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What this calculator does
Predict a likely next period date from the last period and cycle length.
How to use this calculator
- Enter the start date and typical cycle length.
- Review the calculation assumptions before using the result.
- Use the result for calendar planning.
How to calculate
Enter the values requested by the period calculator.
Worked example
With the example values shown above, the next period starts is 2026-01-29, and the next period ends is 2026-02-02, and the estimated ovulation is 2026-02-12.
Understanding your result
Use the result for calendar planning.
- Irregular cycles reduce prediction accuracy.
- Double-check values and units before calculating.
This calculator is educational and is not financial or medical advice.
Frequently asked questions
What does this period calculator do?
Predict a likely next period date from the last period and cycle length.
How do I use this period calculator?
Enter the start date and typical cycle length.
What does the result mean?
Use the result for calendar planning.
What should I check before relying on it?
Irregular cycles reduce prediction accuracy.
How far in advance can I predict future periods?
The calculator projects forward using your average cycle length, but the further out the prediction, the more it can drift from your actual cycle, especially if your cycle length varies month to month.
What if my cycle length varies each month?
Enter your typical or average cycle length — predictions will be less precise for months where your actual cycle runs shorter or longer than that average.