Under the laws of all states in this country a debt owed another person that is readily ascertainable runs interest at the allowable rate per the laws of that particular state.If you do not have a signed promissory note or court order by the person who owes you the money that you are writing about with principle and interest stated, monthly payments and due date, you should consult with an attorney about the need for such and the possible running of the statute of limitations on such a debt if there is no court order in place as to it.