Calculate your average cycle length and track your period timing more easily.
* Required. Adding 3–6 total dates gives the most accurate average cycle length.
A printable 3-month tracker, daily symptom log, cycle length guide and doctor red-flag checklist — sent straight to your inbox.
Menstrual cycle length is the number of days from the first day of one period to the day before the first day of your next period. It is not the length of your bleeding — that is your "period length" or "menstrual flow duration." The full cycle includes four phases: menstrual (bleeding), follicular (egg development), ovulation (egg release), and luteal (progesterone-dominant). Tracking your cycle length helps you predict your next period, identify your fertile window, and notice changes that may warrant medical attention.
The most reliable way to calculate your average cycle length is to mark the first day of each period for 3–6 months, then find the number of days between each pair of consecutive starts. Add those cycle lengths together and divide by the number of cycles. For example:
Jan 1 → Jan 29 = 28 days
Jan 29 → Feb 25 = 27 days
Feb 25 → Mar 26 = 29 days
Average: (28 + 27 + 29) ÷ 3 = 28 days
This average becomes the basis for predicting your next period, estimating ovulation, and identifying your fertile window. This calculator does the math automatically once you enter your dates.
For most adult women, a normal cycle length is between 21 and 35 days. The average is around 28 days, but anything within that range is considered medically normal if it is consistent for you. Teenagers often have cycles between 21 and 45 days during the first few years after menarche. Perimenopausal women may see cycles shorten, lengthen, or become irregular as hormone levels shift.
Cycle length falls into three broad buckets. Where you sit tells you something about how your hormones are behaving month to month.
Short cycle
Under 21 days
Often a short follicular phase or a luteal phase under 10 days. Common with perimenopause, thyroid issues, or high stress.
Normal cycle
21–35 days
Regular ovulatory cycles for most adult women. Small month-to-month shifts of 1–4 days are completely normal.
Long cycle
Over 35 days
Delayed or missed ovulation. Common with PCOS, thyroid imbalance, breastfeeding, or coming off hormonal birth control.
A little variation is normal. These patterns are worth bringing up with a healthcare provider:
These can point to conditions like PCOS, endometriosis, thyroid disorders, fibroids or hormonal imbalance — most of which are very treatable once identified.
Once you know your average cycle length, predicting your next period is straightforward: add that number of days to the first day of your last period. If your average is 29 days and your last period started on March 1, your next period is predicted for March 30. For the most accurate prediction, use an average calculated from at least 3 months of data. This calculator automatically computes your average and projects your next period date.
A printable 3-month tracker, daily symptom log and doctor red-flag checklist — designed for the calculator you just used.
Ovulation typically happens 14 days before your next period, not 14 days after your last one. This means:
The fertile window spans roughly 6 days — the 5 days before ovulation plus ovulation day. Sperm can survive up to 5 days in the reproductive tract, while the egg lives about 12–24 hours after release. This calculator identifies your estimated fertile window based on your average cycle length and last period date.
These two numbers are often confused but measure different things:
A 5-day period inside a 28-day cycle is completely normal. When tracking, log both numbers separately so you can spot changes in either one.
An irregular cycle means your cycle length varies by more than 7–9 days from month to month, or repeatedly falls outside the 21–35 day range. Common causes include:
If your cycles are irregular, this calculator still works — enter as many recent period start dates as you can and use the average as a rough guide rather than a precise prediction. For ongoing irregularity, talk to a healthcare provider.
Consult a healthcare provider if your cycles are consistently shorter than 21 days or longer than 35 days, if they vary by more than 7–9 days month to month, if you miss three or more consecutive periods (and are not pregnant), if you experience very heavy bleeding (soaking through a pad or tampon in under an hour), or if you have severe pain, clots larger than a quarter, or bleeding between periods. These can signal underlying conditions like PCOS, thyroid disorders, fibroids, or hormonal imbalances that are treatable once identified.
Your menstrual cycle length is the number of days from the first day of one period to the first day of the next. A typical cycle is 21–35 days, with 28 days as the textbook average.
Enter two or more recent period start dates and the calculator counts the days between them. Averaging 3–6 cycles smooths out month-to-month variation.
Your average cycle length is also used to project your next period (last period + average length) and your ovulation date (about 14 days before the next period).
Try next
Use our Ovulation Calculator to pinpoint your most fertile days based on the cycle length you just calculated.
Find your most fertile window for conception or natural family planning.
Calculate your luteal phase length and check if it's within the normal range.
Estimate your due date from your last period and average cycle length.
Discover which menstrual phase you're in today and how to support your body.
Learn what irregular cycles can mean and when to speak with a healthcare provider.
Log periods, symptoms, and moods over time for pattern insights.
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a healthcare provider with questions about your menstrual cycle, fertility, or reproductive health.
Free printable planners and calculators built for women's wellness — no account needed.
Printable cycle tracker, symptom log and period calendar — everything you need to spot your patterns in one download.
All the tools and guides in this cluster, in one place.
Predict your next period, fertile window, and phases.
OpenFind your average cycle length.
Current pageEstimate when your luteal phase starts.
OpenTrack irregular cycles and spot patterns.
OpenStep-by-step guide to counting cycle days.
OpenWhat's normal for cycle length.
OpenTiming and signs of the luteal phase.
Open