Next Period Calculator

A next-period estimate on this page is a calendar sum. Take the first day of the last period and add the cycle length you typed. A 28-day cycle that started on 1 January has its next start on 29 January. If you also type a bleeding length, the page marks the last day of that next bleed. Five days that begin on 29 January run through 2 February, because the start day counts as day one. The start after that is one more cycle later. The arithmetic is the whole prediction. There is no hidden model and no account.

The American College of Obstetricians and Gynecologists describes a typical adult cycle as about 24 to 38 days, with an average near 28. In the first years after periods begin, ACOG notes a wider span, about 21 to 45 days. This calculator accepts 21 to 45 so a teen and an adult can both use the same box. When the number sits outside 24 to 38, the result says so in a sentence. The page refuses to add fewer than 21 days or more than 45. A cycle outside that span is a reason to ask a clinician, rather than to keep stretching a webpage.

Bleeding length is optional. ACOG patient materials often describe a period as lasting about 2 to 7 days. The form still accepts 2 to 10 days so a longer bleed can draw an end date. Anything past 7 days is labeled as worth a clinical visit. The page does not try to score how heavy a period is. Clots, a soak-through pad, dizziness, and pain that stops daily activity are details for that visit, not for a day counter.

The list under the result is a tracker with a narrow job. Each saved start is written to localStorage in this browser, under the key vsh-period-starts. Two or more dates produce the average gap between them, and that average can be rounded into the cycle box. Dates stay on the device until you remove them. A new phone, a private window, or a cleared browser starts empty. Nothing in the list is uploaded. That is the privacy tradeoff: convenient on one device, gone if the browser storage is cleared.

This countdown is not contraception and not a fertility treatment. Fertile days already have a page on this site, the ovulation calculator, which estimates a fertile window from cycle length. Read how to estimate ovulation and fertile days before you treat any calendar as a birth-control method. Calendar methods used to avoid pregnancy have a real failure rate. If you need contraception, ask a clinician for a method with a known effectiveness.

A missed period, bleeding after menopause, bleeding during pregnancy, and cycles that swing for months are clinical questions. A positive pregnancy test makes this countdown the wrong tool. The pregnancy week counter and the due date calculator start from the last period in a different way, and the pregnancy weight gain page uses the 2009 Institute of Medicine ranges. Breastfeeding, some contraceptives, thyroid disease, and polycystic ovary syndrome all change cycle length. The average of the dates you saved describes those dates. It does not name a condition. The disclaimer states the limit for every health tool here.

Sources

• American College of Obstetricians and Gynecologists. Patient guidance on the menstrual cycle: adult cycles are often about 24–38 days; cycles in the first years after menarche may be about 21–45 days; bleeding is often about 2–7 days.
• Ovulation calculator on this site, for a fertile-window estimate. This period page does not repeat that formula.