Period Calculator: Predict Your Next Period and Cycle Dates
The period calculator is a free online tool that estimates your future period days in calendar form. It uses the first day of your last period, average cycle length and period length to predict your next six cycles. No signup required.
Key Takeaways
- This tool estimates your next six cycles at once, using your last period, cycle length and period length.
- It also works as a menstrual cycle calculator, showing your cycle length, cycle day and estimated phases, so it doubles as a simple period tracker and menstrual cycle tracker.
- A delay of a few days is common and usually not a cause for concern. More than 7 days late is the point worth tracking.
- It works for irregular cycles, PCOS and breastfeeding too, though predictions are rougher the more your cycle varies.
- This calculator gives an estimate of when you might expect your period. It is not a guarantee, as many things can affect your cycle, and it should not be used as a contraceptive.
What Is a Period Calculator?
A free online tool that predicts your next period days and ovulation days on a calendar from three numbers you already know.
What It Shows You
A period calculator estimates when your next periods are likely to start and end. It uses the first day of your last period, your usual cycle length and how many days your period lasts. The results appear on a calendar, so you can see future periods, your likely ovulation days and your fertile window at a glance.
Who It Helps
It helps anyone who wants to plan around their period: for a holiday, a school exam, sports, or a medical visit. It is also useful if you are trying to understand your cycle better, if you are trying to get pregnant, or if you are just starting to have periods. For more on first periods, see our first period guide.
Benefits of Using One
- You can plan ahead and feel less surprised by your period.
- You can see patterns, such as a cycle that keeps getting longer.
- You get a clear record to show a doctor if something seems off.
- You can estimate your ovulation and fertile days in the same view.
The results are based on estimates and never guarantee when you will bleed or ovulate.
How to Use the Period Calculator
Enter the first day of your last period, your usual cycle length and your period length, then read the dates on the calendar.
What You Need Before You Start
- First day of your last period: the first day you saw red blood flow, not spotting.
- Cycle length: the number of days from the first day of one period to the first day of the next.
- Period length: how many days you usually bleed.
No sign-up is needed. If you do not know your cycle length yet, you can start with 28 days and change it later. Read calculating without your cycle length for the simple method.
Step-by-Step Guide
- Pick the first day of your last period.
- Enter your average cycle length in days.
- Enter how many days your period usually lasts.
- Press the button to see your next periods, ovulation days and phases.
- Check the dates on the calendar and save or screenshot them if you want to keep them.
Our full guide to using this calculator has screenshots and examples.
How to Find the First Day of Your Last Period
Count the first day of real bleeding as day 1. Spotting that is light and does not need a pad does not count. If you cannot remember the date, check your phone calendar, your photos or any notes. If you are unsure, use your best guess and treat the results as a rough estimate.
How Does a Period Calculator Work?
It adds your cycle length to your last period date, then works backward from that day to find ovulation.
The Simple Formula Behind the Dates
The first step is easy: first day of your last period + cycle length = first day of your next period. If your last period started on the 1st and your cycle is 28 days, your next period is expected on the 29th. The calculator repeats this to show several cycles ahead.
Ovulation is estimated by counting back from the next period. In many people the time from ovulation to the next period is about 14 days, so in a 28-day cycle ovulation is around day 14. For the full method, see how to calculate your next period.
How to Read Your Results
| Result | What it tells you |
|---|---|
| Next period | The estimated first day of your next period and how many days it may last. |
| Ovulation day | The day you are most likely to release an egg. |
| Fertile window | The days when pregnancy is most likely, from about 5 days before ovulation to 1 or 2 days after. |
| Cycle day | Which day of your cycle you are on today, counted from day 1. |
| Phase | Whether you are in the menstrual, follicular, ovulation or luteal phase. |
Treat each date as a likely day, not an exact day. A cycle can move a few days either way.
What Is the Menstrual Cycle?
The menstrual cycle is the monthly series of changes that prepares your body for a possible pregnancy. It is counted from the first day of one period to the first day of the next.
The menstrual cycle usually starts between ages 12 and 15, when a girl has her first period, and continues until menopause, which happens at around age 52 on average. Doctors count it from the first day of one period to the first day of the next. Rising and falling hormones control every step, which is why the cycle can shift when your stress, sleep, weight or health changes. Our menstrual cycle guide goes deeper.
What Happens Inside Your Body Each Month
Each cycle has two main jobs. First, the lining of the uterus thickens so it can support a pregnancy. Second, an egg matures and is released from an ovary, which is called ovulation.
Ovulation is when you are most fertile. The fertile window runs from about 5 days before ovulation up to 1 or 2 days after it. The days before ovulation count because sperm can survive in the body for several days, while the egg lasts only a short time.
If the egg is not fertilized, pregnancy does not happen. The lining then breaks down and leaves the body as a period, and a new cycle begins.
What Is a Period?
A period, also called menstruation, menses or a menstrual period, is the regular flow of blood and tissue from the lining of the uterus. The blood leaves the body through the cervix and vagina. A period usually lasts between 2 and 7 days. It marks the menstrual phase, the first phase of the cycle, and it starts because the egg from the previous cycle was not fertilized. The days of bleeding are your period days. Read more on what a period is. MedlinePlus also explains what happens during menstruation.
How Long Is a Normal Menstrual Cycle?
Most adult cycles last 21 to 35 days, with 28 days as the average. Small changes from month to month are normal.
Average and Normal Range
The average cycle is 28 days, but many healthy cycles are shorter or longer. In adults, 21 to 35 days is usually considered normal. A cycle is often called regular when your longest and shortest cycles are less than 8 days apart. Read more in what is a normal menstrual cycle.
If Your Cycle Changes Every Month
It is normal for your cycle to change by a few days. If it changes by much more, write down the first day of every period for 3 to 6 months, as Mayo Clinic suggests. Then use the average, or use the shortest and longest cycle to see a likely range. Our period calculator for irregular periods explains how.
How Cycle Length Changes With Age
| Life stage | Typical pattern |
|---|---|
| First 1 to 2 years after the first period | Cycles can be irregular and often fall between 21 and 45 days. |
| Adult years | Most cycles are 21 to 35 days. |
| Perimenopause (years before menopause) | Cycles may get shorter, longer or skip as hormones change. |
Pregnancy, breastfeeding, stress and some medical conditions can also change your cycle at any age.
What Happens in Each Phase of Your Cycle?
Your cycle has four phases: menstrual, follicular, ovulation and luteal. Hormones control each step.
The menstrual cycle is the monthly series of changes that prepares your body for a possible pregnancy. It is counted from the first day of one period to the first day of the next, and hormones rise and fall to run each step. See the full menstrual cycle phases for more.
Menstrual Phase
This is your period. The uterus sheds its lining, with blood and tissue leaving through the vagina. It starts when the egg from the last cycle was not fertilized and usually lasts 2 to 7 days.
Follicular Phase
The follicular phase starts on day 1 of your period and ends at ovulation. The ovaries get follicles ready, one egg matures, and the uterus lining begins to thicken again.
Ovulation Phase
An ovary releases an egg. This is when you are most fertile. The egg lives for a short time, but sperm can survive for several days, which is why the fertile window starts before ovulation.
Luteal Phase
After ovulation the lining thickens further to support a possible pregnancy. If there is no pregnancy, hormone levels fall and the next period starts. Our luteal phase guide explains why its length matters.
Which Phase Are You In Today?
Count from day 1 of your last period. Roughly: days 1 to 5 are the menstrual phase, days 6 to 13 follicular, around day 14 ovulation and days 15 to 28 luteal. The calculator marks your phase for you, and the days shift if your cycle is shorter or longer than 28 days.
What Are the Signs Your Period Is Coming?
Bloating, cramps, tender breasts and mood changes are common signs in the days before a period.
Many people notice changes 1 to 10 days before bleeding. Not everyone has all of them. See signs your period is coming soon for more.
Physical Signs
- Cramps or a heavy, achy feeling in the lower belly
- Bloating, which you can read about in our guide to period bloating
- Tender or swollen breasts
- Headaches, tiredness or trouble sleeping
- Acne or oily skin
- Changes in appetite or food cravings
Emotional Signs
- Mood swings or feeling irritable
- Feeling anxious, low or tearful
- Trouble concentrating
If these feelings are severe and affect your daily life, speak with a doctor. Severe symptoms can be a sign of a condition called PMDD.
How Long Does a Period Last?
A period usually lasts 2 to 7 days, with about 5 days as the average.
Typical Period Length
The average period is about 5 days, but 2 to 7 days is usual. The first days are often heavier and the later days lighter. You may also feel cramps or tiredness. Read how long a period lasts for what is normal.
How Much Blood Is Normal?
Most people lose only a few tablespoons of blood over a whole period, even though it can feel like much more. Bleeding may be too heavy if you soak through a pad or tampon every hour or two, pass large clots, or bleed for more than 7 days. See our heavy periods guide if this sounds like you.
At What Age Does Menstruation Usually Begin?
Menstruation usually begins between ages 12 and 15, but it can start earlier or later for each girl. It is an important sign of puberty, when the body goes through many changes. After periods begin, the cycle may be irregular for some time and often becomes more regular over time. See our first period guide.
When Do Periods Start and Stop in Women?
Periods start at puberty and stop at menopause, which happens at about age 52 on average. Menopause is confirmed after 12 months in a row with no period. Periods also stop during pregnancy and often do not return in the early months of breastfeeding, although the timing differs from person to person.
Why Is My Period Late or Irregular?
Stress, weight changes, illness, hormone conditions and pregnancy are common reasons for a late or irregular period.
Common Causes of a Late Period
A delay of a few days is common. Pregnancy is the first thing to rule out if you had sex and your period is late. Other causes include stress, big changes in weight, illness, travel, hormonal birth control, breastfeeding and conditions such as PCOS or a thyroid problem. If you are more than 7 days late, take a pregnancy test and track the delay. See what counts as a late period for more.
Stress, Illness and Weight Changes
Your brain controls the hormones that trigger ovulation, so stress and low energy intake can delay it. Read how stress affects your period, weight changes and your cycle and thyroid and your cycle.
Using the Calculator With PCOS
With PCOS, cycles can be long or skip months, so the dates are less reliable. Use a longer average cycle, and look at PCOS and period calculators.
Using the Calculator While Breastfeeding
Periods often return later while you breastfeed, and the first cycles can be unpredictable. Predictions will be rough until your cycles settle. Read periods while breastfeeding.
How Accurate Is a Period Calculator?
Accurate for regular cycles, less accurate for irregular ones. It is an estimate, not a medical test.
What Affects Accuracy
- How regular your cycle is
- Whether you entered your real cycle length and last period date
- Stress, illness, travel, sleep and weight changes
- Hormonal birth control, breastfeeding, PCOS and perimenopause
If your cycle changes by only a day or two, the next period is usually close to the prediction. If it varies by a week or more, the dates are rough.
Calculator or Tracker App: Which Should You Use?
A calculator gives a quick estimate with no sign-up. A tracker app stores your history and learns your pattern over time. Using both works best. Read period tracker vs period calculator.
How Do I Find My Ovulation Date and Fertile Window?
Ovulation is usually about 14 days before your next period. The fertile window is the 5 days before it plus the day after.
How the Calculator Estimates Them
The tool counts back from your next expected period to estimate ovulation. In a 28-day cycle that is about day 14. The fertile window runs from about 5 days before ovulation to 1 or 2 days after. See how the fertile window is estimated and how to calculate ovulation day. For fertility dates, you can also use the Ovulation Calculator.
Why It Is Not a Birth Control Method
Ovulation can move, and sperm can survive for several days, so a calendar estimate can miss your fertile days. Do not rely on this tool to prevent pregnancy. Read how reliable safe days are before using the Safe Days Calculator.
When Should I See a Doctor?
See a doctor for very heavy bleeding, no period for 3 months, severe pain, or cycles that are suddenly very different for you.
Warning Signs You Should Not Ignore
- Soaking through a pad or tampon every hour or two
- Bleeding for more than 7 days, or passing large clots
- No period for 3 months in a row (and you are not pregnant)
- Periods suddenly much more or less frequent than usual
- Severe pain that stops you from doing daily activities
- Bleeding after sex, between periods or after menopause
- No first period by age 15
This page gives general information and does not replace medical advice.
Frequently Asked Questions
What is a period calculator and who is it for?
Can I calculate my next period without knowing my cycle length?
Who controls the menstrual cycle?
How many days does an average cycle last?
What happens to the inner lining of the uterus each month?
What is cycle day 1?
What is the difference between cycle length and period length?
How many days before my period do I ovulate?
Is it normal for my cycle to change from month to month?
How do I know if my period is too heavy?
What causes heavy periods?
Does the calculator work if I use hormonal birth control?
Can I use a period calculator to avoid pregnancy?
At what age does menopause usually happen?
Is it normal to miss a period once?
Medical Disclaimer
This page and calculator provide estimates for general cycle tracking only. They do not diagnose any condition, do not confirm ovulation, and are not a form of contraception. Cycle length varies between people and from cycle to cycle. For personal advice, speak with a healthcare provider.
Our Free Cycle & Pregnancy Calculators
Sources & References
- American College of Obstetricians and Gynecologists (ACOG). Your First Period. Patient FAQ stating that most girls start their periods between ages 12 and 13, though some start earlier or later, the basis for the first-period age on this page. acog.org
- Mayo Clinic. Menopause: Symptoms & Causes. States that menopause is typically confirmed after 12 consecutive months without a period, with the average age in the US around 51 and most women reaching it between 49 and 52. mayoclinic.org
- Cleveland Clinic. Menstrual Cycle: What’s Normal, What’s Not. Source of the standard last-period-plus-cycle-length counting method this calculator automates, and a plain-language reference for what counts as a normal versus irregular cycle. clevelandclinic.org
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). Irregular Periods: Condition Information. Estimates that 14% to 25% of women experience irregular menstrual cycles, and outlines common causes including stress, weight changes, and PCOS. nichd.nih.gov
- NHS. Periods and Fertility in the Menstrual Cycle. UK national health service guidance on typical cycle length, ovulation timing, and the fertile window, broadly consistent with the ACOG figures this calculator uses. nhs.uk/fertility-in-the-menstrual-cycle
- U.S. Office on Women’s Health (OASH). Period Problems. Defines an irregular cycle as one shorter than 24 days or longer than 38 days, and outlines when heavy bleeding, missed periods, or other period problems are worth a doctor’s visit. womenshealth.gov
- Centers for Disease Control and Prevention (CDC). About Heavy Menstrual Bleeding. Defines the specific warning signs of heavy menstrual bleeding (soaking through protection hourly, periods beyond 7 days, large clots) that this page flags as worth discussing with a doctor. cdc.gov
- MedlinePlus (U.S. National Library of Medicine). Menstruation. Plain-language definition of menstruation and the menstrual cycle used as the baseline description of what a period calculator is actually tracking. medlineplus.gov
- National Institutes of Health (NIH). NIH Partners With Apple and Harvard University on Women’s Health Study. News release on an NIH-backed research partnership using cycle and fertility tracking apps to study PCOS, menopause, and fertility at scale, cited as evidence that everyday period tracking feeds into real research, not just personal planning. nih.gov
- World Health Organization (WHO). WHO Statement on Menstrual Health and Rights. Frames menstrual health as one continuous picture spanning from first period through menopause, rather than a series of unrelated life stages, the basis for this page’s framing of cycle changes across a lifetime. who.int
- Better Health Channel (Victoria State Government, Australia). Puberty. Confirms that irregular periods are very common in the first few years after a first period, and explains the basic shed-and-rebuild cycle mechanics, supporting this page’s note that early cycle predictions are naturally less reliable. betterhealth.vic.gov.au
- NHS. Periods. Describes typical period length of about 2 to 7 days within the broader 21 to 35 day cycle, and when irregular cycles are worth discussing with a doctor. nhs.uk
- Mayo Clinic. Menstrual Cycle: What’s Normal, What’s Not. Describes how to count cycle length from the first day of one period to the first day of the next, the 24 to 38 day range, and when to see a healthcare professional. mayoclinic.org
- Cleveland Clinic. Myth or Truth: Do Periods Really Sync Between Close Friends? Explains that closeness does not change cycle timing, the basis for this page’s section on period syncing. clevelandclinic.org
- Fehring RJ, Schneider M, Raviele K. Variability in the Phases of the Menstrual Cycle. Journal of Obstetric, Gynecologic & Neonatal Nursing. 2006;35(3):376-384. Analysis of fertility-chart data finding that cycle-to-cycle variation comes mainly from the follicular phase, while the luteal phase stays comparatively consistent, part of why this calculator’s “estimate, not a guarantee” framing matters. pubmed.ncbi.nlm.nih.gov/16700687
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles. npj Digital Medicine. 2019;2:83. Analysis of 612,613 real-world cycles found an average follicular phase of 16.9 days versus an average luteal phase of 12.4 days, confirming that most cycle-length variation comes from the first half of the cycle, not the second. nature.com
- Epstein DA, Lee NB, Kang JH, et al. Examining Menstrual Tracking to Inform the Design of Personal Informatics Tools. Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems. 2017. A human-computer-interaction study of how people actually use menstrual-tracking tools day to day, informing why a fast, low-friction calculator like this one is useful alongside more detailed tracking apps. dl.acm.org
- Worsfold L, Marriott L, Johnson S, Harper JC. Period Tracker Applications: What Menstrual Cycle Information Are They Giving Women? Women’s Health. 2021;17:17455065211049905. A review of popular period-tracking apps assessing how accurately and clearly they communicate cycle, ovulation, and fertile-window estimates to users. journals.sagepub.com (doi 17455065211049905)
- Broad A, Biswakarma R, Harper JC. A Survey of Women’s Experiences of Using Period Tracker Applications: Attitudes, Ovulation Prediction, and How the Accuracy of the App in Predicting Period Start Dates Affects Their Feelings and Behaviours. Women’s Health. 2022;18:17455057221095246. Survey of period-tracker users finding that prediction accuracy directly affects how much users trust and rely on an app, supporting why this page is upfront that predictions are estimates, not guarantees. journals.sagepub.com (doi 17455057221095246)
- Kelly BG, Habib M. Missed Period? The Significance of Period-Tracking Applications in a Post-Roe America. Sexual and Reproductive Health Matters. 2023;31(4):2238940. Discusses the shifting privacy and legal stakes around period-tracking data in the US, part of why this page is explicit that nothing entered here is stored or shared. tandfonline.com
- Woodland C, et al. The Benefits and Harms of Using a Period Tracking App: A Qualitative Study of Women’s Experiences. Patient Education and Counseling. 2026:109755. Qualitative interviews on both the upsides (self-knowledge, planning) and downsides (anxiety when predictions are wrong, data-privacy worry) women report from using period-tracking tools. pubmed.ncbi.nlm.nih.gov/42475997
- Zwingerman R, Chaikof M, Jones C. A Critical Appraisal of Fertility and Menstrual Tracking Apps for the iPhone. Journal of Obstetrics and Gynaecology Canada. 2020;42(5):583-590. Found that a notable share of popular fertility/menstrual tracking apps contained errors or unsupported claims in how they calculated fertile windows, part of why this page is upfront that predictions are estimates, not guarantees.
- Hong M, et al. Menstrual Cycle Management and Period Tracker App Use in Millennial and Generation Z Individuals: Mixed Methods Study. Journal of Medical Internet Research. 2024;26:e53146. Mixed-methods study of how younger generations actually use period-tracker apps day to day and what they expect from them. jmir.org
- Moglia ML, et al. Evaluation of Smartphone Menstrual Cycle Tracking Applications Using an Adapted APPLICATIONS Scoring System. Obstetrics & Gynecology. 2016;127(6):1153-1160. A structured quality review of smartphone menstrual-tracking apps, scoring them on accuracy, usability, and the quality of health information provided. journals.lww.com
- Levy J, Romo-Avilés N. “A Good Little Tool to Get to Know Yourself a Bit Better”: A Qualitative Study on Users’ Experiences of App-Supported Menstrual Tracking in Europe. BMC Public Health. 2019;19(1):1213. Qualitative interviews with European menstrual-app users on how tracking tools shape their understanding of their own cycles and bodies. bmcpublichealth.biomedcentral.com
- Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, Menstrual Cycle. In StatPearls. Treasure Island, FL: StatPearls Publishing; 2024. Foundational reference on the four cycle phases (menstruation, follicular, ovulation, luteal) and the hormonal shifts driving each one. ncbi.nlm.nih.gov
- Mihm M, Gangooly S, Muttukrishna S. The Normal Menstrual Cycle in Women. Animal Reproduction Science. 2011;124(3-4):229-236. Peer-reviewed review of normal cycle-length variation and why counting from the first day of one period to the first day of the next is the standard convention. pubmed.ncbi.nlm.nih.gov
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262. Shows that fertile window timing can be unpredictable even when cycles are usually regular, so calendar estimates are best treated as a guide. doi.org