Online Period Calculator

The period calculator answers the common question of when the next period is due. You enter the first day of your last period and your usual cycle length, counted from the first day of one period to the day before the next. If you don't know your cycle length, you can enter the start dates of your last few periods and it averages them. It then shows your expected next period, an estimated ovulation day about 14 days before it, the six-day fertile window, and a forecast you can stretch to 3, 6 or 12 months.

For irregular cycles there is a separate mode that takes your shortest and longest cycles and shows a range of possible dates rather than a single day. The page explains the formula step by step, includes a worked example and a quick reference table by cycle length, and has a section on when the standard formula doesn't fit.

The site has 14 tools in all: cycle phase, a period calendar and tracker, cycle length, irregular periods, a first-period quiz, late and missed period checks, a pregnancy test timing tool, and ovulation, conception, implantation and due date calculators. Guides cover topics such as why a period might be late and how to tell one is ending. Pages credit Mira Chen as founder and editor, with review dates and a page explaining how sources are cited.

The calculators run in your browser. The dates you enter and any periods you log are saved in this browser's storage, not sent to the site's server, and the privacy page says they are kept out of ad and analytics requests. Shared links put the dates after the # in the address, which browsers don't send to the server, though anyone you share the link with can see them. The site does carry Google AdSense on every page, and Google receives normal request data such as your IP address and may set cookies. Results are estimates, and the site says they are not a diagnosis.