Montana governs the interest rates in the state through Title 31, Chapter 1, Section 106 of Montana Code.  Section 31-1-106 prescribes a maximum interest rate of ten percent (10 %) per annum if there is no written contract for the same.  Pursuant to Section 31-1-107, parties may agree in writing for the payment of any rate of interest that does not exceed more than fifteen percent (15 %).

Penalty for usury is prescribed under Section 31-1-108 of Montana Code.  According to this Section any person taking, receiving, reserving, or charging a rate of interest greater than lawfully allowed must forfeit a sum double the amount of interest.  Pursuant to Section 25-9-205, interest on a judgment recovered in the courts of this state is at the rate of ten percent (10 %) per annum.

Certain exemptions are laid down under Section 31-1-112 for charging interest rate greater than the lawful rate.  A regulated lender is exempted from all limitations on the lawful rate of interest.  A finance operation that finances transactions between merchants is also exempted from usury limits.

Mont. Code Anno., § 31-1-106

Legal interest.
(1)  Except as otherwise provided by the Uniform Commercial Code, 31-1-111 and 31-1-112, or 31-1-817, unless there is an express contract in writing fixing a different rate or a law or ordinance or resolution of a public body fixing a different rate on its obligations, interest is payable on all money at the rate of 10% a year.

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Mont. Code Anno., § 31-1-107

Interest rate allowed by agreement.
(1)  Parties may agree in writing for the payment of any rate of interest that does not exceed the greater of 15% or an amount that is 6 percentage points per annum above the prime rate of major New York banks, as published in the Wall Street Journal edition dated 3 business days prior to the execution of the agreement. Interest must be allowed according to the terms of the agreement.
(2)  A loan that is not usurious when made is lawful for the duration of the loan, provided the loan agreement is not substantially changed. This subsection does not apply to loan renewals.
(3)  The provisions of this section do not apply to regulated lenders as defined in 31-1-111.

Mont. Code Anno., § 31-1-108

Penalty for usury — action to recover excessive interest.
(1)  The taking, receiving, reserving, or charging a rate of interest greater than is allowed by 31-1-107 must be considered a forfeiture of a sum double the amount of interest that the note, bill, or other evidence of debt carries or  that has been agreed to be paid on the note, bill, or other evidence of debt.
(2)  When a greater rate of interest has been paid, the person by whom it has been paid or the person’s heirs, assigns, executors, or administrators may recover from the person, firm, or corporation taking, receiving, reserving, or charging interest a sum double the amount of interest paid, provided that  the action must be brought within 2 years after the payment of  the interest, and provided that, before any suit may be brought to recover the usurious interest, the party bringing suit makes written demand for return of the interest paid.

Mont. Code Anno., § 25-9-205

Amount of interest.
(1)  Except as provided in subsection (2), interest is payable on judgments recovered in the courts of this state and on the cost incurred to obtain or enforce a judgment at the rate of 10% per year. The interest may not be compounded.
(2)  Interest on a judgment recovered in the courts of this state involving a contractual obligation that specifies an interest rate must be paid at the rate specified in the contractual obligation.

Mont. Code Anno., § 31-1-112

Interest rate limitation exemption — regulated lenders — merchant finance.
(1)  A regulated lender is exempt from all limitations on the rate of interest that it may charge and is exempt from the operation and effect of all usury statutes.
(2)  A finance operation that finances transactions between merchants, as defined in 30-2-104, is also exempt from usury limits.