The calendar method helps you predict your fertile days by tracking the length of your menstrual cycles over several months.
How do I use the calendar method?
Before you can use the calendar method as birth control, you need to keep track of the length of your menstrual cycles for at least 6 periods. You can do this with a regular calendar or our period tracking app.
Here’s how you do it
Mark the first day of your period (this is day 1). Then mark the first day of your next period. Count the total number of days between each cycle (the number of days between the first days of each period).
You must chart at least 6 cycles, but charting a few more months is even better. Here’s an example:
|First Day of Period||Number of Days in Cycle||First Day of Period||Number of Days in Cycle|
|January 20||29||May 12||26|
|February 18||29||June 9||28|
|March 18||29||July 9||30|
|April 16||29||August 5||27|
Note: If all of your cycles are shorter than 27 days, the calendar method won’t be accurate for you.
What are the safe days to have sex when using the calendar method?
To predict the first fertile day (when you can get pregnant) in your current cycle:
- Find the shortest cycle in your past record
- Subtract 18 from the total number of days in that cycle
- Count that number from day 1 of your current cycle, and mark that day with an X. (Include day 1 when you count.)
- The day marked X is your first fertile day.
For example: if your shortest cycle is 26 days long, subtract 18 from 26 — you get 8. Then, count 8 days starting from day 1 (the first day of your period). If day 1 was on the 4th of the month, you’ll mark X on the 11th. So the 11th is your first fertile day of this cycle — you should stop having vaginal sex on this day or start using another method of birth control.
To predict the last fertile day in your current cycle:
- Find the longest cycle in your record
- Subtract 11 from the total number of days in that cycle
- Count that number from day 1 (the first day of your period) of your current cycle, and mark that day with an X. (Include day 1 when you count.)
- The day marked X is your last fertile day.
For example, if your longest cycle is 30 days long, subtract 11 from 30 — you get 19. Then, count 19 days starting from day 1. If day 1 was on the 4th of the month, you’ll mark X on the 22nd. So the 22nd is your last fertile day of this cycle — you can start having unprotected sex the next day.
The calendar method can only predict what is most likely to be safe and unsafe days — it can’t tell you for sure exactly when you’re fertile. So it’s hard to use if your cycles are not always the same length, and you can’t use it at all if all of your cycles are shorter than 27 days.
The calendar method is most effective when you combine it with other fertility awareness methods, like the temperature and cervical mucus methods.