To calculate your period, count from the first day of bleeding in one cycle to the first day of bleeding in your next cycle. That number is your cycle length. The average is 28 days, but anywhere from 21 to 35 days is normal for adults.
How to Count Your Cycle Length
Day 1 is always the first day you notice actual bleeding, not spotting. Mark that date on a calendar or app, then count every day until your next period starts. That first day of new bleeding becomes Day 1 of your next cycle. The total number of days between those two Day 1s is one cycle length.
For example, if your period starts on March 3 and your next period starts on March 31, your cycle length is 28 days. If your next period after that starts on April 27, that cycle was 27 days. Both are normal. You want to track at least six cycles before drawing conclusions about your pattern, and 12 cycles gives you an even more reliable baseline.
What Your Cycle Length Tells You
Once you have several months of data, you can estimate when your next period will arrive by adding your average cycle length to the start date of your most recent period. If your last three cycles were 29, 30, and 28 days, your average is 29 days. Count 29 days from your last Day 1, and that’s your best estimate for the next one.
Your cycle has two main phases. The first half, before ovulation, can vary quite a bit in length from month to month. The second half, after ovulation, is more consistent, typically lasting 12 to 14 days (with a normal range of 11 to 17 days). This is why cycles that vary by a few days are still considered regular. The variation almost always comes from the first half.
Estimating Your Ovulation Window
If you’re tracking your period to understand fertility, ovulation typically happens about 14 days before your next period starts. For a 28-day cycle, that puts ovulation around Day 14. For a 32-day cycle, it’s closer to Day 18. Since the second half of the cycle is the more predictable part, counting backward from your expected period is more accurate than counting forward from Day 1.
Calendar math alone isn’t precise enough to pinpoint ovulation reliably. Two additional body signals can help:
- Basal body temperature: Your resting temperature rises by about 0.5 to 1°F after ovulation and stays elevated until your period arrives. To catch this shift, take your temperature every morning before getting out of bed, eating, or drinking. The catch is that this method only confirms ovulation after it has already happened, so it’s more useful for understanding your pattern over several months than for predicting a specific day.
- Cervical mucus: Just before ovulation, mucus becomes noticeably thinner, more slippery, and more abundant. After ovulation, it thickens and decreases. Two consecutive dry days (little to no noticeable mucus) generally indicate that the fertile window has passed.
If your cycles consistently fall between 26 and 32 days, days 8 through 19 of each cycle are generally considered the most fertile window.
Cycles in the First Few Years of Menstruation
Teens who have recently started their period often have longer and less predictable cycles. A range of 21 to 45 days is considered normal for adolescents, compared to the 21-to-35-day range for adults. It can take up to six years after your first period for cycles to settle into a regular pattern.
Even with irregular timing, tracking is still worthwhile. Mark each Day 1 on a calendar and note how many days of bleeding you have. Over time, a pattern usually emerges, and having that history is useful if you ever need to discuss your cycle with a doctor. The simplest method is to mark an X on each day you bleed, count the first X as Day 1, and keep counting until your next period starts.
Cycles During Perimenopause
In the years leading up to menopause, cycles often become less predictable again. You might notice cycles getting shorter or longer than usual, periods arriving at random intervals, or flow that’s heavier or lighter than what you’re used to. Some months you may skip a period entirely.
Keeping a journal of start dates, stop dates, and flow intensity during this phase helps you distinguish normal perimenopausal changes from something that needs attention. Periods arriving less than 21 days apart during perimenopause are worth bringing up with a healthcare provider to rule out other causes.
Signs Your Cycle May Be Irregular
Some variation from month to month is expected. A 28-day cycle followed by a 30-day cycle is completely normal. But certain patterns fall outside the typical range:
- Cycles consistently shorter than 21 days or longer than 35 days (45 days for teens)
- Cycle lengths that swing by more than 9 days from one month to the next, such as a 28-day cycle followed by a 37-day cycle
- Missing three or more periods in a row
- Bleeding that lasts longer than 7 days
- Going more than 90 days between periods, even once
Any of these patterns is worth investigating. They don’t necessarily signal something serious, but conditions like thyroid imbalances, polycystic ovary syndrome, and hormonal shifts can alter cycle length, and most are straightforward to manage once identified. The tracking data you’ve collected will be one of the first things a provider asks about, so having several months of records makes that conversation much more productive.