Menstrual Cycle Basics: How to Count Cycle Days
Educational information · Not independently reviewed by a medical professional
Day 1 of a menstrual cycle is the first day of menstrual bleeding. A cycle runs until the day before the next period begins. Cycle length is the number of days between the start dates of two periods.
Why cycles vary
Cycle length is not always identical month to month. Age, health conditions, stress, and other factors may affect timing. An average is a useful description of previous cycles, not a promise about future dates.
How our calculator works
You can enter a cycle length manually, or enter four consecutive period start dates. Four dates produce three intervals; the calculator averages these three intervals and rounds to the nearest whole day.
When to seek guidance
Consult a qualified healthcare professional about persistent cycle changes, unusual bleeding, or concerns about your health.