How menstrual cycle calculation works
The menstrual cycle starts on the first day of bleeding (day 1) and ends the day before the next period begins. Average cycle length is 28 days, but anything from 21 to 35 days is medically normal. This calculator takes your last period start date and your average cycle length to predict future periods.
A 25-year-old woman with a 30-day cycle who had her last period on May 1 can expect her next period around May 31, the one after that around June 30, and so on. The calculator shows three months of periods by default — you can extend to six.
Period duration and what’s normal
Most periods last 3–7 days. Flow is typically heaviest in the first 2 days. The default of 5 days in this calculator is the population average. If your periods consistently last fewer than 2 days or more than 7 days, that’s worth discussing with a gynaecologist — it can indicate hormonal imbalances, fibroids, thyroid issues, or other conditions.
Clots smaller than a 10-rupee coin during the first 2 days are normal. Larger clots, soaking through a pad or tampon every hour for multiple hours, or periods lasting longer than 7 days consistently are signs of heavy menstrual bleeding (menorrhagia), which is common and treatable.
Why cycles get irregular
Stress is the most common cause of a late or missed period in women without underlying conditions. Significant weight changes, intense exercise, travel, illness, and hormonal shifts can all push the cycle off by days or weeks. PCOS (polycystic ovary syndrome) causes irregular cycles in 1 in 5 women in India — irregular ovulation means irregular periods.
Thyroid issues, both hypo and hyperthyroid, affect cycle regularity. Perimenopause (typically 40s) causes increasing irregularity before periods stop entirely. If your cycle length varies by more than 7–9 days most months, tracking with this calculator will give rough estimates rather than reliable predictions.
The connection to fertility
The fertile window shown alongside each period prediction is calculated as 5 days before ovulation through ovulation day. Ovulation is estimated as 14 days before the next expected period. For a 28-day cycle, that’s day 14. For a 32-day cycle, it’s around day 18.
This is the same method used in the ovulation calculator. If you’re tracking for conception, that calculator gives more detail on the fertile window and conception probability by day.
Sources
- ACOG (American College of Obstetricians and Gynecologists) — “Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign,” 2015
- FOGSI — guidance on normal menstrual cycle parameters for Indian women
- Dasharathy SS et al. — “Menstrual bleeding patterns among regularly menstruating women,” American Journal of Epidemiology, 2012