SDCL § 54-3-5: Interest on moneys after they become due--Exception for express contracts or interest rate on bill, statement, or invoice--Limitation on interest rate.

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. Any interest rate appearing on a bill, statement, or invoice may not exceed eighteen percent.