Cycle Length Calculator
Find out your average menstrual cycle length in seconds. Enter the start dates of your last 3 periods below.
Cycle Length Calculator
Enter the first day of your last 3 periods to calculate your average cycle length.
For automatic cycle length tracking across all cycles, use PeriodTracker.
Track My Cycles FreeCycle Length Calculator FAQs
How do I calculate my menstrual cycle length?
Count the number of days from the first day of one period to the first day of the next period. For example, if your last period started July 1 and the previous one started June 4, your cycle length is 27 days. Our calculator does this automatically when you enter your period start dates.
What is a normal cycle length?
A normal menstrual cycle length ranges from 21 to 35 days, with 28 days being the commonly cited average. Research shows that only about 13% of cycles are exactly 28 days — most people's cycles vary within the normal range and cycle to cycle.
Why does my cycle length change every month?
Cycle length naturally varies due to stress, illness, travel, significant weight changes, intense exercise, and hormonal fluctuations. Minor variations of ±2–4 days are normal. Variations greater than 7–9 days from cycle to cycle may warrant medical evaluation.
How many cycles should I track for an accurate average?
Tracking 3–6 consecutive cycles provides a reasonable average. The more cycles you log, the more accurate and personalized your average becomes. PeriodTracker automatically calculates this from your logged period history.
Does my cycle length affect when I ovulate?
Yes. Ovulation typically occurs approximately 14 days before your next period. So if your cycle is 28 days, ovulation is around Day 14; if 32 days, around Day 18. Knowing your cycle length helps estimate your ovulation date and fertile window.