How Our Period Predictions Work

What you see without an account

Enter the first day of your last period, your usual cycle length and how many days your period lasts. You'll get three dates:

• Next period. The day your next period is expected to start.
• Ovulation. The day you're most likely to ovulate. Ovulation usually happens 12 to 16 days before your next period. Some of our calculators let you set your own luteal phase if you know it.
• Fertile window. Six days: the five days before ovulation and the day of ovulation. Sperm can survive for several days, which is why the window starts before ovulation.

The calculators run in your browser and don't need an account.

What changes when you log your periods

With a free account you can log each period. After you log 3 to 4 periods, your predictions are based on your own cycles instead of the number you typed in, and they keep updating every time you log. A missed log or a break in tracking won't throw your dates off.

Your calendar shows your next period, ovulation and fertile window. Dates further ahead are shown with softer edges because they are less certain.

If your cycles are irregular

When your cycles vary a lot, any single predicted date is less likely to be exact, so treat it as a rough guide. Cycles often vary more in the first few years after a first period and as you get closer to menopause. For those groups the tracker shows a likely range instead of one date.

The tracker can also let you know when your cycles fall outside the usual range for your age or when a period is much later than expected.

Why a calendar can't confirm ovulation

A calendar can tell you when ovulation is likely. It can't tell you it happened. Ovulation can move from one cycle to the next because of stress, illness, travel or other changes. Ovulation tests and basal body temperature charts give you signs from your own body.

Not for birth control

Do not use these estimates as birth control. Predictions based on the calendar can be off by several days, and a few days is enough to matter. If you want to avoid pregnancy, talk to a doctor or nurse about methods that are designed for it.

When predictions pause

If you log a pregnancy or pause tracking, predictions stop until you log a new period, so you don't see dates that don't apply to you. If your age group is set to Under 18, fertility details are left out of your calendar, emails and monthly reports.