How the prediction works
Your cycle is counted from the first day of one period to the first day of the next. The calculator simply adds your average cycle length to your last start date, repeatedly, to estimate the next few periods. A typical cycle is 21–35 days, with 28 being the average.
What affects your cycle
Cycle length can shift with stress, travel, illness, weight changes, exercise, and hormonal changes. A little month-to-month variation is completely normal. Tracking over several months gives you a more accurate personal average than any single cycle.
When to see a doctor
Consider checking in with a healthcare professional if your cycles are very irregular, suddenly change, are extremely heavy or painful, or stop unexpectedly. Track your dates over time so you have useful information to share.
Limitations
This projects forward using a single average cycle length, so its accuracy depends entirely on how consistent your actual cycles are — predictions get progressively less reliable for cycles further in the future, and considerably less reliable if your cycle length varies a lot month to month.