Period calculator
Enter your last period, see when the next ones are due.
How do I calculate when my next period is due?
Add your cycle length to the first day of your last period. With a 28-day cycle and a period that started September 1, the next one is due September 29.
What is a normal cycle length?
For adults, cycles typically run 21 to 35 days, counted from the first day of one period to the first day of the next (ACOG). In the first years after periods start, 21 to 45 days is typical.
How long does a period usually last?
Up to about 7 days (ACOG).
Why was my period early or late?
Cycles vary from month to month, so the dates here are estimates. Tracking a few cycles and using your average length makes them closer.
Can this tell me when I am fertile?
No — for the fertile window, use the ovulation calculator. Neither should be used as birth control.
Worked example: the same start date with two cycle lengths
Say the last period began on September 1 and lasted 5 days. With a 28-day cycle the next one starts 28 days later, on Sep 29, and runs through Oct 3 (the start date plus 4 more days). The one after that starts on Oct 27, and the third on Nov 24. If the cycle is 29 days instead, the first date moves to Sep 30 and by the third cycle the dates differ by three days: Nov 24 against Nov 27. A one-day error in the cycle length grows with each cycle you project forward, which is why the later dates are rougher than the first.
How is cycle length counted, and what is the usual range?
A cycle is counted from the first day of one period to the first day of the next, so the first day of bleeding is day 1. The U.S. Office on Women’s Health describes a typical cycle as 28 days, with a normal range of 24 to 38 days and periods still regular if they usually arrive within that range. That range is a little wider than the 21 to 35 days quoted by ACOG above; the sources define “typical” differently. For a few years after the first period, cycles longer than 38 days are common, and girls usually settle into more regular cycles within three years.
When should I talk to a doctor about my cycle?
The Office on Women’s Health advises talking to a doctor or nurse if cycles are longer than 38 days or shorter than 24 days, if you go 90 days without a period and are not pregnant or breastfeeding, or if bleeding often lasts longer than eight days. A calculator cannot tell you why a cycle changed, and it should not replace that conversation.
Common mistakes with period predictions
Counting from the last day of the previous period instead of the first day. Using one unusual cycle: if your last four cycles were 27, 30, 29, 31 days, the average is 29.3 days, so enter 29 rather than the length of a single cycle. Mixing up cycle length (start to start) with period length (days of bleeding). Expecting an exact day: the dates are estimates, and cycles vary from month to month. And relying on the dates for birth control, which they cannot provide.
Sources and related calculators
Sources: U.S. Department of Health and Human Services, Office on Women’s Health, “Your menstrual cycle” (typical cycle, normal range, how cycles change, when to call a doctor); American College of Obstetricians and Gynecologists (adult and adolescent cycle ranges quoted above). Related: ovulation calculator, due date calculator and pregnancy week calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.