A regular period cycle lasts 21 to 35 days, counted from the first day of one period to the first day of the next. The average falls around 28 to 29 days, but cycles on the shorter or longer end of that range are completely normal. Your actual bleeding typically lasts 2 to 7 days within each cycle.
How Cycle Length Is Measured
Day one of your cycle is the first day you see actual bleeding, not spotting. You count every day from that point until the day before your next period starts. That total is your cycle length. So if you start bleeding on March 1 and your next period arrives on March 29, your cycle was 28 days.
It helps to track a few cycles rather than relying on one. Most people find their cycle varies by a few days from month to month, and that’s expected. A variation of up to about nine days between your shortest and longest cycle is still considered regular. If the gap between your shortest and longest cycles is consistently more than nine days, that points toward irregular periods.
What Happens During Those Days
Your cycle has two main halves, separated by ovulation. The first half, called the follicular phase, starts on day one of your period and lasts until you ovulate. This phase is the one that varies the most from person to person and cycle to cycle, which is why total cycle length differs so much. Someone with a 25-day cycle likely has a shorter follicular phase than someone with a 33-day cycle.
The second half, the luteal phase, runs from ovulation to the start of your next period. This phase is more consistent, averaging 12 to 14 days, with a normal range of 10 to 17 days. Because the luteal phase stays relatively fixed, most of the variation in your overall cycle length comes from how long it takes your body to ovulate in the first half.
How Cycle Length Changes With Age
Your cycle doesn’t stay the same throughout your life. A large study from the Harvard T.H. Chan School of Public Health tracked how cycles shift across the reproductive lifespan and found a clear pattern. People under 20 averaged 30.3-day cycles. By the late 30s, the average dropped to about 28.7 days. Those in their early to mid-40s had the shortest cycles, averaging around 28.2 days, before cycles lengthened again after age 50.
Regularity follows a similar arc. Cycles tend to be less predictable in the first few years after a first period, become most regular through the 20s and 30s, and then grow increasingly irregular again after age 45 as ovarian function gradually declines.
The First Few Years After Your First Period
Teens operate on a wider normal range. ACOG considers cycles of 21 to 45 days normal for adolescents, compared to 21 to 35 days for adults. The average cycle in the first year after a first period is about 32 days. The hormonal system that controls ovulation is still maturing during this time, so skipping ovulation in some cycles is common and leads to longer or less predictable timing. By the third year after a first period, 60 to 80 percent of cycles fall into the typical adult range of 21 to 34 days.
Your 40s and the Shift Toward Perimenopause
A noticeable change in your cycle length is often the first sign of perimenopause. Your period might start arriving more frequently (shorter cycles) or less frequently (longer gaps). Cycles can also become unpredictable in ways they weren’t before. This transition typically begins in the early to mid-40s, though the timing varies. These changes reflect a natural decline in ovarian function and don’t necessarily signal a problem, but a sudden shift in your pattern is worth noting.
When a Cycle Counts as Irregular
A cycle is considered irregular if it consistently falls outside the 21-to-35-day window, if you miss three or more periods in a row, or if your bleeding lasts longer than seven days. The nine-day variability rule matters too: if one cycle is 28 days and the next is 37, that kind of swing happening regularly suggests irregularity rather than normal fluctuation.
An occasional off cycle is not the same as having irregular periods. Stress, illness, travel, significant weight changes, and intense exercise can all delay ovulation in a given month, pushing that one cycle longer than usual. One odd cycle in a string of otherwise predictable ones is rarely a concern. A pattern of irregular cycles, or a sudden and lasting change from what’s been normal for you, carries more clinical significance.
Tracking Your Own Pattern
The 28-day cycle gets treated as the default, but only a fraction of people land on exactly 28 days every month. What matters more than matching a specific number is knowing your own baseline. Track the start date of each period for at least three to six cycles. Over time you’ll see your personal range, whether that’s 26 to 29 days or 31 to 34 days. Either pattern is normal as long as it stays within the broader 21-to-35-day window and doesn’t swing wildly from month to month.
A phone app works fine for this, but even a simple calendar note does the job. The goal is to notice your pattern so you can recognize when something actually deviates from it, rather than comparing yourself to an average that may not reflect your body.