September 2026
SMTWTFS
Tap each date your period started, oldest to newest — needs at least two.
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Cycle length calculator
Work out your average cycle length and variability from past period start dates.
Estimate, not a diagnosis. This tool gives an educational estimate based on the numbers you enter. It cannot account for your individual health history — consult a healthcare professional for anything clinical.
Why this matters
Knowing your average cycle length and its variability helps interpret ovulation timing, fertility windows, and when it's worth discussing irregular cycles with a doctor.
Example
Dates 2026-01-01, 2026-01-29, 2026-02-26 → intervals 28, 28 → average 28 days.
Related reading
- Cycle variability: what the science says
- Tracking vs. understanding
- Managing PMOS with cycle awareness
- Perimenopause: what's actually changing
Frequently asked questions
How is average cycle length calculated?
From the intervals between the start dates of consecutive periods you enter — for example, three period start dates give two intervals, which are averaged to get your typical cycle length.
What counts as normal cycle-to-cycle variability?
A typical adult cycle runs 24 to 38 days, and some month-to-month variation is normal for almost everyone. A difference of more than about 7-9 days between your shortest and longest cycle is worth discussing with a doctor.
How many past cycles should I enter for an accurate average?
At least three to four period start dates (giving two to three intervals) is enough for a rough picture. More cycles give a more reliable average and a clearer sense of your real variability.