Attorneys have computer programs for this. I would suggest that you contact the Office of the Attorney General or a private attorney to collect the support owed to you for your child or children so that the interest can be calculated accurately. The 12% interest rate is on unconfirmed arrearages. That figure changed, by statute to 6% in January 2002. Also, you refer to a judgment. I take by that comment that you had your arrearages confirmed and obtained a judgment at some point in the past.
This response in general in nature and not specific to any facts in your case. This response to this general question does not establish or an attorney-client relationship between the person making the inquuiry or the attorney responding to the inquiry.