Menstrual Cycle Calculator

It is a reasonable approximation for people with regular cycles but has significant limitations. A study in the New England Journal of Medicine found that only about 30% of women have their fertile window entirely within days 10-17 of a 28-day cycle, as traditionally assumed. Tracking cervical mucus and basal body temperature alongside dates substantially improves accuracy.

Calendar-based methods alone have typical-use failure rates of around 12-24% per year, which is much higher than hormonal or barrier methods. If you are relying on fertility awareness for contraception, you need formal training in the symptothermal method, which combines multiple fertility signs. This calculator is not a contraceptive tool.

Not necessarily. A late period can result from stress, illness, significant weight change, over-exercise, travel, or hormonal fluctuations. Take a home pregnancy test 14 days after the expected period if pregnancy is possible, or contact your healthcare provider for guidance.

The luteal phase (from ovulation to the next period) is relatively fixed at approximately 14 days for most people. Variability in cycle length is almost entirely due to differences in the follicular phase before ovulation, not the luteal phase. Subtracting 14 from cycle length is therefore the standard method for estimating ovulation day.