Period & Menstrual Cycle Calculator

What a Normal Cycle Looks Like

A typical menstrual cycle ranges from 21 to 35 days, with 28 days being the average. The cycle begins on the first day of your period. Cycles shorter than 21 days or longer than 35 days may warrant discussion with a GP. Cycle length can vary month to month by up to 7–9 days and still be considered normal — a consistent 28-day cycle is actually less common than variable cycles.

Ovulation and the Fertile Window

Ovulation typically occurs 14 days before your next period, not 14 days after your last one. For a 28-day cycle this is around day 14, but for a 35-day cycle it is around day 21. The fertile window includes the 5 days before ovulation and the day of ovulation itself — sperm can survive in the reproductive tract for up to 5 days. Ovulation predictor kits detect the LH surge 24–36 hours before ovulation and are the most reliable home method.

Cycle Tracking for Health

Tracking your cycle over 3+ months reveals patterns that can indicate hormonal health. Consistently short cycles (under 24 days) may suggest a shortened luteal phase. Very heavy periods with clotting may indicate fibroids or endometriosis. Absence of periods in non-pregnant women should always be investigated. Apps like Clue, Flo, or Natural Cycles can track symptoms alongside dates for a fuller picture.

Limitations of Period Calculators

This calculator uses average cycle length and assumes ovulation 14 days before your next period. In reality, ovulation day varies — stress, illness, significant weight change, and hormonal conditions like PCOS can shift it significantly. This calculator is for general guidance only and should not be used as a method of contraception. For fertility planning, use ovulation tests or consult a healthcare provider.