Robertson cty tn. took x son in law to court for monies owed to me from 12/15. 4/16 won case plus cost interest at5.5%. how is the interest computed

Dec 2015 paid x son in laws car repairs. Was to be repaid within a few days. After a year of his stalling I filed a civil warrent12/16. Went to court in 4/17. Won the case. Judge ordered him to pay all cost plus interest at5.5%. Court permitted him to pay biweekly at $50.00. As of 3 weeks ago he stopped paying , leaving a outstanding balance. How does one compute the interest? Thanks for any insight