Period Regularity and Cycle Variability Calculator
Compare recent period start dates to describe your average cycle length and how much it varies.
Your details
Enter only the information needed for this estimate.
Complete the fields and select “Calculate my cycle variability” to see your estimate.
Your recent cycle pattern
These figures describe the complete cycle intervals between the dates you entered.
Variation can have many explanations. These numbers describe a pattern, but they do not diagnose perimenopause or any health condition.
Make it useful beyond today
Notice the patterns that matter to you
Use Femy’s daily guidance to support your energy, sleep, and stress patterns.
What this calculator measures
A menstrual cycle runs from the first day of one period to the first day of the next. The calculator measures each complete interval, then reports its average, median, range, and population standard deviation.
It also shows the largest difference between two consecutive cycle lengths. Tracking several dates can make it easier to discuss a changing pattern with a healthcare professional.
How to use the result
Keep recording period start dates using the same definition of day one. Look at the pattern over time alongside symptoms, medications, contraception, pregnancy and postpartum changes, stress, sleep, and illness.
Limits of this estimate
- Four dates provide only three complete cycles, so a short history can be strongly affected by one unusual month.
- The calculation does not assess bleeding volume, pain, ovulation, fertility, or the cause of a changing cycle.
- Hormonal contraception, pregnancy, postpartum recovery, breastfeeding, illness, and reproductive ageing can change cycle patterns.
Common questions
How should I use this estimate?
Use it as a planning starting point, then compare it with your own experience and any guidance from a qualified health professional.
Is this medical advice?
This calculator is an educational description of dates you enter. It does not determine whether a cycle is healthy, diagnose perimenopause, or replace individual medical advice.
What happens to what I enter?
The calculator uses the values in your current browser session to produce the result. It does not save them in browser storage.