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 28-day figure is a convention rather than a measured average, and the 21 to 35 band this page uses is its own: ACOG gives 21 to 34 for adults and FIGO gives 24 to 38.

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 a convention rather than a measured average, and this page said both things until now. In a study of 612,613 ovulatory cycles the mean was 29.3 days and only 13 percent of cycles were actually 28 days long. Yours might be quite different and still completely normal.

CYCLE LENGTH: this page uses 21 to 35 days

• That band is this page's own. ACOG's figure for adults is 21 to 34 days: "by the third year after menarche, 60 to 80% of menstrual cycles are 21 to 34 days long, as is typical of adults". The string 21 to 35 does not appear in that document at all, though it is common as a working definition in research.
• FIGO disagrees with both. Its 2018 revisions define predictable cyclic menses as every 24 to 38 days, with anything under 24 days classed as frequent, and define regularity by variation of 7 to 9 days depending on age. So the three published answers to the same question are ACOG's 21 to 34, FIGO's 24 to 38, and this page's 21 to 35, and a 22-day cycle is normal on two of them and frequent on the third.
• For adolescents the same ACOG opinion gives a much wider range, 21 to 45 days, with a mean interval of 32.2 days in the first gynaecologic year. A 40-day cycle in a teenager is normal by ACOG and would fall outside this page's band.
• Cycle length gets SHORTER with age, not longer. This page previously said the opposite in both directions, and both halves were backwards. A study of more than 600,000 cycles found mean cycle length fell by 0.18 days per year of age from 25 to 45, a difference of 2.9 days between the youngest and oldest groups, and the STRAW+10 staging system defines the run-up to menopause by cycles becoming shorter. Young women have the longest cycles, not the shortest.
• The one honest complication: after that shortening phase, very late perimenopause brings erratic cycles and long gaps, defined by STRAW+10 as an interval of amenorrhoea of 60 days or more. That is a different thing from a longer regular cycle.

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. On how common it is, this page previously said 5 to 8 percent and no source publishes that. The figure that circulates is 3 to 8 percent, and the strongest evidence is narrower: a 2024 systematic review and meta-analysis in the Journal of Affective Disorders pooling 44 studies and 50,659 participants found 3.2 percent for a CONFIRMED diagnosis and 7.7 percent for a provisional one, and its authors warn that studies relying on provisional diagnosis produce artificially high rates. DSM-5 is quoted at 1.8 to 5.8 percent. Every published figure starts lower than 5.

• 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

Related Calculators

• Ovulation Calculator
• Pregnancy Due Date Calculator
• Age Calculator
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