📅 Period Calculator

How Period Prediction Works

Enter the first day of your last period and your average cycle length, and the calculator projects your next period start dates. Ovulation typically occurs about 14 days before the next period, so for a 28-day cycle that is day 14; the fertile window spans the 5 days before ovulation plus ovulation day itself, because sperm survive up to 5 days while the egg survives about 24 hours. The calendar view marks predicted period days, the fertile window, and estimated ovulation for six months ahead.

Cycle length is counted from the first day of one period to the first day of the next. The average is 28 days, but anything from 21 to 45 can be normal for an individual. The calculator uses your average, so if your cycles vary, enter the most typical length. Tracking three or more cycles gives a much better average than guessing from one.

Accuracy and Limits

Calendar prediction works best for regular cycles. Stress, illness, travel, intense exercise, weight changes, and perimenopause can shift ovulation by days, and this calculator cannot see those coming. It is a planning aid, not contraception: the rhythm method alone has a typical-use failure rate around 24 percent per year. For trying to conceive, combine the calendar with ovulation test strips or basal body temperature for confirmation.

When to Talk to a Doctor

See a healthcare provider for cycles consistently shorter than 21 days or longer than 45, missed periods without pregnancy, bleeding between periods, periods lasting over 7 days, or pain that disrupts your life. Irregular cycles can signal thyroid issues, PCOS, or other treatable conditions. Bring your tracked dates: a history is the most useful thing you can hand a doctor. For informational purposes only, not medical advice. Consult a healthcare professional.

What to Track Besides Dates

Dates alone tell half the story. Noting flow heaviness, cramps, headaches, mood shifts, and spotting turns the calendar into a genuinely useful health record. Adult cycles typically run 21 to 35 days; teen cycles can be irregular for the first couple of years. A cycle that suddenly changes length by more than a week, bleeding between periods, or pain that disrupts daily life are all worth a doctor visit, not just calendar watching. Bring your tracked data to the appointment: three months of notes helps a clinician spot patterns far faster than memory. And remember predictions are estimates: stress, illness, travel, and intense training can all shift ovulation and delay a period.