The daily interest rate is derived by dividing the annual interest rate by
the number of days in a given year (365 or, for a leap year, 366). The OPM
back pay interest program (released in September 1999) rounds daily interest
rates to the 9th decimal place.
The Office of Personnel Management (OPM) provides these interest rates as
a convenience for Federal personnel management specialists. Every attempt has
been made to provide accurate data; however, the official rates are
established by the Internal Revenue Service, Department of the Treasury.
Official rates may be obtained by calling the IRS Federal Tax Information
Line (1-800-TAX-1040) and requesting the interest rate paid on tax
overpayments determined under section 6621 of the Internal Revenue Code.