Calculating your menstrual cycle is an easy task that can tell you a lot about your body. By looking at the number of days between the start of your periods, you can get a better idea of when you are most fertile and your overall reproductive health. In addition, tracking your flow, your symptoms, and any irregularities in your cycle can help you get more in tune with your body, and provide you with warnings of possible medical complications.
Method One of Three:
Counting Days Between Periods
Start counting on the first day of your period. To get an accurate depiction of your menstrual cycle, start counting on the first day of your period. Make a note on your calendar or in a menstrual cycle monitoring app when your period begins.
- Smartphone apps such as Clue, Glow, Eve, and Period Tracker are designed to help you monitor your menstrual cycle, ovulation, and other key points in your cycle. They can be an easy and data-driven means of monitoring your cycle length.
Count up to the day before you start your next period. Your count resets on day 1 of your menstrual cycle. This means that your count for each cycle should end on the day before your next period. Include a count for the day before your period starts, but do not include the start date of your period, even if it starts later in the day. 
- If, for example, your cycle started on March 30 and your next period came on April 28, your cycle would be March 30 to April 27, and would total 29 days.
Monitor your cycle for at least 3 months.The length of your menstrual cycle may vary from month to month. If you want an accurate depiction of your average cycle length, monitor your cycle for at least 3 months. The longer you monitor your cycle, the more representative your average will be.
Calculate your average cycle length.Find the average for the length of your cycle using the numbers you collected while counting your period. You can recalculate this every month to get a more accurate depiction of your general cycle length. Remember, though, that the average shows a trend—it doesn’t definitively represent the length of your next cycle. 
- To find the average, add the total number of days of your cycle for each month that you have monitored. Then, divide that total by the number of months you monitored. This will give you your average cycle length.
- For example, you had a 28 day cycle in April, a 30 day cycle in May, a 26 day cycle in June, and a 27 day cycle in July, your average would be (28+30+26+27)/4, equalling a 27.75 day average cycle.
Continue to track your cycle. Keep tracking your cycle every month. Even if you pass a certain target, such as getting pregnant, keeping track of your cycle throughout your life can help you know when something is off. Medical professionals often ask for information about your cycle, as well. Monitoring your periods and cycle length will help you provide the most accurate information possible. 
- If your doctor asks you the date of your last period, the answer is the first day of your last period, not the day it ended.
Method Two of Three:
Monitoring Your Period
Watch your flow. Very heavy menstrual flows can be an indication of other problems. It may even lead to its own problems, such as anemia and lethargy. While you track your cycle, keep an eye on what days your flow is heavy, normal, and light. In most cases, you don’t need to measure the quantity of blood. Instead, estimate by looking at what type of menstrual products you are using (super tampons, regular pads, etc.) and how often you need to change out those products.
- If, for example, you have to change a super tampon every hour, you may have an irregularly heavy flow.
- Keep in mind that most women will have heavier days and lighter days. It is normal to have different levels of flow on different days.
- The severity of flow varies greatly from person to person. A heavier or lighter cycle isn’t inherently problematic. Instead, watch for very heavy cycles or completely skipped periods, which may be an indicator of another medical issue.
Note changes in your mood, energy, and body prior to and during your cycle.PMS and PMDD can do anything from make you a little cranky to make it difficult to function. Knowing when these symptoms are most likely to hit can help you better plan and cope. Take note of any extreme mood changes, changes in energy level and appetite, and physical symptoms such as headaches, cramps, and breast tenderness in the days leading up to and during your cycle. 
- If your symptoms are extreme enough that they make daily functioning difficult, contact your doctor. They may be able to help you find a solution or proper management program.
- If you notice symptoms suddenly happening that you have never experienced before, such as severe lethargy, you may also want to contact your doctor. In some cases, these could be an indicator of a larger medical issue.
Seek medical help for any sudden, major changes. Different people naturally have different cycles. Your cycle isn’t problematic just because it doesn’t follow the same pattern as someone else. Sudden or major changes to your cycle, though, are often an indicator of larger medical problems. Contact your doctor or OB-GYN if your period suddenly gets very heavy or disappears completely. 
- You should also contact a medical professional if you experience severe cramping, migraines, lethargy, or depression in the days leading up to and during your cycle.
- Your doctor will be able to talk to you about your symptoms and run tests as necessary to see if changes in your cycle may be related to medical issues such as endometriosis, polycystic ovarian syndrome, thyroid disorders, or ovarian failure, among others.
Method Three of Three:
Tracking Your Ovulation by Cycle Length
Find the midpoint of your menstrual cycle. Ovulation typically occurs around the midpoint of your menstrual cycle. Count the halfway point in your average cycle to give you an idea of what the midpoint of your next cycle might be. 
- So if you have a 28 day average cycle, your midpoint would be at 14 days. If you have a 32 day average cycle, your midpoint would be at 16 days.
Add 5 days before ovulation. If you are trying to get pregnant, the 5 days before ovulation are just as important as the day of ovulation. Your chances of getting pregnant increase when you engage in sexual activity 5 days prior to ovulation, as well as the probable date of ovulation.
- Your egg can be fertilized up to 24 hours after it is released, and sperm can live in the fallopian tube for up to 5 days after sex. Having sex 5 days prior to ovulation, as well as on the day of ovulation, helps give your egg the best chance of fertilization.
Use an ovulation predictor kit if you have irregular cycles. If your cycles are irregular , monitoring ovulation by charting your cycle length might not be the most accurate. Using an ovulation predictor kit may be the most accurate method if you have irregular periods. 
- Ovulation predictor kits are available from most pharmacies and drug stores, as well as online retailers.
To calculate the length of your menstrual cycle, keep track of your period using a calendar or an app like Clue, Glow, or Period Tracker. The first day of your period is the first day in your cycle and the last day before your next period begins is the last day in your cycle. If your period started on the 1st, for example, and your next period began on the 30th, your cycle would be from the 1st to the 29th and would be 28 days long.