Use this free online period calculator to predict your next six expected period start dates based on the first day of your last period and your average cycle length. It's a quick way to plan ahead using your own menstrual cycle history. This calculator offers general estimates for educational purposes only and does not replace advice from a healthcare professional.
How to Use the Period Calculator
Enter the first day of your last period in the "First Day of Last Period" field.
Enter your average cycle length in days (between 15 and 45) in the "Average Cycle Length (days)" field.
Click "Calculate Periods" to generate your predictions.
Review the list of your next 6 predicted period start dates in the result box.
Frequently Asked Questions
Starting from the first day of your last period, the calculator repeatedly adds your average cycle length in days to generate each subsequent date, producing a list of your next six predicted period start dates.
The calculator accepts an average cycle length between 15 and 45 days. Entering a value outside that range, or leaving it blank, will show a validation message instead of predicted dates.
You'll see an error message asking you to enter the first day of your last period, or to enter a valid date, so you can correct it and try again.
It's completely free with no sign-up required. Because it assumes a regular cycle length, actual dates may vary; the results are general, educational estimates and not a substitute for professional medical advice.