Missouri Revised Statutes Title XXVI. Trade and Commerce § 408.020. When no rate of interest is agreed upon, nine percent allowed as legal interest

Creditors shall be allowed to receive interest at the rate of nine percent per annum, when no other rate is agreed upon, for all moneys after they become due and payable, on written contracts, and on accounts after they become due and demand of payment is made; for money recovered for the use of another, and retained without the owner's knowledge of the receipt, and for all other money due or to become due for the forbearance of payment whereof an express promise to pay interest has been made.