PCOS AllyCycle length calculator
Enter the first day of your last few periods. See how long each cycle was, your average, and how much your cycles vary.
Period start dates
Add at least two dates.
This calculator only does the math on the dates you enter. It does not diagnose anything. If your cycles changed, stopped or worry you, talk to your doctor.
How it works
Each cycle is the number of days from one period start to the next. The calculator averages them, finds the shortest and the longest, and shows the variation: the gap between your longest and shortest cycle, which is how doctors describe how regular cycles are.
What the references say
FIGO (2018): for adults, a normal cycle comes every 24 to 38 days. Cycles count as regular when the shortest and the longest differ by up to 9 days at 18 to 25, 7 days at 26 to 41, and 9 days at 42 to 45.
ACOG: cycles are typically 21 to 35 days. It lists as abnormal cycles longer than 35 or shorter than 21 days, cycle length that varies by more than 7 to 9 days, and no period for 3 to 6 months.
2023 International PCOS Guideline: irregular cycles are normal in the first year after the first period. From 1 to 3 years after, cycles shorter than 21 or longer than 45 days count as irregular; from 3 years on, shorter than 21 or longer than 35 days, or fewer than 8 cycles a year. Any single cycle longer than 90 days counts after the first year.
Sources: FIGO, Munro et al., 2018, FIGO, 2022, ACOG, Abnormal Uterine Bleeding, 2023 International PCOS Guideline.
Questions
How do I count a cycle?
A cycle starts on the first day of your period (the first day of real bleeding, not spotting) and ends the day before your next period starts.
How many dates do I need?
At least two period start dates give one cycle. Four or more give a better picture of how much your cycles vary.
What counts as an irregular cycle?
Definitions differ. For adults, FIGO counts 24 to 38 days as normal and allows the shortest and longest cycle to differ by up to 7 to 9 days, depending on age. ACOG describes 21 to 35 days as typical. The 2023 International PCOS Guideline, from 3 years after the first period, counts cycles shorter than 21 or longer than 35 days, or fewer than 8 periods a year, as irregular.
Are my dates saved?
No. The dates stay in this page, on your device, and disappear when you close it.