Menstrual Cycle Length Calculation Guide

Determining the length of your menstrual cycle (period cycle) can be helpful for many reasons. For example, if you plan to take ovulation tests, you will often need to calculate your cycle length to help you determine the time to start testing.

In this article, you’ll learn how to count your period cycle length.

Tools Needed to Count Menstrual Cycle Length

• Calendar (to count your cycle)
• Calculator (to track the length of your cycle for at least three months)
• Dates of when your recent periods started (preferably at least 3 cycles worth of dates, which you will average together)

Here are some examples of recent period start dates:

• Period started: October 31, 2022
• Next period started (November 29, 2022)…which gives us one month
• Then the next period started (December 29, 2022)…which gives us two months
• And finally the next period started (January 27, 2023)….which gives us three months

You could go beyond that to six month’s worth of period dates or even a year and then average it out. The more dates you use, the better; but three months will work just fine to determine your average menstrual cycle length.

How to Calculate Period Cycle Length (Menstrual Cycle)

To calculate menstrual cycle length, you’ll start with the first month you’re using for your period dates, and look at the day your period started. This first day that your period started is counted as cycle day 1.

Note: the day you count as the actual day your period starts is the day you had an actual flow of menstrual blood, NOT just spotting.

Once you have cycle day 1, you keep counting up until the day BEFORE your next period started, because this is the LAST day of that cycle.

Example: The first month your period started was on October 31, 2022, and your next period was November 29, 2022.

In this example above, we’ll count October 31 as cycle day one, and keep counting to include November 28, 2022 (the day before the second month’s period that started on November 29). This will give us the cycle length for that month to be 29 days.

Then count the cycle length for the next two months.

Second month: November 29-December 28: counting from November 29 day one to December 28 which is the last day of that cycle (it ends the day before the next period started): 30 days

Third month: December 29-January 26. This will give you a period cycle length of 29 days:

Calculating Menstrual Cycle Length Average

Again, here are the cycle lengths for each month we calculated:

• First month: 29 days cycle length
• Second month: 30 days cycle length
• Third month: 29 days day cycle length

To calculate menstrual cycle length average, add all three cycle lengths together (29+30+29=88). Next, we divide by the total number of months we used (3), and we get an average menstrual cycle length of 29.33 days (rounded to 29 days).

Therefore, in the example above, we had an average menstrual cycle length of 29 days.

Menstrual Cycle Questions and Answers

What should a normal menstrual cycle be? A normal menstrual cycle should last anywhere between 21-35 days, and it can vary month to month. For example, some months could be 28 days long, some may be 27 days long, or some may be 26 days long, but it’s still considered normal because it falls within 21-35 days range. The average menstrual cycle length is around 28 days.

What is considered an irregular menstrual cycle? If your cycle is less than 21 days or greater than 35 days, it is usually considered irregular. This can be happen if you are missing menstrual cycles or if the bleeding portion of your period is lasting  longer than 7 days (it should last 3-7 days with an average of 5 days). Therefore, counting your cycle length is important to help you determine if your menstrual cycles are normal.

Why do you want to know your cycle length? It can be beneficial to know the length of your menstrual cycle for several reasons. For example, it helps you determine whether your cycle is regular or not. It can also help you track when to expect your next period if you want to avoid certain situations (such as swimming) while menstruating (bleeding).

Finally, it can also help track ovulation, which tends to happen 14 days after the start of your period (for a typical 28-day cycle). Knowing the time of ovulation can help increase chances of pregnancy or could even be used as a birth control method known as the “rhythm method” to prevent pregnancy. However, keep in mind that the rhythm method is not 100% reliable, and pregnancy could still occur.