The most common method used is to -
Take the first day of your last period and +9 months and +7days
ie period 1st Dec due on 8th Sept
or
If you don't know your period date -
Take the day of actual conception (or unprotected sex) and take off 14 days and then add 9months and 7 days
ie sex on 18th Dec take 14 = 4th add 9m+7days = 11th Sept
Take the first day of your last period and +9 months and +7days
ie period 1st Dec due on 8th Sept
or
If you don't know your period date -
Take the day of actual conception (or unprotected sex) and take off 14 days and then add 9months and 7 days
ie sex on 18th Dec take 14 = 4th add 9m+7days = 11th Sept
