Predict your upcoming period dates based on your cycle history.
Fill in the fields on the left, then press Calculate to see your result instantly. This tool provides general estimates for informational purposes and does not replace advice from your doctor or midwife.
Tracking your menstrual cycle helps predict upcoming periods, plan around your cycle for travel or events, and notice irregularities early that might be worth discussing with a healthcare provider. This calculator projects your next several expected period start dates by adding your average cycle length repeatedly from the first day of your last period, following the standard definition of cycle length as the number of days from the first day of one period to the first day of the next. A cycle length between 21 and 35 days is generally considered within the normal range for adults, according to guidelines from bodies like the American College of Obstetricians and Gynecologists, though what's "normal" for any individual is really their own typical, consistent pattern rather than a specific number within that range. Cycle length naturally varies somewhat month to month even for people with generally regular cycles — influenced by stress, illness, travel, and hormonal fluctuations — so treat projected future dates as reasonable estimates rather than guaranteed exact dates, particularly for cycles projected several months into the future.
A menstrual cycle is measured from the first day of one period to the first day of the next — not, as sometimes assumed, from the end of one period to the start of the next — and this full-cycle measurement is what most tracking apps and calculators, including this one, use as the basis for predicting future cycles.
Major medical bodies including the American College of Obstetricians and Gynecologists (ACOG) consider a cycle length anywhere from 21 to 35 days to fall within the normal adult range, a considerably wider window than the commonly assumed "textbook" 28 days. In practice, very few people have a cycle that's exactly 28 days every single month, and that number functions more as a population average used for medical calculations than as a personal benchmark to match.
What matters more for an individual than matching a specific number is consistency with their own typical pattern. Someone with a reliably regular 24-day cycle and someone with a reliably regular 33-day cycle are both within normal medical parameters; what draws more clinical attention is a meaningful, sustained change from a person's own established pattern, or cycles that vary widely and unpredictably from month to month.
Some month-to-month variation is expected even in generally regular cycles — stress, significant weight changes, travel across time zones, illness, and normal hormonal fluctuation can all shift cycle length by a few days without indicating anything concerning. Consistently large swings (commonly cited as more than 7 to 9 days of variation between your shortest and longest cycles over several months) or a sudden, unexplained departure from a previously stable pattern are the kinds of changes more worth discussing with a healthcare provider.
Because this calculator projects future dates by repeatedly adding your average cycle length, its accuracy naturally decreases the further into the future it projects, since normal cycle-to-cycle variation compounds over multiple projected cycles. Tracking your actual cycle over several months and updating the average used gives considerably more reliable near-term predictions than relying on a single reported cycle length indefinitely.
Cycle lengths between 21 and 35 days are generally considered typical, though what's normal varies by individual.
Significant irregularity is worth discussing with a doctor, as it can have underlying hormonal causes worth investigating.
A variation of a few days from cycle to cycle is common and generally not a cause for concern. Variation of more than 7-9 days between your longest and shortest cycles over several months, or a sudden significant change from your usual pattern, is generally what providers consider worth discussing rather than occasional minor shifts.