Predicting your cycle from a simple pattern
Menstrual cycle prediction relies on one core assumption: that your cycle length is reasonably consistent from month to month. Given the first day of your last period and your typical cycle length, this calculator projects forward to estimate your next period, the one after that, and your likely ovulation day in between.
Why ovulation is calculated from the end, not the start
Ovulation timing is more consistent relative to the end of a cycle (the next period) than the beginning — it typically occurs about 14 days before the next period starts, regardless of how long the total cycle runs. That's why this calculator works out ovulation by counting backward from the predicted next period, rather than forward a fixed number of days from the last one.
When predictions get less reliable
Stress, illness, travel, significant weight changes, and several health conditions can all shift cycle timing in a given month, even for someone with an otherwise consistent pattern. Treat these predictions as a useful estimate to plan around, not a guarantee — and if cycles are consistently irregular, that's worth discussing with a healthcare provider.