About this tool

Period Tools is a cycle calculator that answers one question directly: when is the next period likely to arrive. Enter the first day of your last period, your typical cycle length and how long bleeding usually lasts, and it projects the next six cycles onto a calendar, marking the estimated ovulation day and the six-day fertile window on each. The method is the standard one — add your cycle length to the last start date, then subtract fourteen days for ovulation, because the luteal phase is the most stable part of the cycle. The site explains that method on the page rather than hiding it, and is equally direct about when predictions drift: cycles that vary by more than a week, recently coming off hormonal birth control, unusual stress or travel, and breastfeeding or the postpartum period. A companion tracker lets you log cycles on a private calendar so it can learn your own pattern. Privacy is structural, not a promise: there is no account, and cycle data is kept in the browser on your own device rather than on a server. The site also carries pregnancy and genetics calculators and a blog. Results are estimates, not medical advice.