Period Calculator
Predict your next period, track your menstrual cycle, and view a 6-month calendar. Includes PMS prediction and cycle length analysis.
Medical Disclaimer: This calculator provides estimates based on average cycle patterns. Menstrual cycles vary due to stress, health conditions, and lifestyle factors. This tool is not contraception. Consult a healthcare provider for personalized medical advice.
📊Cycle History (for better accuracy)
Add your past period start dates to calculate your true average cycle length.
About This Calculator
When will I get my period? The Period Calculator helps you predict your next menstrual period, track your cycle patterns, and understand your body with confidence. Whether you're asking "when is my next period due?" or "why is my period late?", this tool provides answers based on your personal cycle data.
Unlike basic calculators that only show one prediction, our tool provides a comprehensive 6-month calendar view, PMS window predictions, fertile window estimates, and cycle length analysis. The average menstrual cycle lasts 28 days, but normal ranges from 21-35 days - and only about 13% of women actually have a perfect 28-day cycle. This calculator learns your unique pattern.
Why track your period? Over 420 million people worldwide use period tracking apps for good reason: understanding your cycle helps you plan around high-energy and low-energy phases, predict PMS symptoms before they hit, identify health patterns, and even optimize fertility awareness. The more cycles you track, the more accurate your predictions become.
Privacy first - no app required, no account needed. Your data stays private in your browser using localStorage. Nothing is sent to any server. You can track multiple cycles, see patterns in your cycle length, and print a calendar for offline reference - all without creating an account or sharing personal health information.
Whether you're dealing with irregular periods, trying to plan around an important event, or just want to know what to expect, this calculator uses proven methods based on the same principles doctors use.
Disclaimer: This calculator provides estimates based on average cycle patterns. Menstrual cycles can vary due to stress, PCOS, thyroid issues, medications, and lifestyle factors. Consult a healthcare provider for concerns about irregular periods or reproductive health. This is not a reliable method of contraception.
How to Use the Period Calculator
- 1**Enter your last period start date**: Select the first day of your most recent menstrual period - this is Day 1 of your cycle, when bleeding begins.
- 2**Set your average cycle length**: The typical cycle is 28 days, but normal ranges from 21-35 days. If unsure, use 28 days as a starting point.
- 3**Enter your average period duration**: How many days does your period typically last? Most periods last 3-7 days.
- 4**Add cycle history (optional)**: For more accurate predictions, enter the start dates of your last 3-6 periods. This helps calculate your true average cycle length.
- 5**Enable PMS prediction (optional)**: Toggle this on to see when PMS symptoms might begin, typically 7-10 days before your period.
- 6**View your 6-month calendar**: See predicted periods, fertile windows, and PMS days color-coded on an interactive calendar.
- 7**Print or save**: Use the print button to save a calendar for offline reference.
Understanding Your Menstrual Cycle
Your menstrual cycle is counted from the first day of one period to the first day of the next. A "normal" cycle can range from 21 to 35 days in adults, with the average being 28 days.
The Four Phases of Your Cycle:
| Phase | Days (28-day cycle) | What Happens |
|---|---|---|
| Menstruation | Days 1-5 | Uterine lining sheds (your period) |
| Follicular | Days 1-13 | Egg develops in ovary, estrogen rises |
| Ovulation | Day 14 | Egg released from ovary (fertile window) |
| Luteal | Days 15-28 | Uterus prepares for pregnancy or next period |
What Affects Cycle Length:
- Stress: Can delay ovulation and extend your cycle
- Weight changes: Significant gain or loss affects hormones
- Exercise: Intense training can disrupt cycles
- Age: Cycles often become more regular after your 20s
- Birth control: Hormonal contraceptives regulate or stop periods
- Health conditions: PCOS, thyroid issues, and other conditions affect cycles
What's a Normal Cycle Length?
While 28 days is often cited as "normal," only about 13% of women actually have a perfect 28-day cycle. Here's what research shows:
Cycle Length Distribution:
| Length | Classification | Notes |
|---|---|---|
| 21-24 days | Short but normal | More common in older reproductive years |
| 25-30 days | Typical range | Most women fall here |
| 31-35 days | Long but normal | More common in teens and perimenopause |
| <21 or >35 days | Irregular | May warrant medical evaluation |
Cycle Variation Is Normal: Your cycle length can vary by 2-7 days from month to month and still be considered regular. Factors like travel, illness, stress, and sleep changes commonly cause minor variations.
When to See a Doctor:
- Periods consistently shorter than 21 days or longer than 35 days
- Suddenly irregular after years of regularity
- Extremely heavy bleeding (soaking through a pad/tampon hourly)
- Severe pain that interferes with daily activities
- No period for 3+ months (when not pregnant)
PMS vs PMDD: Understanding Premenstrual Symptoms
Premenstrual Syndrome (PMS) affects up to 75% of menstruating women to some degree. Symptoms typically begin 7-10 days before your period and resolve within a few days of bleeding starting.
Common PMS Symptoms:
| Physical | Emotional |
|---|---|
| Bloating | Mood swings |
| Breast tenderness | Irritability |
| Headaches | Anxiety |
| Fatigue | Food cravings |
| Acne flare-ups | Difficulty concentrating |
| Cramps | Feeling overwhelmed |
PMDD (Premenstrual Dysphoric Disorder):
PMDD is a severe form affecting 3-8% of women. Unlike regular PMS, PMDD significantly impacts daily life and relationships. Symptoms include:
- Severe depression or hopelessness
- Intense anger or conflict with others
- Marked anxiety or tension
- Decreased interest in usual activities
- Difficulty concentrating
- Physical symptoms similar to PMS
If symptoms severely impact your quality of life, talk to your healthcare provider about treatment options including lifestyle changes, supplements, or medications.
Tracking Your Cycle: Why It Matters
Cycle tracking isn't just about knowing when your period will arrive - it provides valuable health insights and helps you:
1. Plan Important Events Know which weeks you'll feel your best for job interviews, athletic events, vacations, or social occasions.
2. Identify Health Patterns Regular tracking can reveal:
- Cycle irregularities that might indicate hormonal issues
- Symptom patterns (when you get headaches, mood changes, etc.)
- How lifestyle factors affect your cycle
3. Understand Fertility Your fertile window occurs around ovulation (typically mid-cycle). Knowing this timing helps whether you're trying to conceive or avoid pregnancy.
4. Improve Doctor Visits Arriving at your gynecologist with cycle data makes consultations more productive. They can spot patterns you might miss.
5. Connect Body and Mind Many women report that understanding their cycle helps them:
- Schedule demanding work during high-energy phases
- Plan rest and self-care during premenstrual days
- Understand emotional fluctuations as hormone-related, not personal failings
Tracking Methods:
- This calculator: Simple predictions based on cycle length
- Apps: Detailed tracking with symptom logging
- Basal body temperature: Confirms ovulation after the fact
- Cervical mucus monitoring: Physical signs of fertility
The Four Phases of Your Menstrual Cycle Explained
Understanding the four phases of your menstrual cycle helps you predict not just your period, but your energy levels, mood, and fertility throughout the month.
Phase 1: Menstruation (Days 1-5)
This is your period - Day 1 is the first day of actual bleeding (not spotting).
| Aspect | What to Expect |
|---|---|
| Duration | Typically 3-7 days |
| Flow | Heaviest days 1-2, then tapers |
| Hormones | Estrogen and progesterone at lowest |
| Energy | Often lower; rest is beneficial |
| Symptoms | Cramps, fatigue, possible headaches |
Phase 2: Follicular Phase (Days 1-13)
This phase overlaps with menstruation and continues after. It's when your body prepares eggs for potential fertilization.
| Aspect | What to Expect |
|---|---|
| Duration | About 13-14 days (most variable phase) |
| Hormones | Estrogen rising; 11-20 eggs begin developing |
| Energy | Increasing; many feel their best |
| Mood | Often positive, creative, social |
| Best for | Starting new projects, exercise, socializing |
Pro tip: Many women report peak mental clarity and motivation during the late follicular phase, making it ideal for important meetings, exams, or challenging workouts.
Phase 3: Ovulation (Day 14, ~24 hours)
An egg is released from the ovary - this is your fertile window.
| Aspect | What to Expect |
|---|---|
| Duration | About 24 hours |
| Timing | Approximately day 14 (varies by cycle length) |
| Cervical mucus | Clear, stretchy, egg-white consistency |
| Libido | Often peaks during this time |
| Energy | High energy; peak physical performance |
Fertile window: Sperm can live up to 5 days, so your fertile window is actually about 6 days: 5 days before ovulation and 1 day after.
Phase 4: Luteal Phase (Days 15-28)
The "PMS phase" - your body prepares for either pregnancy or the next period.
| Aspect | What to Expect |
|---|---|
| Duration | Consistent 14 days in most women |
| Hormones | Progesterone rises then falls |
| Days 15-21 | Often calm; some feel nesting instinct |
| Days 22-28 | PMS symptoms may appear |
| Energy | Gradually decreasing toward period |
Luteal phase symptoms: Tender breasts, bloating, mood changes, acne, and food cravings are all triggered by progesterone fluctuations during this phase.
Why Is My Period Late? 12 Common Causes
A late period is one of the most common reasons women search for period calculators. Here are the most common causes beyond pregnancy:
Stress and Lifestyle Factors
| Cause | How It Affects Your Cycle |
|---|---|
| Stress | Cortisol can delay or prevent ovulation, pushing back your period |
| Weight changes | Losing or gaining significant weight affects hormone production |
| Excessive exercise | Can suppress menstruation (secondary amenorrhea) |
| Travel | Jet lag and schedule changes can disrupt hormonal rhythms |
| Sleep changes | Poor sleep affects the hypothalamus, which controls your cycle |
Hormonal and Medical Causes
| Cause | What to Know |
|---|---|
| PCOS | Affects up to 10% of women; causes irregular or missed periods |
| Thyroid disorders | Both hyperthyroidism and hypothyroidism affect cycles |
| Perimenopause | Irregular periods are the hallmark sign; starts mid-40s |
| Breastfeeding | Prolactin suppresses ovulation; periods may not return for months |
| Birth control | Coming off hormonal contraception can cause temporary irregularity |
| Hormonal imbalance | Low estrogen or progesterone can delay periods |
When Your Period Is Considered "Late"
| Situation | What It Means |
|---|---|
| 1-5 days late | Usually normal variation |
| 6-7 days late | Consider a pregnancy test if sexually active |
| 1-2 weeks late | Warrants attention; test if pregnancy is possible |
| 35+ days since last period | Considered a missed period |
| 3+ months without period | Amenorrhea - see a doctor |
Signs You Should See a Doctor
- Periods consistently shorter than 21 days or longer than 35 days
- Suddenly irregular after years of regularity
- No period for 3+ months (and not pregnant)
- Very heavy bleeding (soaking through protection hourly)
- Severe pain that affects daily activities
- New symptoms like excessive hair growth or acne (possible PCOS)
Understanding Your Fertile Window
While this calculator isn't designed for conception planning or contraception, understanding your fertile window provides valuable health insight.
When Is Your Fertile Window?
| Timing | Fertility Level |
|---|---|
| 5 days before ovulation | Fertile (sperm can survive) |
| 2-3 days before ovulation | Most fertile |
| Day of ovulation | Peak fertility (egg released) |
| 1 day after ovulation | Declining (egg only lives 12-24 hours) |
| 2+ days after ovulation | Not fertile until next cycle |
How to Estimate Ovulation
For a 28-day cycle, ovulation typically occurs around day 14. But the formula adjusts for different cycle lengths:
| Your Cycle Length | Estimated Ovulation Day |
|---|---|
| 21 days | Day 7 |
| 24 days | Day 10 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 35 days | Day 21 |
Formula: Ovulation typically occurs 14 days BEFORE your next period (not 14 days after your last one).
Physical Signs of Ovulation
| Sign | What to Look For |
|---|---|
| Cervical mucus | Clear, stretchy, egg-white consistency |
| Basal body temperature | Slight rise (0.5-1°F) after ovulation |
| Mittelschmerz | Mild one-sided lower abdominal pain |
| Libido changes | Often increases around ovulation |
| Breast tenderness | Some women notice this |
Ovulation Prediction Methods
| Method | Accuracy | Notes |
|---|---|---|
| Calendar counting | Low-moderate | Only reliable with very regular cycles |
| Ovulation predictor kits | High | Detect LH surge 24-36 hours before ovulation |
| Basal body temperature | Moderate | Confirms ovulation after it happens |
| Cervical mucus tracking | Moderate | Requires learning to interpret changes |
| Multiple methods combined | Highest | Best approach for accuracy |
Important: This calculator provides estimates only. Do NOT rely on cycle tracking alone for contraception or fertility planning.
How to Track Your Period Effectively
Effective period tracking goes beyond just marking start dates. Here's how to get the most valuable insights from your cycle data.
What to Track Each Cycle
| Data Point | Why It Matters |
|---|---|
| First day of bleeding | This is Day 1 of your cycle - the foundation of all calculations |
| Flow intensity | Changes may indicate hormonal shifts or health issues |
| Period duration | Tracks if your period is getting longer or shorter |
| Symptoms | Identifies PMS patterns, helps predict future symptoms |
| Mood/energy | Reveals which cycle phases affect you most |
| Discharge changes | Can help identify fertile window |
How to Calculate Your Cycle Length
- Mark the first day of your period (Day 1)
- Count all days until your next period starts
- That total is your cycle length
- Track 3-6 cycles to find your average
Example:
- Period 1 starts: November 1
- Period 2 starts: November 29
- Cycle length: 28 days
Dealing with Irregular Cycles
| Cycle Type | Tracking Approach |
|---|---|
| Regular (varies <7 days) | Use average cycle length for predictions |
| Somewhat irregular (varies 8-14 days) | Track 6+ cycles; expect less accurate predictions |
| Very irregular (varies 15+ days) | Focus on symptom tracking; predictions less reliable |
Signs Your Tracking Is Working
After 3-6 cycles of consistent tracking, you should be able to:
- Predict your period within 1-3 days
- Anticipate when PMS will start
- Recognize your high-energy vs. low-energy phases
- Identify patterns in symptoms
- Have useful data for doctor visits
Common Tracking Mistakes
| Mistake | Why It Matters |
|---|---|
| Counting spotting as Day 1 | Day 1 is actual bleeding, not spotting |
| Inconsistent tracking | Gaps make averages less accurate |
| Only tracking period dates | Missing valuable symptom data |
| Assuming 28-day cycle | Only 13% of women have this cycle length |
| Not noting stress/travel | These affect predictions |
PCOS and Irregular Periods: What You Need to Know
Polycystic Ovary Syndrome (PCOS) is the most common cause of irregular periods, affecting up to 10% of women of reproductive age. Understanding PCOS helps you know when irregular cycles are just normal variation vs. a sign to see a doctor.
What Is PCOS?
PCOS is a hormonal disorder that causes:
- Irregular or missed periods
- Excess androgen (male hormone) levels
- Polycystic ovaries (multiple small cysts)
You only need two of these three criteria for diagnosis.
How PCOS Affects Your Cycle
| Normal Cycle | PCOS Cycle |
|---|---|
| Regular ovulation | Infrequent or no ovulation |
| Predictable periods | Fewer than 8 periods per year |
| 21-35 day cycle | Cycles often 35+ days |
| Consistent flow | Variable, sometimes heavy |
PCOS Symptoms Beyond Irregular Periods
| Symptom | Cause |
|---|---|
| Excess facial/body hair | High androgens |
| Acne (especially jawline) | High androgens |
| Hair thinning on scalp | High androgens |
| Weight gain (especially abdomen) | Insulin resistance |
| Dark skin patches | Insulin resistance |
| Difficulty getting pregnant | Irregular ovulation |
Should You Get Tested for PCOS?
Consider seeing a doctor if:
- Fewer than 8 periods per year
- Cycles consistently longer than 35 days
- Excessive hair growth or severe acne
- Difficulty losing weight despite diet/exercise
- Trouble conceiving after 12 months of trying
PCOS Management and Treatment
| Approach | What It Does |
|---|---|
| Lifestyle changes | Weight loss can restore regular cycles |
| Birth control pills | Regulate periods and reduce androgens |
| Metformin | Addresses insulin resistance |
| Anti-androgens | Reduce hair growth and acne |
| Fertility medications | Help induce ovulation when trying to conceive |
Good news: Many women with PCOS can achieve regular cycles through lifestyle modifications like maintaining a healthy weight, exercising regularly, and following a low-glycemic diet.
Pro Tips
- 💡Track at least 3-6 cycles for the most accurate predictions. Your average cycle length becomes clearer with more data. The more cycles you track, the better the predictions.
- 💡The first day of your period is Day 1 of your cycle - this is when actual bleeding begins, not spotting. Getting this right is fundamental to accurate tracking.
- 💡Stress can delay ovulation and push your period back by days or even weeks. If you're experiencing unusual stress, your period may be later than predicted.
- 💡Regular exercise and a balanced diet can help regulate irregular cycles over time. Even moderate lifestyle changes can improve cycle regularity.
- 💡Keep a symptom diary alongside cycle tracking to identify patterns in PMS, energy levels, and mood. Many women find they perform better on certain tasks during specific cycle phases.
- 💡Your fertile window is approximately 6 days: 5 days before ovulation and 1 day after. This calculator provides estimates only - do NOT rely on it for contraception.
- 💡Print your 6-month calendar before traveling to areas with limited internet access. Being prepared helps you pack appropriately and plan activities.
- 💡Use the follicular phase (days 6-14) for important meetings, job interviews, or challenging workouts - many women feel most energetic and focused during this time.
- 💡If your cycles vary by more than 7-9 days consistently, this may indicate a hormonal issue like PCOS or thyroid problems. Consider seeing a doctor.
- 💡Track whether your period comes on time to help your doctor diagnose issues faster. Detailed cycle data makes medical consultations more productive.
- 💡Cycle changes are normal during major life transitions: after pregnancy, when starting/stopping birth control, during perimenopause, or after significant weight changes.
- 💡If you have severe PMS or PMDD symptoms, schedule self-care and reduced responsibilities during your luteal phase (days 15-28) when possible.
Frequently Asked Questions
For women with regular cycles (21-35 days with consistent length), predictions are typically accurate within 1-3 days. Accuracy improves significantly when you enter multiple past cycles to calculate your true average. For irregular cycles, predictions are less reliable - consider tracking more data points or consulting a healthcare provider.
