How to Calculate Ovulation on a Calendar

Mark on the calendar when your period begins and ends each month.

If your period begins every 28 days, you probably have a regular cycle and can use the counting method to determine your most fertile day. Counting the day you began your period as day 1, find day 15 and circle it on the calendar. This is the day on which you're likely to be the most fertile.

If you do not have a regular menstrual cycle, or you wish to make your calculations more accurate, you will need to measure your basal body temperature with a basal body temperature thermometer.

Plot your basal body temperature on a calendar for each day of your menstrual cycle.

Locate the day on which your basal body temperature reaches its high point. Normally, you will be at your most fertile two or three days before this temperature spike occurs.