Menstrual Cycle Calculator — Next Period & Ovulation
Calculate your next period, ovulation date, and fertile window. See a calendar view of the next 3 months. 100% private, no sign-up.
100% Private – your data never leaves this device.
Next Period
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Ovulation Date
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Fertile Window
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Frequently Asked Questions
Your next period is calculated by adding your cycle length to the first day of your last period. For a 28-day cycle starting May 1, your next period is expected on May 29. This assumes a regular cycle.
The fertile window is about 6 days: 5 days before ovulation and the day of ovulation. For a 28-day cycle, ovulation is around day 14, making the fertile window roughly days 9-14.
Ovulation is when a mature egg is released from the ovary, typically around day 14 of a 28-day cycle. The egg survives 12-24 hours, but sperm can survive up to 5 days, creating a fertile window of about 6 days.
These are estimates based on regular cycles. Actual lengths vary due to stress, illness, travel, and other factors. This calculator should not be used as a sole method of contraception or fertility planning.
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Last updated: June 2026