Period Calculator
Enter your last period and cycle length to predict the next 6 periods.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator predicts your next 6 menstrual periods based on your last period date and cycle length. Here is how to use it:
- Enter the first day of your last period. Use the date picker to select the day bleeding started (not spotting). This is the anchor for all calculations.
- Set your cycle length. A typical cycle is 28 days, measured from the first day of one period to the first day of the next. Normal cycles range from 21 to 35 days. Enter your average if you know it.
- Set your period duration. The average period lasts 5 days, with a normal range of 2-7 days. Enter yours to see accurate end dates.
- Read your results. The calculator shows the next period date and lists all 6 upcoming periods with start and end dates.
About the Period Calculator
Menstrual cycle length is measured from the first day of one period to the first day of the next. The average is 28 days, but anywhere from 21 to 35 days is considered normal. Cycles shorter than 21 or longer than 35 days, or significant variation month to month, may warrant evaluation by a healthcare provider.
This calculator assumes a consistent cycle length. In practice, cycles can vary by a few days due to stress, travel, illness, or hormonal fluctuations. Use it as a planning guide rather than a precise tracker. For irregular cycles, a period tracking app may be more helpful.
Frequently Asked Questions
What is a normal menstrual cycle length?
A normal menstrual cycle ranges from 21 to 35 days, measured from the first day of one period to the first day of the next. The average is 28 days. Most women's cycles vary by 1-3 days month to month. Cycles consistently shorter than 21 days or longer than 35 days, or variations of more than 7-9 days between cycles, may indicate a hormonal issue worth discussing with a doctor.
Why is my period late if I'm not pregnant?
Many factors can delay a period other than pregnancy: significant stress, sudden weight changes, over-exercising, thyroid issues, polycystic ovary syndrome (PCOS), perimenopause, and certain medications. A period is typically considered late if it has not arrived within 5 days of the expected date. If your period is more than a week late and a pregnancy test is negative, consult your healthcare provider.
How accurate are period predictions?
Period predictions are most accurate for women with very regular cycles. For a cycle that varies by 2-3 days each month, predictions within 3-4 days of the actual start are typical. For irregular cycles with variations of a week or more, calendar-based predictions are less reliable. Tracking your actual period dates over 3-6 months and updating your average cycle length improves accuracy.
What is the difference between cycle length and period duration?
Cycle length is the number of days from the first day of one period to the first day of the next. It includes the days of bleeding plus the days between periods. Period duration is just the number of days of active bleeding, typically 3-7 days. For example, a 28-day cycle with a 5-day period means 23 days between the end of one period and the start of the next.