A typical menstrual cycle lasts 24 to 38 days, with the average across adults falling right around 28 to 29 days. To find your specific number, count from the first day of one period to the day before your next period starts. That full span of days is your cycle length.
How to Count Your Cycle
Day 1 is the first day of real bleeding, not spotting. You count forward from there until the day before your next period begins. If you start bleeding on March 3 and your next period arrives on March 31, your cycle is 28 days. The key is counting to the day before your next period, since that next bleed marks the start of a new cycle.
One cycle alone won’t tell you much. Track at least three consecutive cycles to get a reliable picture. You’ll likely notice your number shifts slightly each month, and that’s completely normal.
What Counts as Normal
Any cycle between 24 and 38 days falls within the medically accepted range for adults. A large Harvard study analyzing over 165,000 cycles found the overall average was 28.7 days, which means roughly half of all cycles are longer than 28 days and half are shorter. The “textbook 28-day cycle” is an average, not a standard you need to hit.
Your cycle can also vary from month to month and still be regular. If you’re between 26 and 41, your cycles are considered regular as long as your shortest and longest cycles don’t differ by more than about 7 days. For younger adults (18 to 25) and those approaching menopause (42 to 45), that window is slightly wider, up to 9 days of variation. So if your cycles bounce between 27 and 33 days, that’s a normal pattern, not an irregular one.
Why Your Number Changes Month to Month
Your cycle has two main halves. The first half, before ovulation, is the part that varies. It can last anywhere from 14 to 21 days depending on how quickly your body prepares and releases an egg. The second half, after ovulation, stays relatively fixed at about 14 days. This is why your cycle length shifts: you’re not ovulating on the exact same day every month.
Several things can push your cycle shorter or longer in a given month. Significant stress is one of the most common culprits, because stress hormones can delay ovulation. Gaining or losing a noticeable amount of weight also affects cycle timing. Intense exercise routines, particularly those that drop body fat very low (common in long-distance runners, dancers, and gymnasts), can lengthen cycles or cause missed periods altogether.
Cycles During the Teen Years
If you’re in your teens, your cycles will likely be longer and less predictable than adult norms. For adolescents, the normal range stretches from 21 to 45 days. The hormonal system that controls ovulation takes time to mature after your first period, which means some months you may ovulate late or not at all, producing a longer cycle.
This gradually settles. By the third year after your first period, 60 to 80 percent of cycles fall into the adult range of 21 to 34 days. If your cycles are still consistently longer than 45 days after a few years of menstruating, that’s worth bringing up with a doctor.
Cycles in Your 40s and Perimenopause
As you approach menopause, cycles start shifting again. In early perimenopause, you may notice your cycles getting slightly shorter, then occasionally much longer. Research tracking women through this transition found that the most dramatic lengthening happens in the final year before menopause, when average cycle length jumped to around 80 days. In the years before that, the changes are more subtle: averages of about 30 days four years before menopause, 35 days three years before, and 45 days two years before.
During this time, shorter ovulatory cycles and longer anovulatory ones often alternate unpredictably. A cycle of 26 days followed by one of 50 days is a common perimenopause pattern, not necessarily a sign of a separate problem.
When Your Cycle Length May Signal a Problem
Cycles consistently shorter than 24 days or longer than 38 days in adults fall outside the expected range. Cycles over 40 days have been linked to higher rates of fertility difficulties and metabolic health concerns. A sudden, persistent change in your pattern (your reliable 30-day cycle jumping to 45 days for several months, for example) is more meaningful than a single odd month.
The most useful thing you can do is track your cycles over time so you know your own baseline. A pattern that’s always been 35 days is very different from one that recently shifted from 28 to 35. Knowing your personal range gives you real information to share with a healthcare provider if something feels off.