes the averages here more accurate.

Common Mistakes When Predicting Periods

  • Counting from the last day of bleeding. Cycles are measured from the first day of flow. Counting from the end shortens every estimate by the length of your period.
  • Using the calendar as birth control. Ovulation can move by several days, and sperm can survive up to five days, so there is no reliably safe day. This tool is not contraception.
  • Relying on one cycle. An unusually short or long month skews the prediction. Average at least three cycles.
  • Ignoring a late period. If today is past the expected date, the calculator says so. A pregnancy test is reasonable if pregnancy is possible, and repeated missed periods deserve a medical check.

Frequently Asked Questions

Common questions about the Period Calculator (Next Period & Cycle Length)

Add your average cycle length to the first day of your last period. With a 28-day cycle and a period that started March 1, the next one is expected around March 29.

Sources & References

Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign

Typical cycle length ranges (21-45 days in adolescents) and when irregular cycles warrant evaluation.
→Source: American College of Obstetricians and Gynecologists (ACOG)

Your menstrual cycle

How cycle length is counted from day 1 of a period, typical 21-35 day adult cycles, and ovulation timing.
→Source: Office on Women's Health, U.S. Department of Health and Human Services

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