A normal menstrual cycle lasts between 21 and 35 days, counted from the first day of one period to the first day of the next. The bleeding itself typically lasts two to seven days. While 28 days is often cited as the standard, most people find their cycles vary by a few days from month to month, and that’s completely normal.
What Counts as Day One
Your cycle starts on the first day of full bleeding, not spotting. From there, the count continues until the day before your next period begins. So if you start bleeding on March 1 and your next period arrives on March 29, that’s a 28-day cycle. If it arrives on April 2, that’s a 32-day cycle. Both fall within the normal range.
The key number to pay attention to isn’t whether your cycle is exactly 28 days. It’s whether your cycle length stays reasonably consistent. A cycle that’s usually 30 days but occasionally 33 or 27 is normal. A cycle that swings from 25 days to 40 days regularly is worth investigating.
The Four Phases of Your Cycle
Your menstrual cycle has four distinct phases, each driven by different hormones.
Menstrual phase (days 1 through 5, roughly): This is your period. The lining of your uterus sheds because no pregnancy occurred. Flow can range from light to heavy, and total blood loss for the entire period is typically under 60 milliliters, or about four tablespoons. Losing more than 80 milliliters is considered excessive bleeding.
Follicular phase (overlaps with your period and continues until ovulation): Rising estrogen levels cause the uterine lining to thicken again, building a fresh layer. At the same time, follicle-stimulating hormone triggers your ovaries to develop small fluid-filled sacs, each containing an egg.
Ovulation (around day 14 in a 28-day cycle): A surge of luteinizing hormone causes one ovary to release a mature egg. This is your most fertile window. If your cycle is shorter or longer than 28 days, ovulation shifts accordingly, so it doesn’t always land on day 14.
Luteal phase (from ovulation until your next period): Progesterone rises to prepare the uterine lining for a potential pregnancy. If the egg isn’t fertilized, progesterone drops, and the lining begins to break down. This phase is fairly consistent at about 14 days for most people, which means variation in cycle length usually comes from the follicular phase being shorter or longer.
How Flow Volume Varies
Most periods produce less blood than people expect. Under 60 milliliters across the entire period is considered normal. Between 60 and 100 milliliters is moderately heavy, and anything over 100 milliliters is excessive. A practical way to gauge this: if you’re soaking through a pad or tampon in an hour or less, or if you’re passing blood clots larger than a quarter, your flow is heavier than typical.
Flow usually isn’t the same each day. The first two or three days tend to be heaviest, then taper off. The color can range from bright red to dark brown, and both are normal. Brown blood is simply older blood that took longer to leave the uterus.
Normal Looks Different at Different Ages
Teenagers have a wider range of normal. In the first few years after a first period, cycles can run anywhere from 21 to 45 days. This happens because the hormonal system that controls ovulation is still maturing, and many early cycles don’t involve ovulation at all. By the third year after a first period, 60 to 80 percent of cycles settle into the adult range of 21 to 34 days.
For adults in their 20s and 30s, cycles tend to be the most predictable. This is when the 21-to-35-day window applies most reliably.
In your 40s, things start shifting again. Perimenopause, the transition toward menopause, brings changes that can feel like a return to the unpredictability of adolescence. Cycles may get shorter, then longer. Flow might be heavier some months and barely there others. You may skip periods entirely. If the gap between your periods consistently changes by seven days or more compared to your usual pattern, you’re likely in early perimenopause. Going 60 or more days between periods suggests late perimenopause. Once you’ve gone a full 12 consecutive months without a period, you’ve reached menopause.
Signs Your Cycle May Not Be Normal
Some variation is expected, but certain patterns warrant attention:
- Cycles shorter than 21 days or longer than 35 days (or 45 days for teens)
- Bleeding that lasts longer than seven days
- Missing three or more periods in a row when you’re not pregnant or breastfeeding
- Cycle length that varies by more than nine days from one month to the next
- Severe pain, nausea, or vomiting during your period
- Spotting or bleeding between periods or after sex
- No period for 90 days or more (outside of pregnancy, breastfeeding, or menopause)
Any of these can point to conditions like hormonal imbalances, thyroid problems, polycystic ovary syndrome, or structural issues in the uterus. They’re not always serious, but they’re worth getting checked.
How to Find Your Own Normal
The ranges above are population averages. What matters most is establishing your personal baseline, because a shift from your own pattern is more meaningful than whether you match a textbook number. Tracking your cycle for several months gives you useful data. You can use an app or a simple calendar, and the most helpful things to record are the start date of each period, how many days bleeding lasts, flow heaviness, and any symptoms like cramps, headaches, mood changes, or breast tenderness.
Over time, patterns emerge. You’ll learn your typical cycle length, when you tend to feel your best, and when PMS symptoms usually appear. That information is useful for planning, for understanding shifts in your energy and mood, and for giving your healthcare provider specific details if something changes. A vague “my periods are irregular” is much harder to evaluate than “my cycles have been 29 to 31 days for two years, and the last three were 22, 38, and 26.”