Knowing when to expect your next period can be helpful, giving you a chance to plan, manage any symptoms, and feel more in tune with your body. By using some basic information about your last period and your menstrual cycle, Flo’s period calculator can predict when your next period might be. You can also learn more about your pre-period and post-period days using Flo. 

Give the period calculator a try (above) and then scroll down for everything you need to know about your menstrual cycle. If you find it helpful, know that Flo’s app uses artificial intelligence or AI-powered tech to give you even more accurate predictions and notifications of what to expect from your cycle, so you’re always prepared. 

Remember that period calculators, ovulation-tracking apps, and period calendars can help you learn more about your menstrual cycle. This tool is based on a regular menstrual cycle. However, menstrual cycles will vary from person to person and month to month.

Many things can affect how regular a cycle is, from stress to polycystic ovary syndrome (PCOS) to breastfeeding. Predictions are estimates, and menstrual cycles vary for everyone. This calculator’s results can help you understand your body better but cannot, and should not, be used to prevent pregnancy.

Key takeaways about using a period calculator

• Flo’s period calculator estimates your future period date based on your last period start date and cycle length, helping you stay prepared.
• Some of us experience symptoms around our periods, such as menstrual cramps, sore boobs, and mood swings. By logging symptoms each month, the Flo app can help you know what to expect. 
• Even if your periods are irregular, regularly tracking your cycle dates and symptoms can help you spot patterns that you can speak to your doctor about.
• This calculator provides an estimate of when you might expect your period, but it’s not a guarantee, as lots of things can affect your cycle. This tool shouldn’t be used as a contraceptive.

How do I use a period calculator?

To use a period calculator like this one, start by entering:

• The date of the first day of your last period: The first day of your period marks the first day of a new cycle. 
• Your average menstrual cycle length: You can figure this out by counting the number of days between the first day of your last period and the day before your next one. The number may be around 28 — this is the average menstrual cycle length. However, if yours is between 21 and 35, then it’ll still be considered typical.

Flo’s period calculator will then estimate the start date of your next period by adding your cycle length to the first day of your last period.

Since menstrual cycles can vary slightly each month, these results are only estimates and can shift by a few days depending on factors like stress or lifestyle changes. 

If your periods or cycle lengths are generally irregular, the results may not be as precise. However, tracking several cycles using an app like Flo over time can help you understand your body better. It may provide you with data to show to your doctor and demystify the different menstrual cycle phases.

How do I understand period calculator results? 

This period calculator provides estimated dates of when your next period might begin and end. 

Based on the information you input — such as your last period start date and cycle length — the calculator predicts when your next period will likely occur. But, since menstrual cycles are influenced by many factors like stress, weight changes, and illness, it’s best to think of your result as a guide to future periods rather than a guarantee. And remember, the results shouldn’t be used as a contraceptive method or a substitute for medical advice.

What is the average cycle length? 

When it comes to our cycles, it’s hard to talk about what’s average because we’re all different. However, the average length of a menstrual cycle is considered to be 28 days, and most people have their period for around two to seven days. But like we say, just like the length of a period, the length of a menstrual cycle will naturally vary from person to person and through your lifetime. 

“A normal menstrual cycle — from day one, which is the first day of bleeding, all the way up to the next period — can be anywhere from 21 to 35 days,” explains obstetrician and gynecologist Dr. Sara Twogood, Cedars-Sinai Medical Group, California, US. Tracking your cycles can help you come to grips with what’s normal for you. 

How do I know if I have irregular periods? 

Your period is still considered “regular” if it varies slightly from cycle to cycle. Here are some examples of what doctors would consider irregular periods:

• Your periods occur fewer than 21 days or more than 35 days apart.
• The length of time between cycles varies by more than seven to nine days. For example, one cycle is 28 days, the next is 37 days, and the next is 29 days. 
• Your periods are particularly heavy, meaning your period soaks through one or more tampons or pads every hour. 

If you’re struggling to use a menstrual cycle calendar because your periods are a bit scattered, don’t panic. It’s not necessarily a cause for concern. Fluctuations in the length of your menstrual cycle are incredibly common. In fact, research shows that between 14% and 25% of women have irregular menstrual cycles. 

Fluctuations can happen for all kinds of reasons, such as weight changes, stress, and conditions like PCOS. If you’re experiencing irregular periods, it could be a good idea to get checked by a health care provider. They may be able to help you understand what’s going on and ensure there aren’t any underlying issues to be concerned about. 

How do I calculate when my last menstrual period was? 

If you haven’t tracked your periods before, it can be tough to remember exactly when your last one was. 

You can calculate when your last menstrual period was by identifying the first day of your most recent period, meaning the day bleeding started. This is considered “day 1” of your cycle. Using a period-tracking app like Flo can keep a handy record of your previous periods, your cycle length, and how long menstruation typically lasts for you. You can also track your cycle using a pen and paper calendar. 

Why might my period be late? 

If you’ve used the period calculator, but your period hasn’t arrived as expected, or you’ve missed the date planned on your menstrual cycle calendar, then you might think your period is late. This can be understandably distressing but try not to worry. 

Generally speaking, a period is technically only late if it happens eight days after you expected it to arrive. “If periods are consistently late, irregular, or have no pattern to them, that’s a good reason for an evaluation [with a health care provider],” Dr. Twogood adds. 

Of course, there could be one obvious reason for a late period. If someone has had unprotected sex, “then a late period for them could mean a pregnancy and a reason to take a pregnancy test,” Dr. Twogood explains. Flo’s pregnancy test calculator can help you figure out the best time to take a pregnancy test for the most accurate results. 

Other reasons for a late period include stress, changing your birth control, weight changes, and conditions like PCOS. 

What are some possible menstruation symptoms?

Knowing when your next period might be can help you prepare and plan. If you feel differently just before your period starts, then you’re not alone. You might notice:

Menstrual cramps

Period pain, or menstrual cramps, is also called dysmenorrhea. Around 60% of us experience mild cramps around the time of our period, and for 5% to 15%, it is so severe it disrupts daily activities. 

Period pain shouldn’t get in the way of your life — pain relievers like medicines or a heating pad can help a lot. If your cramps are so severe that they keep you from doing your normal activities, visit your doctor. There are lots of things that can help, and you don’t just need to put up with cramps alone. 

Feeling tired 

It’s common to feel more tired than usual during your period due to hormonal fluctuations. But if your periods are really heavy, you might find you’re particularly tired. Heavy periods can cause low iron levels (anemia), and one of the main symptoms is fatigue. If you’re experiencing heavy bleeding, chat with your doctor. They’ll be able to measure and monitor your iron levels and deduce if your tiredness is being caused by anemia. 

Bloating 

Some people experience more bloating than usual during their periods. This is thought to be due to hormone fluctuations affecting our digestive system. Always speak to your doctor if you’re worried about bloating or if it’s been going on for longer than a couple of weeks. 

Mood swings

Changes in your hormone levels around the time of your period can cause heightened emotions, mood swings, and some irritability. This can be very normal. Try to be gentle with yourself and indulge in your favorite activities, whether that’s settling down in front of your comfort show or going to an exercise class. If your mood swings feel severe, though, check in with your doctor. 

Acne-prone skin 

Do you find you’re prone to breakouts on your period? You’re not alone. Hormone fluctuations around this time can affect your sebum levels (the oil on your skin), leading to an increase in acne for a few days for some people. 

Tender breasts 

Hormonal changes can also cause sore boobs. Some people find their breasts get more tender around ovulation, while others experience it before or during their periods. 

Period tracking with Flo 

Knowing when to expect your next period can be invaluable, helping you prepare for physical changes and better understand your cycle.

Flo’s period calculator offers an easy way to estimate your next period, using the date of your last period and your cycle length. However, if you want even more accurate results, Flo learns from each of your logged cycles with the help of AI. We can then provide you with increasingly precise predictions for period dates, pre-period symptoms, and your fertile window. 

“When patients come to me with concerns about their periods, it’s helpful to see their period-tracking data to understand any patterns that they may not have noticed,” says Dr. Twogood. “It also helps turn a general concern like, ‘My period is all over the place’ into specific data, showing obstetricians and gynecologists exactly what ‘all over the place’ means for you!”