Your menstrual cycle runs from the first day of one period to the first day of the next. In adults it usually lasts between 21 and 35 days, and bleeding most often lasts 2 to 7 days. Twenty-eight days is a commonly quoted average, but it is not the only normal. It is also normal for your cycle to shift by a few days from month to month. What matters is knowing your own pattern and being able to notice when it changes.
Keeping a period calendar helps you predict when your next period is due, notice a late period early and give clear information at your appointments. One of the first things a gynecologist asks is the date of your last period. A few months of regular records show how much your cycle length varies, how many days your period lasts and roughly which days you ovulate. If you are trying to conceive, seeing your fertile days is valuable too.
The ovulation and fertile day information in the calendar is an estimate calculated from your past records. Stress, travel, illness, breastfeeding or hormonal changes can shift your cycle. That is why a period calendar is not a method of birth control and should not be used to prevent pregnancy. The more cycles you log, the more personal the predictions become, and a wider range in the first few months is expected.
What you can do
- Estimates a date range for your next period from your last period date, cycle length and period length.
- Marks period days, the fertile window and the estimated ovulation day in different colors on the calendar.
- Shows how many days late your period is when it is overdue.
- Calculates your cycle length from your records and alerts you to cycles shorter than 21 days, longer than 35 days, or periods longer than 7 days.
- Shows ovulation confirmed by basal body temperature or an ovulation test separately on the calendar.
- Lets you mark a past day as a period start and log when your period ended or ran longer.
How to use it
- Enter the first day of your last period and your average cycle and period length.
- Tap 'My Period Started' when your period begins and log the end date when it finishes.
- Follow your fertile window, estimated ovulation day and next period range on the calendar.
- As you log more cycles, the predictions update to match your own cycle length.
Frequently asked questions
How long should a menstrual cycle be?
In adults a menstrual cycle usually lasts between 21 and 35 days. It is counted from the first day of one period to the first day of the next. Twenty-eight days is only an average. Cycles can be more variable in the teenage years and as you approach menopause. If your cycles are consistently outside this range or vary a lot from month to month, it is a good idea to talk to your doctor.
How many days late can a period be?
A period arriving a few days early or late is often normal. Stress, intense exercise, weight changes, travel or illness can shift your period by several days. If there is a chance you could be pregnant and your period is past its expected date, you can take a pregnancy test. If you are not pregnant and your period has not come for more than three months, or delays are becoming frequent, you should see a gynecologist.
How do I calculate my next period?
Add your average cycle length to the first day of your last period to find the estimated start of your next one. For example, with a 28-day cycle, a period that started on March 1 would be followed by the next period around March 29. If your cycle varies, it is more realistic to think in terms of a range instead of a single day. A few months of records make the average clearer, so logging every period start improves the estimate.
Is a period tracker reliable for birth control?
No. The fertile day and ovulation information in a period calendar is an estimate based on past cycles. Ovulation can shift from month to month even in the same person, and sperm can survive in the body for several days. So picking 'safe days' from a calendar does not remove the risk of unintended pregnancy. If you want to prevent pregnancy, choose a suitable method together with your doctor.