Menstrual cycle length calculator
Instead of guessing, enter period start dates from your app or diary to get a real number to use in pregnancy and ovulation calculators.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter period start dates (one per line).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Dates are sorted; each cycle = next start - this start. Average = sum of cycles ÷ number of cycles. Next period = last date + average rounded to the nearest day.
Example
Result: Average cycle length: 28.3 days; Shortest cycle: 28 days; Longest cycle: 29 days.
Limits and caveats
- More cycles (6 or more) give a more reliable average.
- A cycle after childbirth or stopping the pill may be unusual and skew the average.
Questions about this tool
Do I enter every bleeding day?
No, only the first day of each period, one date per period.
How much variation is normal?
A few days of difference is common. If the spread is large or cycles keep going past 35 days, talk to a doctor.
Where do I use the result?
In the ovulation, due date and safe days calculators, which all depend on cycle length.