How to Calculate Your Menstrual Cycle?

Question:

Is the menstrual cycle supposed to be 28 days from the first day of one period to the start of the next, or is it 28 days from the last day of one period to the start of the next?

Answer :

To calculate menstrual cycle length, you count the number of days from day one of your period (cycle day 1) up to, and including, the day before the next period starts.

The count does not include the first day of the next menstrual period, because that's Cycle Day 1 of the next cycle.

Example :

Period starts June 3rd, and lasts for 5 days. The next period starts June 27.

The start day of the next period, June 27, is not included as it starts the new cycle

Answer for this example: 
Cycle Length = 24 days

Note: 
The period length of 5 days does not effect the cycle length calculation. Had this period been 4 days or 6 days, cycle length would still be 24 days.

Source : Thanx to WhenMyBaby.com