Not all judgments include interest, so you need to read your specific judgment to see what it says.
However, if interest is included, the statutory interest in PA is 6%.
Further, contrary to the other post, not all interest starts on the day of the judgment. Some jurisdictions allow the debtor a 30 day grace period to pay the judgment without interest. If this is your case, then the judgment accrual wouldn't start until then.
Once you have confirmed that interest applies, you need to calculate the accrued interest. PA allows calculation of interest to be compounded. The formula for this is pretty complicated and the formula for simple interest is much simpler (pun?).
Interest per day = Amount x APR / 365
Total interest due = interest per day x number of days
Here is a site for compound interest, just be sure to use a daily rate since payment will not usually be in 'whole' months or years.
http://math.about.com/library/howto/htinterest.htm