Interest on unpaid invoices in South Dakota: 15% a year if no rate was agreed; the most you can set depends on the deal

South Dakota's rate when no rate was agreed

15% a year. Money due on a written instrument or on a settlement of accounts bears the Category F rate (15% a year) unless a written contract fixes a different rate or a rate clearly appears on the bill, statement or invoice. Where there is an obligation to pay interest but no rate is specified, SDCL 54-3-4 sets a maximum of the Category C rate (12%).

• “Unless there is an express contract in writing fixing a different rate or the interest rate clearly appears on the bill, statement, or invoice, interest is payable on all moneys at the Category F rate of interest as established in § 54-3-16 after they become due on any instrument of writing, and on moneys lent, or due on any settlement of accounts, from the day on which the balance is ascertained, and on moneys received to the use of another and detained from that other.” — SDCL § 54-3-5, read November 6, 2026
• “Category F rate of interest is fifteen percent per year; and” — SDCL § 54-3-16, read November 6, 2026
• “Under an obligation to pay interest, no rate being specified, interest is payable from date of incurrence of debt, unless the parties have otherwise agreed, at a maximum rate of the Category C rate of interest as established in § 54-3-16, and in the like proportion for a longer or shorter term.” — SDCL § 54-3-4, read November 6, 2026
• “Category C rate of interest is twelve percent per year;” — SDCL § 54-3-16, read November 6, 2026

The most you can set in South Dakota

For homeowners and businesses alike: No maximum by written agreement (SDCL § 54-3-1.1); a rate printed on a bill, statement or invoice may not exceed 18%. Consumer installment sales contracts: finance charge 'as agreed upon by the creditor and the consumer by written agreement' (54-3A-3). The 18% limit applies to a rate that merely appears on the bill/statement/invoice (54-3-5).

• “Unless a maximum interest rate or charge is specifically established elsewhere in the code, there is no maximum interest rate or charge, or usury rate restriction between or among persons, corporations, limited liability companies, estates, fiduciaries, associations, or any other entities if they establish the interest rate or charge by written agreement.” — SDCL § 54-3-1.1, read November 6, 2026
• “Any interest rate appearing on a bill, statement, or invoice may not exceed eighteen percent.” — SDCL § 54-3-5, read November 6, 2026
• “Notwithstanding the provisions of any other law, a creditor may contract for and receive, pursuant to an installment sales contract, a finance charge at a rate as agreed upon by the creditor and the consumer by written agreement.” — SDCL § 54-3A-3, read November 6, 2026

Late fees in South Dakota

What the text says: Consumer installment sales contracts: if the contract provides, a delinquency charge on an installment at least 10 days in arrears of up to the greater of 5% of the installment or $5, once per installment; a written delinquency notice is required before the next installment is due. Consumer installment sales contracts (goods or services bought by a natural person for personal, family or household purposes, paid in installments with a finance charge).

• “The holder of a contract authorized by this chapter may, if the contract so provides, collect a delinquency and collection charge on each installment in arrears for a period of not less than ten days in an amount not in excess of five percent of each installment or five dollars, whichever is greater. Any delinquency or collection charge may be collected only once on any one installment however long the installment remains delinquent.” — SDCL § 54-3A-12, read November 6, 2026
• “If the buyer has not paid an installment on an installment sales contract on or before the tenth day after its due date, as originally scheduled or as deferred, the creditor, if he chooses to impose a penalty for the delinquency, shall before the due date of the next scheduled installment, mail or deliver to the buyer at his last known address a written notice informing the buyer: (1) That the installment is delinquent;” — SDCL § 54-3A-11, read November 6, 2026
• “an arrangement other than a revolving charge account, entered into in this state evidencing any consumer transaction in which a consumer purchases goods or services from a creditor, under which arrangement a finance charge may be imposed and the consumer agrees to pay for the goods or services in one or more installments” — SDCL § 54-3A-1 (definition of "Consumer installment sales contract"), read November 6, 2026
• “Any interest rate appearing on a bill, statement, or invoice may not exceed eighteen percent.” — SDCL § 54-3-5, read November 6, 2026

Does South Dakota want it agreed in advance?

Agreed in advance: yes, in the cases below. Above the statutory rate, interest needs an express written contract or a rate that 'clearly appears on the bill, statement, or invoice' (max 18%); the consumer installment delinquency charge is allowed only 'if the contract so provides'.

• “Unless there is an express contract in writing fixing a different rate or the interest rate clearly appears on the bill, statement, or invoice, interest is payable on all moneys at the Category F rate of interest as established in § 54-3-16 after they become due on any instrument of writing, and on moneys lent, or due on any settlement of accounts, from the day on which the balance is ascertained, and on moneys received to the use of another and detained from that other.” — SDCL § 54-3-5, read November 6, 2026
• “Any interest rate appearing on a bill, statement, or invoice may not exceed eighteen percent.” — SDCL § 54-3-5, read November 6, 2026
• “The holder of a contract authorized by this chapter may, if the contract so provides, collect a delinquency and collection charge on each installment in arrears for a period of not less than ten days in an amount not in excess of five percent of each installment or five dollars, whichever is greater. Any delinquency or collection charge may be collected only once on any one installment however long the installment remains delinquent.” — SDCL § 54-3A-12, read November 6, 2026

Homeowners and businesses in South Dakota

The delinquency-charge rule and written-notice duty apply to consumer installment sales contracts (natural persons, personal/family/household purposes).

• “a natural person who seeks or acquires, or is offered property, services, or credit for personal, family, or household purposes” — SDCL § 54-3A-1 (definition of "Consumer"), read November 6, 2026
• “an arrangement other than a revolving charge account, entered into in this state evidencing any consumer transaction in which a consumer purchases goods or services from a creditor, under which arrangement a finance charge may be imposed and the consumer agrees to pay for the goods or services in one or more installments” — SDCL § 54-3A-1 (definition of "Consumer installment sales contract"), read November 6, 2026

Prompt payment on private construction in South Dakota

Not found in our check for South Dakota.

Work out a late charge in South Dakota

Starts at South Dakota's 15%, the rate when none was agreed, not a maximum.

What was not checked for South Dakota

• No South Dakota prompt-payment law for private construction was found on the state's site.

Check South Dakota's own text

Before you charge: read SDCL § 54-3-5 and the other South Dakota texts quoted on this page, or ask a lawyer. As of November 6, 2026.

This is a plain-words summary of each state's own laws as of November 6, 2026, not legal advice. Rules have exceptions. Check the state's own text, linked on every state's page, or a lawyer before you charge a late fee or interest.

Compare South Dakota with every state's answer, or with its neighbours just below.

Next door to South Dakota

• Iowa: 5% if none agreed, depends by agreement
• Minnesota: 6% if none agreed, depends by agreement
• Montana: 10% if none agreed, depends by agreement
• Nebraska: 12% if none agreed, 16% for homes (two answers)
• North Dakota: 6% if none agreed, depends by agreement
• Wyoming: 7% if none agreed, depends by agreement

Official sources for South Dakota

• SDCL § 54-3-5 — read November 6, 2026
• SDCL § 54-3-16 — read November 6, 2026
• SDCL § 54-3-4 — read November 6, 2026
• SDCL § 54-3-1.1 — read November 6, 2026
• SDCL § 54-3A-3 — read November 6, 2026
• SDCL § 54-3A-12 — read November 6, 2026

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