Note the first day of your period
Note the first day of your next period
Count the number of days between the two first days
The total number of days is your period cycle length
Repeat for several cycles to find your average cycle length
Note the first day of your period
Note the first day of your next period
Count the number of days between the two first days
The total number of days is your period cycle length
Repeat for several cycles to find your average cycle length