Monthly Period Cycle Calculator

To calculate your monthly period cycle, count the days from the first day of one period to the first day of the next. For example, if your period starts March 1 and the next one starts March 29, your cycle length is 28 days. Do this for 3 months to find your average. Then use this monthly period cycle calculator: enter your last period start date and average cycle length, and it predicts your next period date instantly.

The only way to know your monthly cycle length is to track it. Count from day one of bleeding to day one of your next period. Repeat for 3 monthly cycles and average the numbers. Many people assume 28 days because that is the textbook number, but studies show the average monthly cycle is closer to 29 days. Monthly cycles of 28 and 30 days are both completely normal. This monthly period cycle calculator works for any length between 21 and 35 days.

Both can be normal. A 21-day monthly cycle (about 3 weeks) sits at the short end of the normal range. A 28-day monthly cycle (4 weeks) is the classic average. The medical definition of a normal monthly menstrual cycle period is 21-35 days. If your monthly cycle consistently falls in that range, it is considered regular. If your monthly period arrives more often than every 21 days or less often than every 35 days, talk to your doctor.

Several factors can shift your monthly period cycle. Stress is the most common cause, as it affects the hormones that trigger ovulation. Other factors include significant weight changes (gaining or losing more than 10 pounds), intense exercise, travel across time zones, illness with fever, new medications, and changes in sleep patterns. Birth control starting or stopping also changes cycle timing. Most of these shifts are temporary. If your period is consistently irregular for more than 3 months, see a doctor.

For regular monthly cycles, this monthly period cycle calculator is accurate within 1-3 days. Accuracy depends on how consistent your monthly cycles are. If your monthly period arrives within a 2-day window each month, the prediction will be very close. If your monthly cycle varies by a week or more, the prediction gives you a reasonable window but not an exact date. Tracking for 3+ months and using your actual average monthly cycle length (not the default 28) makes this a more accurate period calculator for your body.

Yes, with a small adjustment. If your monthly cycles vary, calculate your average from the last 3-6 months. For example, if recent monthly cycles were 26, 32, and 29 days, your average is 29 days. Enter that number. The prediction will be less precise than for regular monthly cycles, but it gives you a useful window. For very irregular monthly cycles, pairing this menstrual cycle period calculator with our [ovulation calculator](/ovulation-calculator/) can help you understand your monthly cycle phases better.

Grab a calendar or open your phone. Mark the first day of your monthly period (day one of bleeding). When your next monthly period starts, count every day from the previous day one to this new day one. That total is your monthly cycle length. For example: monthly period starts January 5, next monthly period starts February 2. Count January 5 through February 1 = 28 days. Your monthly cycle length is 28 days. Repeat this for 3 months and average the results for the most accurate number to use in this monthly cycle calculator.

A normal monthly menstrual cycle runs 21-35 days, measured from the first day of one monthly period to the first day of the next. The average monthly cycle across all age groups is about 29 days. Teens often have longer monthly cycles (up to 45 days) in the first few years after their period starts. Monthly cycles tend to shorten slightly and become more regular in your 20s and 30s. A variation of up to 7 days between monthly cycles is considered normal. Monthly cycles shorter than 21 days or longer than 35 days consistently may need medical attention.

Pregnancy is possible during your fertile window, which typically opens about 5 days before ovulation. For a 28-day cycle, ovulation usually happens around day 14, so pregnancy is possible from roughly day 9 onward. Sperm survive up to 5 days inside the body, and the egg lives 12-24 hours after release. To see your fertile days mapped out, try our [fertility calculator](/fertility-calculator/). This monthly period cycle calculator focuses on predicting your next period, while the fertility tool shows conception timing.

A late monthly period does not always mean pregnancy. Common causes include stress, recent illness, significant weight change, intense physical activity, travel, and medication changes. Hormonal shifts from starting or stopping birth control can delay your monthly period by a week or more. Even a minor sleep schedule change can push ovulation back, which delays your monthly period. If your monthly period is more than 7 days late and you could be pregnant, take a test. If late monthly periods happen regularly, see your doctor to rule out conditions like PCOS or thyroid issues.

If your monthly cycle is regular, you can estimate monthly period dates several months ahead. Take your average monthly cycle length and count forward from your last period start date. For a 28-day monthly cycle starting January 1: next monthly period around January 29, then February 26, then March 26. This period cycle calculator for 3 months ahead works best when your monthly cycles stay consistent. Keep in mind that real monthly cycles can shift by a few days each month, so treat future predictions as estimates rather than exact dates.