27 C.F.R. § 46.108 – Interest on unpaid tax.

(a) General. Interest is due on unpaid special tax from the date the tax was required to be paid to the date paid. Interest will be charged for each day at the rate prescribed by law in effect on that day. Interest accruing after December 31, 1982, is compounded daily.
(b) Adjusted interest rates. Adjusted interest rates, determined in accordance with the procedure prescribed by 26 U.S.C. 6621(b), are announced quarterly by the Commissioner of Internal Revenue. The appropriate TTB officer will provide information, when requested, regarding interest rates applicable to specific time periods.
(26 U.S.C. 6601, 6621)