Free tool
Cycle length calculator
Cycle length is measured from the first day of one period to the first day of the next — not day one to the last day of bleeding. Enter three or more period start dates below to see your median, shortest and longest interval. Everything is calculated locally; the page sends nothing anywhere.
Enter the first day of each period (oldest to newest). Everything is calculated on this page — nothing is sent or stored anywhere.
What the numbers mean
Median vs a single cycle. A single cycle length tells you almost nothing on its own — one long or short cycle is common for reasons unrelated to PCOS. The median across several cycles is a much steadier signal. Browse the cycle length hub to read about specific lengths and what they tend to look like.
Irregular cycles. If your longest and shortest intervals differ by more than seven to nine days, your cycles may be irregular. The NHS has a clear overview of irregular periods and when it is worth speaking to your GP.
The app does this automatically — every period start becomes an interval, with strata, medians and prediction built in.