Period Calculator - Track Your Menstrual Cycle & Predict Next Period

Our free period calculator helps you predict when your next period will start, track your menstrual cycle patterns, and identify your fertile window. Get accurate predictions based on your cycle history and plan ahead with confidence.

What is a Period Calculator?

A period calculator (or menstrual cycle calculator) predicts your next period start date based on your last period and average cycle length. It helps you track patterns, anticipate menstruation, plan activities, and identify fertile days for conception or contraception purposes.

Why Track Your Menstrual Cycle?

Health Monitoring:

Detect irregular cycles that may indicate health issues
Identify patterns in symptoms (PMS, cramps, mood)
Track fertility for conception planning
Monitor hormonal health

Practical Planning:

Plan vacations, events, and activities
Prepare with menstrual products
Anticipate PMS symptoms
Manage birth control timing

Enter Your Information

Understanding the Menstrual Cycle

The Four Phases

Phase 1: Menstruation (Days 1-5)

Period bleeding occurs
Uterine lining sheds
Hormone levels at lowest
Duration: 3-7 days (average 5 days)

Phase 2: Follicular Phase (Days 1-13)

Overlaps with menstruation
Follicles develop in ovaries
Estrogen levels rise
Uterine lining rebuilds
Energy and mood typically improve

Phase 3: Ovulation (Day 14)

Mature egg released from ovary
Most fertile time of cycle
LH surge triggers egg release
Lasts approximately 24 hours
Occurs ~14 days before next period

Phase 4: Luteal Phase (Days 15-28)

After ovulation until next period
Corpus luteum produces progesterone
Uterine lining thickens
PMS symptoms may appear
Duration: Typically 14 days (consistent)

Cycle Length Variations

What’s Normal?

Cycle Length	Classification	Percentage of Women
21-24 days	Short	15%
25-27 days	Slightly short	20%
28-30 days	Average	35%
31-34 days	Slightly long	20%
35-40 days	Long	10%

Average Cycle: 28 days (but 21-35 days is normal) Period Duration: 3-7 days (average 5 days) Cycle Regularity: Varies by 2-3 days month to month is normal

How to Calculate Your Next Period

Simple Method

Formula: Last Period Start Date + Cycle Length = Next Period Date

Example:

Last period: January 1
Cycle length: 28 days
Next period: January 29 (1 + 28)

Tracking for Accuracy

For Regular Cycles:

Track your last 3-6 cycles
Calculate average cycle length
Add to last period start date
Result is predicted next period

Example Calculation:

Cycle 1: 28 days
Cycle 2: 30 days
Cycle 3: 27 days
Average: 28.3 days (round to 28)

For Irregular Cycles:

Track minimum and maximum cycle length
Predict earliest and latest possible start dates
Be prepared across wider window

Digital Tracking Tools

Period Tracking Apps:

Flo, Clue, Period Tracker
Automatic calculations
Symptom logging
Fertility predictions
Cycle pattern analysis

Calendar Method:

Mark first day of each period
Count days between periods
Identify average cycle length
Predict next cycle

Period Calendar and Tracking

What to Track

Essential Data:

First day of period (Day 1)
Last day of period
Flow intensity (light, medium, heavy)
Cycle length (days from Day 1 to next Day 1)

Optional but Helpful:

PMS symptoms and timing
Menstrual cramps severity
Mood changes
Breast tenderness
Bloating
Food cravings
Energy levels
Sexual desire changes

Creating Your Period Calendar

Month 1:

Mark first day of period as Day 1
Track daily until period ends
Note flow and symptoms

Month 2+:

Count days since last Day 1
This number is your cycle length
Predict next period
Verify accuracy when period arrives

After 3-6 Months:

Calculate average cycle length
Identify pattern consistency
Note any irregularities
Improve future predictions

Fertile Window and Ovulation

Predicting Fertile Days

From Your Period:

Ovulation typically occurs 14 days before next period
For 28-day cycle: Ovulation on day 14
For 30-day cycle: Ovulation on day 16
For 35-day cycle: Ovulation on day 21

Fertile Window:

5 days before ovulation
Ovulation day itself
Total: 6 days of possible conception

Cycle Day Fertility Chart

For 28-Day Cycle:

Days	Phase	Fertility
1-7	Menstruation	Not fertile
8-9	Post-period	Low fertility
10-14	Pre-ovulation	High fertility
15	Ovulation day	Peak fertility
16-28	Post-ovulation	Not fertile

Pregnancy Probability:

Days 10-12: 10-15% chance
Days 13-14: 20-30% chance
Day 15 (ovulation): 20-25% chance
Days 16+: <5% chance

Period Symptoms and PMS

Common Menstrual Symptoms

During Period (Days 1-5):

Menstrual cramps (dysmenorrhea)
Lower back pain
Headaches
Fatigue
Bloating
Diarrhea or constipation
Acne breakouts

Severity Scale:

Mild: Noticeable but doesn’t interfere with activities
Moderate: Affects daily activities, manageable with OTC pain relief
Severe: Significantly impacts life, may require medical treatment

PMS (Premenstrual Syndrome)

Timeline: Typically 5-11 days before period

Physical Symptoms:

Breast tenderness and swelling
Bloating and water retention
Headaches or migraines
Fatigue
Food cravings (especially sweet or salty)
Acne breakouts
Digestive changes

Emotional Symptoms:

Mood swings
Irritability or anger
Anxiety
Depression or sadness
Difficulty concentrating
Social withdrawal
Sleep disturbances

PMDD (Premenstrual Dysphoric Disorder):

Severe form of PMS (3-8% of women)
Intense mood symptoms
Significant life disruption
May require medical treatment

Managing Period Symptoms

For Cramps:

Heat therapy (heating pad, warm bath)
OTC pain relievers (ibuprofen, naproxen)
Gentle exercise (yoga, walking)
Massage lower abdomen

For Bloating:

Reduce salt intake
Stay hydrated
Avoid carbonated drinks
Eat potassium-rich foods (bananas, avocados)

For Mood Symptoms:

Regular exercise
Adequate sleep (7-9 hours)
Stress management techniques
Limit caffeine and alcohol
Balanced diet

Irregular Periods: Causes and Concerns

What is an Irregular Period?

Irregular Characteristics:

Cycle length varies by >7-9 days month to month
Periods significantly early or late unpredictably
Cycles shorter than 21 days or longer than 35 days
Missing periods (not pregnant)
Spotting between periods

Common Causes

Hormonal Imbalances:

PCOS (Polycystic Ovary Syndrome): Most common (1 in 10 women)
Thyroid disorders: Hypothyroidism or hyperthyroidism
Perimenopause: Transition to menopause (40s-50s)
Recent birth control changes: Can take 3-6 months to regulate

Lifestyle Factors:

Stress: Disrupts hypothalamic-pituitary-ovarian axis
Extreme weight changes: Significant loss or gain
Excessive exercise: Over-training, especially endurance athletes
Poor nutrition: Eating disorders, restrictive diets

Medical Conditions:

Uterine fibroids or polyps
Endometriosis
Pelvic inflammatory disease
Premature ovarian insufficiency

Medications:

Hormonal birth control
Antidepressants
Antipsychotics
Chemotherapy
Blood thinners

When to See a Doctor

Seek Medical Attention If:

Periods stop for 3+ months (not pregnant)
Suddenly irregular after being regular
Bleeding lasts more than 7 days
Very heavy bleeding (soaking pad/tampon every 1-2 hours)
Severe pain that interferes with daily life
Bleeding between periods
Periods more frequent than every 21 days
Periods less frequent than every 35 days

Period Tracking for Birth Control

Fertility Awareness Method (FAM)

How It Works:

Track fertile window
Avoid intercourse or use protection during fertile days
Requires precise tracking

Effectiveness:

Perfect use: 95-99% effective
Typical use: 76-88% effective
Requires discipline and regular cycles

Not Recommended For:

Irregular cycles
Recent birth control discontinuation
Postpartum or breastfeeding
Perimenopause

Rhythm Method vs. Modern Tracking

Rhythm Method (Outdated):

Calendar-only tracking
Less effective (76% typical use)
Doesn’t account for cycle variations

Modern FAM:

Combines calendar with cervical mucus
Basal body temperature monitoring
Ovulation predictor kits
More accurate (95-99% with perfect use)

Age and Menstrual Cycles

Puberty to 20s

First Period (Menarche):

Average age: 12-13 years
Normal range: 9-16 years
Irregular cycles first 1-2 years
Gradually become regular

Late Teens/20s:

Cycles typically regular
Average 28-30 days
Most fertile years
Consistent ovulation

30s to Early 40s

Peak Fertility Decline:

Cycles remain mostly regular
Subtle changes may begin
Fertility gradually decreases
PMS may intensify

Perimenopause (40s-50s)

Transition Period:

Cycles become irregular
Skipped periods common
Heavier or lighter flow
Shorter or longer cycles
Hormone fluctuations increase

Duration: 4-8 years on average

Menopause

Definition: 12 consecutive months without a period

Average Age: 51 years Normal Range: 45-55 years

After Menopause:

No more periods
No more ovulation
Cannot become pregnant naturally
Any bleeding should be evaluated by doctor

Frequently Asked Questions

How do I calculate my next period?

Add your average cycle length to the first day of your last period. For example, if your last period started January 1 and your cycle is 28 days, your next period should start around January 29.

What is a normal period cycle length?

A normal menstrual cycle is anywhere from 21-35 days, measured from the first day of one period to the first day of the next. The average is 28 days, but variation of 2-3 days month to month is completely normal.

How accurate are period calculators?

Period calculators are 80-90% accurate for women with regular cycles, predicting within 1-3 days. Accuracy decreases with irregular cycles. Tracking for 3-6 months improves prediction accuracy.

Can stress delay my period?

Yes. Severe or prolonged stress can delay ovulation, which delays your period. The delay can range from a few days to several weeks. Once the stressful period passes, cycles typically normalize.

Why is my period late if I’m not pregnant?

Common reasons include stress, significant weight changes, excessive exercise, illness, hormonal imbalances (thyroid, PCOS), medications, or irregular ovulation. If your period is more than 5-7 days late and you’re not pregnant, consider seeing a doctor.

How many days late can a period be before worrying?

A period can be 5-7 days late occasionally without concern, especially if you’ve experienced stress or lifestyle changes. If your period is more than a week late and you’re not pregnant, or if irregular periods become a pattern, consult a healthcare provider.

What is considered a heavy period?

Heavy menstrual bleeding (menorrhagia) is defined as soaking through a pad or tampon every 1-2 hours, needing to change protection during the night, passing large blood clots (larger than a quarter), or bleeding lasting more than 7 days.

Can I get pregnant on my period?

It’s unlikely but possible, especially if you have short cycles (21-24 days). Sperm can survive up to 5 days, so if you ovulate early (day 7-10), intercourse during your period could result in pregnancy.

How do I know if my cycle is irregular?

Your cycle is considered irregular if it consistently varies by more than 7-9 days, is shorter than 21 days, longer than 35 days, you skip periods, or you bleed between periods. Occasional variation of 2-3 days is normal.

At what age do periods stop?

Periods typically stop between ages 45-55, with the average age being 51. This is called menopause. The transition period (perimenopause) can begin in the early-to-mid 40s and lasts 4-8 years on average.

Conclusion

Tracking your menstrual cycle is essential for understanding your reproductive health, planning activities, and monitoring for potential health issues. This free period calculator provides accurate predictions to help you stay prepared and informed about your body’s natural rhythms.

Whether you’re tracking for pregnancy planning, birth control, or general health awareness, consistent monitoring helps you identify patterns and recognize when something may be off. Use this calculator alongside symptom tracking for comprehensive cycle awareness.

Calculate your next period now to stay prepared and take control of your menstrual health!

Medical Disclaimer: This period calculator is for informational and educational purposes only. It does not replace professional medical advice, diagnosis, or treatment. If you experience severe pain, heavy bleeding, very irregular cycles, or other concerning symptoms, consult with your gynecologist or healthcare provider.