How to calculate interest on a small claim courts case

What is the name of your state? MI

I won my case in Small Claims Court. The defendant had 21 days to pay, and has not. I'm am filling out a Request for Ganishment form, and I need help to total the interest accrued to date.

Post-Judgment Interest

Without knowing more, I cannot answer. However, here is a nice PDF that likely contains an answer to your question:

http://courts.michigan.gov/scao/resources/other/interest.pdf

Your state’s rate is statutory, is based on the rate for 5-year Treasury Notes and changes twice a year. It’s very possible that the rate was stated in the judgment Order but, if it wasn’t, it seems safe to infer that it was a recent judgment.

Use the suggested PDF link. Find your interest rate, multiply the judgment amount by that rate, divide by 365 to get the daily rate and multiply by 21 days or however many it takes to prepare the paperwork.