Period Due Date Calculator

This period due date calculator estimates when your next menstrual period will begin based on your last period start date and average cycle length. Knowing your next period date helps with planning, travel, and understanding your cycle rhythm. The calculator also shows your expected ovulation window — the days in your cycle when conception is most likely — and counts down the days until your period arrives. Cycle lengths vary naturally between 21 and 35 days, with 28 days being the statistical average. The first day of your period is always day 1 of a new cycle. Ovulation typically occurs approximately 14 days before the start of the next period, so for a 28-day cycle ovulation falls around day 14, and for a 32-day cycle it falls around day 18. These estimates assume a regular cycle; stress, illness, weight changes, and hormonal shifts can all delay or advance your period. Use this calculator as a planning guide and track multiple cycles to understand your personal pattern.

The calculator adds your average cycle length to the first day of your last period to project your next period start date. Ovulation is estimated by subtracting 14 from your cycle length (the luteal phase constant): ovulation day = cycle length − 14. The fertile window spans cycle days (ovulation day − 5) through (ovulation day + 1), accounting for sperm lifespan and egg viability. Days until next period is the difference in days between today and the projected next period date. All calculations assume a consistent cycle; individual cycles naturally vary by a few days each month.