How long is my menstration cycle?

How to u figure out how long ur cycle is? I'm trying to find out my ovulation days and its more accurate if u know exactly how long ur cycle last. do i count the days b/t the end of my end period and the start of my subsequent one or what? Your cycle is from the first day of your end period until the first daytime of the next interval.usually 28 days.
in avarage lssted 5-7 days