Period Calculator

A period calculator projects your next few cycles forward using an average cycle length. It is a convenience, not a medical tool: it takes the one date you know for certain and repeats the pattern, which is genuinely useful for planning travel, appointments and events, and for knowing roughly when to expect the next one.

The menstrual cycle runs from the first day of one period to the first day of the next. A typical cycle is 28 days, but anywhere from 21 to 35 days is considered normal, and cycles between 25 and 30 days cover the majority of people. The period itself usually lasts three to seven days, lighter at the start and end. Ovulation, as described elsewhere on this site, tends to fall about 14 days before the next period rather than 14 days after the last, which is why the ovulation row shifts with cycle length.

How predictable your cycles are determines how much to trust the dates. Someone with a steady 29-day cycle who has tracked it for months can use these predictions with reasonable confidence, give or take a day. Someone whose cycles swing between 25 and 34 days should treat the output as a rough range, because a single average cannot capture that variability. Track your actual start dates for a few months and use the real average rather than a guess.

Finally, treat changes as information. Cycles that suddenly become much longer, much shorter, or stop altogether can reflect stress, significant weight change, thyroid issues, perimenopause or pregnancy. If your cycle changes in a way that concerns you, or your periods become unusually heavy or painful, that is a conversation for a doctor rather than a calculator.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.