Under sections 806 and 806.1 of the Fiscal Code (72 P.S. §§ 806 and 806.1), all underpayments of tax which became due and payable to the Commonwealth of Pennsylvania on or after January 1, 1982, bear interest at the same rate established under section 6621(a)(2) of the Internal Revenue Code. However, the Pennsylvania interest is simple interest (and not compounded like federal interest) and the Pennsylvania rates are adjusted annually, and not quarterly.
These same rates apply to overpayments of tax under Article III of the Tax Reform Code of 1971, but all other overpayments of tax bear interest at a rate that is two percentage points lower.
The following is a table of the rates that have applied since January 1, 1982, expressed both as an annual rate and as a daily factor. To calculate interest for X days, multiply the daily factor by X and the tax underpaid. (See also, 61 Pa.Code 4.2(b).)
| Year | Underpayment Annual Rate | Underpayment Daily Factor | Authority |
|---|---|---|---|
| 1982 | 20% | .000548 | |
| 1983 | 16% | .000438 | |
| 1984 | 11% | .000301 | |
| 1985 | 13% | .000356 | |
| 1986 | 10% | .000274 | |
| 1987 | 9% | .000247 | 16 Pa.B. 4908 |
| 1988 | 11% | .000301 | 17 Pa.B. 5286 |
| 1989 | 11% | .000301 | 18 Pa.B. 5680 |
| 1990 | 11% | .000301 | 19 Pa.B. 5081 |
| 1991 | 11% | .000301 | 20 Pa.B. 5870 |
| 1992 | 9% | .000247 | 21 Pa.B. 6012 |
| 1993 | 7% | .000192 | 22 Pa.B. 6142 |
| 1994 | 7% | .000192 | 24 Pa.B. 81 |
| 1995 | 9% | .000247 | 24 Pa.B. 6486 |
| 1996 | 9% | .000247 | 25 Pa.B. 5788 |
| 1997 | 9% | .000247 | 26 Pa.B. 6224 |
| 1998 | 9% | .000247 | 27 Pa.B. 6866 |
| 1999 | 7% | .000192 | 28 Pa.B. 6351 |
| 2000 | 8% | .000218 | 29 Pa.B. 6377 |
| 2001 | 9% | .000247 | 30 Pa.B. 6494 |
| 2002 | 6% | .000164 | 31 Pa.B. 6990 |
| 2003 | 5% | .000137 | 32 Pa.B. 6220 |
| 2004 | 4% | .000110 | 34 Pa.B. 448 |
| 2005 | 5% | .000137 | 35 Pa.B. 78 |
| 2006 | 7% | .000192 | 35 Pa.B. 7067 |
| 2007 | 8% | .000218 | 36 Pa.B. 8021 |
| 2008 | 7% | .000192 | 37 Pa.B. 6097 |