Period Calculator

A few days is inside ordinary variation, even for regular cycles. Ovulation — not the period — is what actually moves: stress, illness, travel, poor sleep, and hard training shift it, and the period follows about 14 days behind wherever it lands. A week or more past expected is the reasonable threshold for a pregnancy test (if applicable) and, if it recurs, a conversation with a doctor. One late period is a data point; a pattern is what matters.

Count from the first day of one period to the day before the next one starts — first day of bleeding to first day of bleeding — and average the last three cycles. People commonly undercount by measuring from the end of a period rather than the start. If your last three cycles were 27, 30, and 28 days, enter 28. The average matters more than any single cycle, which is why a couple of months of simply noting start dates improves every prediction here.

The next period is the dependable one; each projection after that adds another full cycle of variance, so month six is a planning approximation, not a promise. Concretely: if your cycles vary by ±2 days, the sixth prediction can reasonably drift by a week or more. That's still exactly what you need for booking trips and events around likely dates — just weight the early predictions and treat the later ones as windows rather than days.

The math is identical — apps use the same last-period-plus-average-cycle arithmetic. The difference is where your data lives. Cycle data is among the most sensitive personal information there is, several major tracking apps have been caught sharing it with third parties, and it has been subpoenaed. This page computes everything on your device and stores nothing: close the tab and the data is gone. Jot your start dates somewhere private and enter them fresh.