Period Calculator
Predict your next 6 menstrual periods, fertile windows, and ovulation dates based on your cycle length and last period start date.
Frequently Asked Questions
How accurate is a period prediction calculator?
Predictions are most accurate for people with regular cycles — if your cycle length varies by fewer than 3 days month to month, predictions are typically accurate to within 1–2 days. Irregular cycles (varying more than 7 days) make predictions unreliable. Factors that can shift period dates include significant stress, illness, rapid weight change, intense exercise, hormonal contraceptives, thyroid issues, and approaching menopause.
What is the average menstrual cycle length?
While 28 days is widely cited as average, research shows that most cycles actually fall between 21 and 35 days, and significant variation from month to month is normal. A study of 600,000+ cycles found that only 13% of women have a 28-day cycle. Your personal average — calculated over 3–6 months of tracking — is more useful than the textbook 28 days.
When is the fertile window during a menstrual cycle?
The fertile window spans approximately 5 days before ovulation to the day of ovulation — totalling 6 days. Sperm can survive up to 5 days in the fallopian tubes, while the egg is only viable for 12–24 hours after release. For a 28-day cycle, ovulation typically falls around day 14, making days 10–15 the fertile window — but this shifts with cycle length.
What period length is considered normal?
Normal menstrual periods last 2–7 days, with the heaviest flow on days 1–2. The average blood loss per cycle is 30–80 ml (roughly 2–6 tablespoons). Periods lasting longer than 7 days, blood loss exceeding 80 ml (soaking a pad/tampon hourly for several consecutive hours), or periods occurring less than 21 days apart are considered abnormal and worth discussing with a doctor.
What causes a late or missed period?
Pregnancy is the most common cause, but others include: significant stress (cortisol disrupts the hypothalamus-pituitary axis), low body weight or rapid weight loss, excessive exercise, PCOS (polycystic ovary syndrome), thyroid disorders, perimenopause, breastfeeding (prolactin suppresses ovulation), certain medications, and illness. A cycle 7+ days late without pregnancy warrants a medical check if it happens repeatedly.