Period Calculator

Created by Dominika Śmiałek, MD, PhD candidate and Bogna Szyk
Reviewed by Małgorzata Koperska, MD, Jack Bowater and Adena Benn
Based on research by
Grieger JA, Norman RJ. Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study. Journal of Medical Internet Research (June 2020)See 1 more source
Creinin MD, Keverline S, Meyn LA. How regular is regular? An analysis of menstrual cycle regularity. Contraception (October 2004)
Last updated: Jan 10, 2023

This period calculator is a must-have tool for every woman who wants to understand her menstrual cycle. You can use this period predictor to determine when your next period is. It is especially beneficial if you have an occasion coming up and you would like to plan for a time when you will not be mensurating, for example, to plan a period-free vacation. You can also use it as a period tracker that gives accurate information about the best time to get pregnant.

If you're expecting, check whether the pregnancy is developing properly with our hCG levels calculator.

Menstrual calculator principles

In order for this period calculator to work correctly, you need to input basic information about your menstrual cycle. This includes:

  • Your cycle duration:
    The cycle typically lasts for about 28 days, so if you're unsure about how long yours is, 28 days is a reasonable estimate. If, however, you have been using a period tracker for quite some time, you might know that your cycle is consistently longer or shorter – for example, 27 or 30 days.

  • Period duration:
    Some lucky women only experience bleeding for about three days, while others have periods lasting about six or seven days. Enter the average duration of your period in this field.

  • Last period started on...:
    The last information our period predictor needs is the date when your previous period began. This day is the one we customarily refer to as the first day of your menstrual cycle.

When is my next period?

Once you input all that data into our menstrual cycle calculator, it will automatically estimate the dates when your next period starts and ends.

For example, if your last cycle started on November 16th, and it typically lasts for 29 days, you can expect the next period to begin on December 15th. For a five-day-long period, you can expect the bleeding to stop on December 19th.

Naturally, menstruation defies exact calculations, so it might happen that your period will be a few days early or late. Still, it's a pretty good estimate to help figure out whether you will get your period in the middle of your vacation or not.

Using the period tracker to predict ovulation

You can also use this period calculator to find out what is your fertility window – that is, the time when you're most likely to get pregnant.

As sperm can live up to seven days inside your body, you can safely assume that the fertility window starts seven days before ovulation (see ovulation calculator). It ends only one day after the ovulation, though, as the egg lives for a maximum of 24 hours.

Ovulation is when an egg is released from your ovaries. This is the time when you are most fertile. It usually happens about two weeks before the start of the next period. If you want to have a more precise estimate, you should monitor your body temperature and the changes in cervical mucus.

You can keep track of your menstrual cycle and ovulation with the use of our basal body temperature calculator ⬅️ Check our tool for more details.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

💡 You might also be interested in our pregnancy weight gain calculator.


How do I calculate when my next period will be?

To calculator when your next period will be:

  1. Measure how long your average cycle is, i.e., the time between one period beginning and the next one beginning. If you are unsure, use the average value of 28 days.

  2. Note how long your average period duration is. If you do not know this value, use the average of 6 days.

  3. Add the length of your cycle to the date your last period began. This is the day your next period should begin.

  4. Add the length of your period to the date your next period will begin. This is when your next period should end.

  5. Check your calculations with the Omni Period Calculator.

What is pre period?

Pre period, also known as premenstrual syndrome (PMS), is a syndrome that up to 90% of women will experience in their reproductive lives. It is characterized by headaches, mood swings, anxiety, irritability or upset, tiredness or trouble sleeping, stomach pain, breast tenderness, spotty skin or greasy hair, and changes in libido and appetite. The causes are not fully understood, but some doctors think that changes in hormone levels due to the menstrual cycle is the cause.

How do I calculate my fertility window?

Here's how to calculate your fertility window:

  1. Calculate your ovulation day. Take your average cycle duration, subtract 14 days from this value, and add the remaining number of days to the date your last period began.

  2. Sperm can survive inside a woman's body for 7 days, so subtract 7 days from your ovulation day. This is when your fertile window begins.

  3. As the egg can only survive one day outside of the ovaries, add 1 day to your ovulation day. This is the end of your fertile window.

  4. This time between these two dates is your fertile window.

  5. Check your results, as well as find them for the next 3 months, with the Omni Ovulation Calculator.

When do periods stop?

Periods stop temporarily while a woman is pregnant. They also stop permanently after menopause.

Menopause usually occurs when a woman is in her late 40s to mid-50s. Periods will either become less and less frequent over a few months or may suddenly stop.

Some symptoms of menopause are hot flashes, night sweats, vaginal dryness, difficulty sleeping, increased anxiety, and problems with memory and concentration.

How long should my period last?

Periods can last anywhere between 3 and 8 days, but they usually last for 5 days. Bleeding tends to be heaviest during the first 2 days, and the blood tends to be typically red. On lighter days, blood may be pink, brown, or black. Brown or black periods are due to the iron in your bloody reacting (oxidizing) more with the air due to the slower flow.

How long is my fertility window?

The fertility window has a length of 8 days. The fertility window begins when sperm can theoretically survive within your body (7 days) long enough to fertilize an egg. The fertility window ends with the maximum lifespan of an egg. As an egg can only survive 1 day outside of the ovaries, the window ends 1 day after ovulation.

What age do girls get their first period?

Girls generally get their first period around the age of 10-12, although it can happen as early as 8. A sign that you will begin your periods soon is the growth of underarm and pubic hair, and periods typically start two years after breasts begin to grow and one year after the appearance of white vaginal discharge.

Do periods sync up?

Current studies suggest that the periods of women living together do not sync up, but the verdict is still out. The popular belief is that by being around someone who is menstruating, their pheromones will influence you and eventually cause your cycles to sync up. This was further supported by a study of 135 university women living in the same dorm by McClintock. However, later studies seem to disprove and prove the theory, so it is difficult to tell.

Can you get pregnant during your period?

You can get pregnant during your period, just as you can get pregnant at any time of the month if you have unprotected sex. You can also get pregnant if you have never had a period before, during your first period, or after the first time you have sex. The chances of getting pregnant during your period are increased if your menstrual cycle is less than 28 days, as your ovulation day is closer to your period.

How much blood is lost during a period?

The amount of blood lost during a period will vary from woman to woman, but, on average, a woman will lose about 6-8 teaspoons (30-40 ml) of blood per period. A loss of 80 ml or more, having a period longer than seven days, or both, is considered heavy. However, most women have a good idea of whether or not their bleeding is within the normal range for them.

Dominika Śmiałek, MD, PhD candidate and Bogna Szyk
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