How the period tracker works
Add the first day of each period, and the last day too if you want to track how long it lasts. The tool works out your average cycle length from your own history and shows the next expected period, the estimated ovulation day and the fertile window, both in the summary and on the month calendar. Use the arrows to look at other months, and tap a day to copy it into the start date box.
Reading the calendar
- Solid red days are periods you have added.
- Dashed red days are expected periods.
- Green days are the estimated fertile window, with a dot on the estimated ovulation day.
- The blue outline marks today.
Good to know
- The more periods you add, the better the predictions. Three or more cycles give a useful average.
- Cycles are counted from the first day of one period to the first day of the next.
- Your dates are stored only in this browser, so nobody else can see them.
Expecting a baby? Use the Pregnancy Due Date Calculator. For other dates, try the Date Difference Calculator.
Frequently asked questions
How does the tracker predict my next period?
It takes the first day of your last period and adds your average cycle length. Once you have added two or more periods it uses the average of your last six cycles. Until then it uses the cycle length in the settings, which starts at 28 days.
How is ovulation worked out?
Ovulation usually happens about 14 days before the next period. The tool counts back 14 days from the expected period date. The fertile window is the five days before ovulation and the day after. These are estimates, because ovulation can move from cycle to cycle.
Can I use this to avoid or plan a pregnancy?
No. The dates are estimates and should not be used as birth control. To plan a pregnancy or to prevent one, speak to a doctor. For a due date after a positive test, use the pregnancy due date calculator.
Is my data private?
Yes. The dates you add are saved only in this browser on this device. They are not sent to our servers and are not added to your account. If you clear your browser data or use another device, they will not be there, so you may want to note them elsewhere too.
What is a normal cycle length?
A cycle is counted from the first day of one period to the first day of the next. Cycles between 21 and 35 days are common in adults, and a few days of difference from month to month is normal. If your cycles are often outside that range, or your periods stop or are very painful or heavy, see a doctor.
Why is my period late?
Stress, illness, travel, changes in weight or routine, breastfeeding, some medicines and pregnancy can all change the timing. A single late period is usually not a problem. If you could be pregnant, take a test, and speak to a doctor if you miss several periods.