Period / Menstrual Cycle Calculator

Add your average cycle length to the start date of your last period. Example: last period started 1 May, cycle length 28 days. Next period: 29 May. For 30-day cycles, next would be 31 May. The calculation works best with regular cycles. With irregular cycles, calendar predictions become unreliable — apps that learn from your data over multiple cycles do better than fixed math. Track 3–6 cycles before relying on predictions. Cycle length is counted from day 1 of one period to day 1 of the next, not from the end of one period to the start of the next.

Normal menstrual cycle length is 21 to 35 days, with 28 being the textbook average. Cycles outside this range — shorter than 21 days or longer than 35 — may indicate hormonal imbalance, thyroid issues, PCOS, or other concerns. Cycles vary slightly month to month even in healthy women — variation up to 7 days is normal. Bigger variation persistently warrants evaluation. Period (bleeding) duration is typically 3 to 7 days. The first cycles after starting periods (menarche) and approaching menopause (perimenopause) tend to be more variable. Stress, weight changes, and travel can all affect cycle length temporarily.

Less reliably. Calendar-based calculators assume regular cycles; with irregular cycles, predictions may be off by 5 to 14 days or more. Cycle tracking apps that learn from your historical data give better estimates over time, especially after 3–6 months of input. For irregular cycles, also track ovulation signs (basal body temperature, cervical mucus, ovulation strips) — they help anchor the prediction. If your cycles vary by more than 7–10 days month to month, see a gynaecologist. PCOS, thyroid disorders, hormonal imbalances, and other treatable conditions often cause irregular cycles. Predicting periods is more useful when the underlying issue is addressed.

Ovulation typically occurs about 14 days before your next expected period — the luteal phase is fairly fixed. So in a 28-day cycle, ovulation happens around day 14 (counting from day 1 of your period). In a 30-day cycle, ovulation is around day 16. In a 32-day cycle, day 18. Subtract 14 from your cycle length, count from day 1 of your last period. Example: cycle length 30 days, last period started 5 May. Ovulation around 19 May. The fertile window includes 5 days before ovulation plus ovulation day itself, because sperm survive up to 5 days.

Several common reasons. Stress, especially physical or emotional, suppresses ovulation and delays periods. Significant weight change (gain or loss). Intense exercise, especially with low calorie intake. Hormonal imbalances like PCOS, thyroid dysfunction, or perimenopause. Recent illness, travel across time zones, or sleep disruption. Polycystic ovary syndrome causes irregular cycles in many women. If pregnancy tests are reliably negative and you've missed multiple periods, see a gynaecologist for evaluation. Tests typically include thyroid panel, prolactin, and ultrasound. A single late period is usually nothing serious; persistent irregularity needs investigation. Don't assume — get checked.

No. Period calculators are useful for tracking and planning, not for preventing pregnancy. Cycle variation, sperm survival up to 5 days, and unpredictable ovulation make calendar-based contraception unreliable — typical use failure rates are 12–24% per year. Modern fertility awareness methods that combine multiple tracking signs (calendar, basal temperature, cervical mucus, sometimes ovulation strips) have lower failure rates with perfect use (around 1–5%) but still much higher than reliable contraception. For preventing pregnancy, use medically established methods — barriers, hormonal contraception, IUDs, or sterilisation. The calculator is a planning tool only.