Introduction
This free period calculator helps you track your menstrual cycle and predict when your next period will start. Just enter the first day of your last period, how long your period lasts, and your average cycle length. The tool then builds a color-coded calendar that shows your upcoming periods, fertile window, and estimated ovulation day for up to 12 cycles ahead.
Knowing when your period is due can help you plan ahead and feel more in control. The fertile window and ovulation dates shown here can also help if you are trying to get pregnant. Keep in mind that this calculator gives estimates based on average cycle length. It works best for people with regular cycles. It is not a form of birth control and should not be used to prevent pregnancy. If your cycle is irregular or you have concerns about your menstrual health, talk to your doctor.
How to Use Our Period Calculator
Enter a few details about your menstrual cycle below. The calculator will show you a calendar with your predicted period dates, fertile window, and ovulation day for upcoming cycles.
First Day of Last Period: Pick the date your most recent period started. You can also click "Use Today's Date" if your period started today.
Period Duration: Use the plus and minus buttons to set how many days your period usually lasts. Most periods last between 3 and 7 days.
Average Cycle Length: Set the number of days from the first day of one period to the first day of the next. A typical cycle is 28 days; the calculator accepts 21 to 44 days.
Cycles to Project: Choose how many future cycles you want to see on the calendar. You can pick 3, 6, 9, or 12 cycles.
Luteal Phase Length (Advanced): Open "Advanced Options" to change this number. The luteal phase is the time between ovulation and your next period. Most people can leave this set to 14 days.
Click Calculate to see your results. The color-coded calendar and timeline chart will show your predicted period days, fertile window, ovulation day, and pre-period days.
What Is a Period Calculator?
A period calculator helps you predict when your next period will start, when you are most fertile, and when you will ovulate. It uses the date of your last period, how long your period lasts, and the length of your menstrual cycle to estimate these dates for future months.
How the Menstrual Cycle Works
A menstrual cycle is the monthly process your body goes through to prepare for pregnancy. It starts on the first day of your period and ends the day before your next period begins. Cycle length varies from person to person; the standard dating convention assumes 28 days with ovulation on day 14.3
Each cycle has a few key phases:
- Period (Menstruation): The lining of your uterus sheds and leaves your body as blood. This usually lasts 3 to 7 days.
- Fertile Window: The days when you are most likely to get pregnant: the days before ovulation and the day of ovulation itself, which an ovulation calculator can pinpoint more precisely. This calculator shows the window from 5 days before ovulation to 1 day after, following ACOG's guidance on when pregnancy is possible.1
- Ovulation: The day one of your ovaries releases an egg. This typically happens about 14 days before your next period starts. An egg can survive in the fallopian tube for about 24 hours after ovulation.1
- Luteal Phase: The time between ovulation and the start of your next period, usually around two weeks.
Why Track Your Period?
Tracking your period helps you know what to expect each month. It can help you plan ahead, notice if something is off with your cycle, and understand your body better. If you are trying to get pregnant, knowing your fertile window and ovulation day is very important, and once you do conceive a due date calculator can estimate when your baby may arrive. If you are not trying to get pregnant, this information can still help you stay aware of your health.
What to Know About Irregular Cycles
Not everyone has a cycle that is the same length every month. Stress, diet, exercise, age, and health conditions can all cause your cycle to be shorter or longer than usual. Maintaining a balanced diet and proper hydration can support cycle regularity. If your cycle changes a lot from month to month, the predictions from any period calculator will be less accurate. Even among women with regular cycles, a study of 696 cycles found the fertile window entirely within days 10 to 17 in only about 30%.2 Talk to a doctor if your periods are very irregular, very painful, or suddenly change.
Important Reminder
This calculator gives estimates based on averages. It is not a medical tool and should not be used as a form of birth control. Every person's body is different. For medical advice about your menstrual cycle, fertility, or reproductive health, always speak with a healthcare provider.