Write down the date of the first day of your last period. The more accurate you are on the date, the more accurate your calculations are. Try looking around the house for reminders -- such as a receipt for feminine hygiene products -- if you can't remember the exact day.
Add one week to the first day of your last period. For example, if August 17 was the first day of your last period, one week later is August 24. Mark the date "start date."
Count the number of weeks between the start date and the current date on a calendar. Every time you pass four weeks from the start date, you are another month pregnant. For example, if your start date is August 24 and it is now October 31, you are around two months and one week pregnant.