15 U.S. Code § 1606 - Determination of annual percentage rate
(a) “Annual percentage rate” defined
The annual percentage rate applicable to any extension of consumer credit shall be determined, in accordance with the regulations of the Bureau,
(1) in the case of any extension of credit other than under an open end credit plan, as
(A) that nominal annual percentage rate which will yield a sum equal to the amount of the finance charge when it is applied to the unpaid balances of the amount financed, calculated according to the actuarial method of allocating payments made on a debt between the amount financed and the amount of the finance charge, pursuant to which a payment is applied first to the accumulated finance charge and the balance is applied to the unpaid amount financed; or
(B) the rate determined by any method prescribed by the Bureau as a method which materially simplifies computation while retaining reasonable accuracy as compared with the rate determined under subparagraph (A). 
(2) in the case of any extension of credit under an open end credit plan, as the quotient (expressed as a percentage) of the total finance charge for the period to which it relates divided by the amount upon which the finance charge for that period is based, multiplied by the number of such periods in a year.
(b) Computation of rate of finance charges for balances within a specified range
Where a creditor imposes the same finance charge for balances within a specified range, the annual percentage rate shall be computed on the median balance within the range, except that if the Bureau determines that a rate so computed would not be meaningful, or would be materially misleading, the annual percentage rate shall be computed on such other basis as the Bureau may be regulation require.
(c) Allowable tolerances for purposes of compliance with disclosure requirements
The disclosure of an annual percentage rate is accurate for the purpose of this subchapter if the rate disclosed is within a tolerance not greater than one-eighth of 1 per centum more or less than the actual rate or rounded to the nearest one-fourth of 1 per centum. The Bureau may allow a greater tolerance to simplify compliance where irregular payments are involved.
(d) Use of rate tables or charts having allowable variance from determined rates
The Bureau may authorize the use of rate tables or charts which may provide for the disclosure of annual percentage rates which vary from the rate determined in accordance with subsection (a)(1)(A) of this section by not more than such tolerances as the Bureau may allow. The Bureau may not allow a tolerance greater than 8 per centum of that rate except to simplify compliance where irregular payments are involved.
(e) Authorization of tolerances in determining annual percentage rates
In the case of creditors determining the annual percentage rate in a manner other than as described in subsection (d) of this section, the Bureau may authorize other reasonable tolerances.
 So in original.
Source(Pub. L. 90–321, title I, § 107,May 29, 1968, 82 Stat. 149; Pub. L. 96–221, title VI, § 607,Mar. 31, 1980, 94 Stat. 170; Pub. L. 111–203, title X, § 1100A(2),July 21, 2010, 124 Stat. 2107.)
2010—Pub. L. 111–203substituted “Bureau” for “Board” wherever appearing.
1980—Subsec. (c). Pub. L. 96–221, § 607(a), substituted provisions relating to allowable tolerances for purposes of compliance with disclosure requirements, for provisions relating to rounding off of annual percentage rates which are converted from single add-on or other rates.
Subsec. (e). Pub. L. 96–221, § 607(b), struck out reference to subsection (c) of this section.
Subsec. (f). Pub. L. 96–221, § 607(c), struck out subsec. (f) setting forth requirements for form of expressing percentage rates prior to Jan. 1, 1971.
Effective Date of 2010 Amendment
Amendment by Pub. L. 111–203effective on the designated transfer date, see section 1100H ofPub. L. 111–203, set out as a note under section 552a of Title 5, Government Organization and Employees.
Effective Date of 1980 Amendment
Amendment by Pub. L. 96–221effective on expiration of two years and six months after Mar. 31, 1980, with all regulations, forms, and clauses required to be prescribed to be promulgated at least one year prior to such effective date, and allowing any creditor to comply with any amendments, in accordance with the regulations, forms, and clauses prescribed by the Board prior to such effective date, see section 625 ofPub. L. 96–221, set out as a note under section 1602 of this title.