July 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.