Period Calculator

How to predict your next period

Predicting your next period requires two pieces of information: the first day of your last period, and your average cycle length.

CYCLE LENGTH = the number of days from the first day of one period to the first day of the next. The average is 28 days, but anywhere from 21 to 35 days is considered normal.

FORMULA: Next period start = First day of last period + Cycle length

EXAMPLE: If your last period started on the 1st of the month and your cycle is 28 days, your next period is expected on the 29th.

The calculator above tracks this for you and projects up to 12 months ahead. Remember: these are predictions based on averages. Actual periods can vary by a few days each cycle due to stress, illness, exercise, diet, sleep, and many other factors.

Understanding the menstrual cycle

The menstrual cycle has four phases, each with different hormonal patterns:

MENSTRUAL PHASE (Days 1-7, varies by individual):

• The lining of the uterus sheds, causing your period
• Estrogen and progesterone are at their lowest
• Common symptoms: cramps, fatigue, lower mood
• Typical duration: 3-7 days

FOLLICULAR PHASE (Days 1-13, overlapping with menstrual):

• Pituitary gland releases FSH (follicle-stimulating hormone)
• Eggs in ovaries mature in their follicles
• Estrogen rises, building up the uterine lining
• Energy levels typically increase as the phase progresses

OVULATION (around Day 14 in a 28-day cycle):

• A surge in LH (luteinizing hormone) triggers ovary to release a mature egg
• This is the most fertile time of the cycle
• The egg lives for 12-24 hours
• Some people feel mild cramping ('mittelschmerz') or notice changes in cervical mucus

LUTEAL PHASE (Days 15-28):

• The empty follicle (corpus luteum) produces progesterone
• Uterine lining thickens to prepare for possible pregnancy
• If no pregnancy, hormones drop and trigger the next period
• PMS symptoms (irritability, bloating, fatigue) tend to peak in the last few days

Cycle length variations

A '28-day cycle' is just an average — yours might be quite different and still completely normal.

NORMAL CYCLE LENGTH: 21 to 35 days

• Anything in this range is considered medically normal
• Shorter cycles (21-25 days) are more common in younger women
• Longer cycles (28-35 days) are more common in those approaching menopause

CYCLE IRREGULARITY:

• Most people have cycles that vary by 1-3 days each month
• Major variations (over 7 days different month-to-month) may indicate hormonal imbalance
• Common causes: stress, weight changes, exercise intensity changes, travel, illness, breastfeeding, perimenopause

WHEN TO SEE A DOCTOR:

• Cycles consistently shorter than 21 days or longer than 35 days
• Periods that last longer than 7 days or are unusually heavy (changing pads/tampons every 1-2 hours)
• Missed periods (3+ in a row) if not pregnant
• Severe pain that interferes with daily activities
• Bleeding between periods
• Sudden, persistent changes from your normal pattern

These can be signs of conditions like PCOS (polycystic ovary syndrome), endometriosis, thyroid issues, or other treatable conditions.

Tracking ovulation and fertility

Ovulation typically happens 14 days BEFORE your next period (NOT 14 days after your last one). For a 28-day cycle, that's around day 14. For a 35-day cycle, ovulation is around day 21.

FORMULA: Ovulation Day = Cycle Length - 14

FERTILE WINDOW: The 6 days leading up to and including ovulation. Sperm can survive 3-5 days in the female reproductive tract, so sex during the days before ovulation can still result in pregnancy.

SIGNS OF OVULATION:

• Cervical mucus becomes clear, slippery, and stretchy (like raw egg whites)
• Basal body temperature rises slightly (about 0.5°F or 0.3°C)
• Mild abdominal pain on one side (mittelschmerz, German for 'middle pain')
• Slight increase in libido for some people
• Breast tenderness
• Light spotting (less common)

OVULATION PREDICTOR KITS (OPKs): Detect the LH surge that triggers ovulation, usually 24-36 hours before egg release. Useful for those actively trying to conceive.

LIMITATIONS OF CALENDAR-BASED TRACKING:

• Only reliable for those with regular cycles
• Doesn't account for occasional anovulatory cycles (no ovulation occurs)
• Can be inaccurate after coming off birth control, postpartum, or during perimenopause
• For contraception, calendar methods have a 25% failure rate with typical use — not recommended as the sole birth control method

Common period concerns

PMS (Premenstrual Syndrome): Symptoms that appear 1-2 weeks before your period.

• Common: bloating, breast tenderness, mood swings, food cravings, fatigue, mild cramps
• Most people experience some PMS; severity varies enormously
• Lifestyle helpers: regular exercise, less caffeine and salt, more sleep, stress management

PMDD (Premenstrual Dysphoric Disorder): A severe form of PMS affecting 5-8% of menstruating people.

• Symptoms significantly disrupt daily life
• Often includes severe depression, anxiety, or anger before periods
• Treatable — see a doctor if symptoms feel overwhelming

HEAVY PERIODS (menorrhagia): Bleeding heavy enough to soak through a pad or tampon every hour for several hours.

• Common causes: hormonal imbalance, fibroids, polyps, IUDs (especially copper)
• Persistent heavy bleeding warrants medical evaluation — risks include anemia

MISSED PERIODS (amenorrhea):

• Most common cause: pregnancy (take a test if there's any possibility)
• Other causes: significant weight loss, intense exercise, stress, thyroid issues, PCOS, perimenopause, certain medications, breastfeeding
• See a doctor if you miss 3+ periods and aren't pregnant

PAINFUL PERIODS (dysmenorrhea):

• Mild to moderate cramps are normal and caused by uterine contractions
• Severe pain that prevents daily activities is NOT normal — can indicate endometriosis or fibroids
• Most people respond well to NSAIDs (ibuprofen, naproxen), heat, and gentle exercise
• Persistent severe pain should be evaluated by a doctor

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