Why irregular cycles need a different approach
A single fixed cycle length doesn't capture the reality of irregular periods. This calculator averages your last several cycle lengths and widens the predicted period and fertile window into a range, giving you a more honest estimate than a single date ever could.
When to talk to a doctor
If your cycles are consistently shorter than 21 days, longer than 45 days, or your period disappears for three or more months without pregnancy, it's worth checking in with a healthcare provider.
Frequently asked questions
What counts as an irregular cycle?+
A cycle is generally considered irregular if its length varies by more than about 7–9 days from month to month, or falls outside the typical 21–35 day range for adults.
How does this calculator handle irregular cycles?+
Toggle "My cycles are irregular" above and enter up to six of your recent cycle lengths. We average them to predict your next period, and show a range from your shortest to longest recent cycle instead of one fixed date.
What causes irregular periods?+
Common causes include stress, significant weight changes, intense exercise, hormonal conditions like PCOS or thyroid disorders, perimenopause, and certain medications. Persistent irregularity is worth discussing with a doctor.