Period Calculator

Enter a few past period start dates to see each cycle's length, the average, shortest and longest cycle, and the next six expected start dates with a range. You can also check whether a trip or exam may meet an expected period.

How to use

1. Pick a "Period start date" and press "Add", or tap a day on the "Calendar" (tap again to remove it). The first dates shown are an example.
2. To enter many dates at once, open "Paste dates", paste one date per line and press "Add pasted dates".
3. With only one start date, fill in "Usual cycle length". "Period length (optional)" marks the period days.
4. Add a plan under "Check a trip or exam" to see whether it meets the expected dates. "Print year calendar" prints the year on one A4 or Letter page.

How it works

• A cycle is the number of days from one start date to the next. The next start is the last start plus the average cycle rounded to whole days.
• The range is the previous expected start plus the shortest to the longest recorded cycle.
• All results are estimates and cycles can vary. It is not medical advice.
• Records are kept in this browser only while "Save records on this device" is ticked; "Clear all records" deletes them at once.

Questions and answers

How do I calculate my cycle length?

Count the days from the first day of one period to the first day of the next. From January 31 to March 1, 2026 that is 29 days.

How is the next period date predicted?

The average cycle is rounded to whole days and added to the last start date. An average of 28.5 days adds 29 days.

Does it work with only one date?

Yes. With one start date it uses the "Usual cycle length"; from two dates on it uses your recorded average.

Which date formats can I paste?

Forms such as 2026-07-02, 2026.7.2 and 7/2/2026 are read; short dates follow the page language's order. A date without a year counts as this year, or last year if it would be in the future.