Effective December 21, 2000, the post-judgment interest rate is the weekly average one-year constant maturity Treasury yield for the calendar week preceding the date of the entry of the judgment.  Rates for judgments entered prior to December 21, 2000, are based on the auction of 52 week T-bills.

The current interest rate is provided by the Federal Reserve and published each Monday for the prior week (unless Monday is a holiday, in which case the rate is published on the next business day).  The current rate may be found at the link below.  The rate is found in the table under the two columns headed “Week Ending.”  The two dates under those two columns refer to the Friday averages of the last two weeks.  Under those columns, go down to the “1-year” row under “U.S. government securities, Treasury constant maturities, Nominal.”  Follow that row to where it meets the “Week Ending” columns to find the rate to use for judgments entered during the two weeks listed.

http://www.federalreserve.gov/releases/H15/Current

Historical rates for judgments entered since December 21, 2000, may be found here.

Historical rates for judgments entered prior to December 21, 2000, are listed here.