Period Cycle Length Calculator

Your period cycle length is the number of days from the first day of one period to the first day of the next. To calculate it manually: note the start date of one period, note the start date of the following period, then count the days between them. For example, if one period started January 1st and the next started January 29th, your cycle length is 28 days. To calculate average cycle length over several months, add up the lengths from 3 or more cycles and divide by the number of cycles. This period cycle length calculator does that math automatically when you enter your dates.

A normal menstrual cycle length falls between 21 and 35 days. The average is 28 days, but only around 15% of people actually have a 28-day cycle. Your personal normal cycle length is whatever is consistent for you. A 24-day cycle that stays at 24 days is just as normal as a 32-day cycle that stays at 32 days. What flags concern is cycles that vary by more than 7-9 days from month to month, or cycles that fall below 21 days or exceed 35 days consistently. If you have questions about your pattern, a healthcare provider can help.

You can only find out by tracking. Count from the first day of bleeding in one period to the first day of bleeding in the next. If that span is 21 days, your cycle is 21 days. If it is 28 days, it is 28 days. Most people assume they have a 28-day cycle because that is the standard used in apps and textbooks, but the range from 21 to 35 days is entirely normal. Track 3 or more cycles using this period cycle length calculator to get a reliable number. A single cycle does not tell you enough because one-time disruptions can shift timing.

For irregular periods, the method for how to calculate average cycle length is the same, but you need more data. Track your period start dates for at least 3 months, ideally 6. Add up all the cycle lengths and divide by the number of cycles. For example, if your last four cycles were 26, 31, 24, and 28 days, add those together to get 109, then divide by 4 to get an average of 27.25 days. Use that average when entering your cycle length into this irregular period cycle length calculator. Your prediction will not be exact, but it gives you a reasonable window of when to expect your next period.

Accuracy depends on how consistent your cycles are. If your period comes on the same schedule every month, predictions based on your average cycle length will be accurate within 1-2 days. If your cycle varies by a week or more each month, any prediction is an estimate. This calculator predicts your next period date based on the cycle length you enter. It cannot account for ovulation shifting early or late in any given cycle. For greater accuracy, track 6 or more cycles and use your personal average rather than the standard 28-day default.

Start by recording the first day of each period in a calendar or notebook. After 3 months, subtract each start date from the next to get individual cycle lengths. Add those cycle lengths together and divide by the number of cycles measured. This gives your average cycle length for irregular periods. For example: cycles of 23, 30, 27, and 25 days add up to 105, divided by 4 gives 26.25. Use 26 days as your working estimate. Over time, as you track more cycles, you can update that average. The more months you track, the more accurate your calculation becomes.

Both are within the normal range. A 3-week cycle (21 days) and a 4-week cycle (28 days) are both considered normal period cycle lengths. The full normal range spans 21 to 35 days. A 3-week cycle means you have more periods per year (about 17 instead of 13) but each one can be just as typical as a longer cycle. What matters is that your cycle is consistent from month to month. If your cycle recently shifted from 4 weeks to 3 weeks and stays there for several months, mention it to a doctor to check for any underlying causes.

Yes. Ovulation typically happens about 14 days before your next period, not 14 days after your last one. So to estimate ovulation from your cycle length: subtract 14 from your cycle length to find the approximate day in your cycle when ovulation occurs. For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, it is around day 18. For a 24-day cycle, ovulation is around day 10. Use our [ovulation calculator](/ovulation-calculator/) for a more detailed fertile window calculation, or the [fertility calculator](/fertility-calculator/) if you are trying to conceive. Your cycle length is the starting point for both.

Your period cycle length determines when ovulation happens, which sets your fertile window. Shorter cycles mean earlier ovulation, which means your fertile window starts sooner after your period ends. For a 24-day cycle, your most fertile days might begin just a week after your period ends. For a 35-day cycle, you might not reach your fertile days until nearly 3 weeks into the cycle. Understanding your average cycle length is the first step before using any tool that predicts fertility or ovulation timing. The [cycle phases calculator](/cycle-phases-calculator/) can break down each phase of your cycle once you know your cycle length.

See a doctor if your period cycle length drops below 21 days or exceeds 35 days consistently for several months. Also seek care if your cycle length was previously regular and has shifted by more than 7 days per cycle and stayed at the new timing for 3 or more cycles. Sudden major changes in period cycle length can sometimes indicate thyroid issues, PCOS, perimenopause, or other conditions that benefit from early diagnosis. A one-time shift is usually nothing to worry about. A pattern of changes is worth investigating.