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 judgment. 
Effective December 21, 2000, the rate of interest that may be added to a judgment is subject to the provisions of 28 U.S.C. §1961, 18 U.S.C. §3612, and 40 U.S.C. §3116 (formerly cited as 40 U.S.C. §258e-1, August 21, 2002).

The Board of Governors of the Federal Reserve System releases Selected Interest Rates (Weekly) - H.15, which lists current releases, a daily update, and historical data.  Also available is the National Judgment Network (NJN) Post-Judgment Interest Calculator.