Period Calculator

First day of your last period

Average cycle length (days)

Not sure? 28 days is a common average — anywhere from 21 to 35 days is considered typical.

Next period expected

day cycle

Next 4 predicted periods

This estimate assumes a regular cycle length and won't be precise if your cycles vary from month to month. A late period can mean many things beyond pregnancy — stress, illness, travel and hormonal changes can all shift timing. If you've missed a period and might be pregnant, our Due Date Calculator can help once you know more.

How this is calculated

This calculator simply repeats your average cycle length forward from the first day of your last period, one cycle at a time, until it reaches a date in the future. That date becomes your next expected period. If today falls after that date, the calculator counts how many days you are past it; if today is before it, it counts down instead.

The "next 4 predicted periods" list keeps adding your cycle length again and again from that point, so each entry is simply your average cycle length multiplied out further into the future. It's a straightforward projection, not a personalized prediction — it has no way to know about a particular cycle running long or short.

Common edge cases

The biggest limitation is cycle-to-cycle variation. Even people with "regular" 28-day cycles often see some natural swing from month to month, and conditions like PCOS or thyroid issues can make cycles considerably less predictable, which reduces how useful a single average number is.

Stress, illness, travel, significant weight changes, and intense exercise can all delay or advance a period by days without anything being wrong. The months after stopping hormonal birth control, and the return of periods after pregnancy or while breastfeeding, are also commonly irregular as your body's natural rhythm re-establishes itself. Cycles also tend to become less predictable during perimenopause.

A period that's late by more than a week or two is worth paying attention to — it can be an early pregnancy sign, but it can also reflect any of the factors above. If pregnancy is a possibility, our Pregnancy Test Calculator can help you figure out when a home test would give a reliable result.

Sources

The cycle-length projection method used here reflects standard menstrual cycle tracking guidance. For more on menstrual cycles, what's considered typical, and when irregular cycles are worth discussing with a doctor, see the American College of Obstetricians and Gynecologists (ACOG), Mayo Clinic, and the NHS.