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Period Calculator

Enter the first day of your last period and your average cycle length to get predictions for your next several cycles. Quick, private, and no sign-up required.

How Period Prediction Works

Your menstrual cycle is counted from the first day of one period to the first day of the next. According to the American College of Obstetricians and Gynecologists (ACOG), a typical menstrual cycle ranges from 21 to 35 days, with 28 days being the commonly cited average. This calculator uses your reported cycle length and the date of your last menstrual period (LMP) to project when your next periods are likely to begin.

Cycle regularity varies from person to person. Factors such as stress, weight changes, travel, illness, and hormonal shifts can all cause your cycle to be shorter or longer than usual. If your cycle length varies by more than 7 to 9 days from month to month, ACOG considers this clinically irregular, and it may be worth discussing with your OB-GYN or healthcare provider. Tracking your periods over several months gives you a clearer picture of your personal pattern.

This calculator provides estimates for planning purposes and is not a substitute for professional medical advice. If you experience unusually heavy bleeding, cycles shorter than 21 days or longer than 35 days, or if your period stops for 90 days or more, contact your healthcare provider. Regular cycle tracking is one of the simplest ways to stay informed about your reproductive health.

FAQ

How accurate is a period calculator?
Period calculators are most accurate when your cycles are regular. If your cycle length is consistent within a day or two each month, predictions will be quite reliable. However, if your cycles vary significantly, the calculator gives a best estimate based on your reported average. Tracking several months of data in the Cycora app improves accuracy over time.
What if my cycle length changes every month?
It is completely normal for your cycle to vary by a few days. To get the best result, enter the average of your last three to six cycles. If your cycles swing by more than 7 to 9 days regularly, consider speaking with your OB-GYN, as this could indicate conditions like PCOS or thyroid imbalances.
Can I use this calculator if I'm on birth control?
Hormonal birth control regulates your cycle artificially, so your withdrawal bleed is not the same as a natural period. This calculator is designed for natural menstrual cycles. If you are on hormonal contraception, your pill pack or device schedule is a more reliable guide for predicting bleeding.
What is considered a normal cycle length?
ACOG defines a normal menstrual cycle as 21 to 35 days for adults and 21 to 45 days for adolescents in the first few years after menarche. Cycles shorter than 21 days or longer than 35 days in adults are considered outside the typical range and are worth discussing with a healthcare provider.

Get Smarter Period Predictions

The Cycora app learns your unique cycle patterns and sends reminders so you are never caught off guard.

Download on the App Store