Tex. Finance Code § 306.003

(a) In addition to any other method otherwise permitted under this title, a creditor and an obligor may agree to compute an annual interest rate on a commercial loan on a 365/360 basis or a 366/360 basis, as applicable, determined by applying the ratio of the percentage annual interest rate agreed to by the parties over a year of 360 days, multiplied by the outstanding principal balance, multiplied by the actual number of days the principal balance is outstanding. A creditor and an obligor may also agree to compute the term and rate of a commercial loan based on a 360-day year consisting of 12 30-day months. Each interest rate ceiling under Chapters 302 and 303 expressed as a rate per year may mean a rate per year computed in accordance with this section.

(b) A creditor and an obligor may agree that one or more payments of interest due or that are scheduled to be due with respect to a commercial loan may be paid on a periodic basis when due wholly or partly by adding to the principal balance of the loan the amount of unpaid interest due or scheduled to be due, regardless of whether the interest added to the principal balance is evidenced by an existing or a separate promissory note or other agreement. On and after the date an amount of interest is added to the principal balance under this subsection, that amount no longer constitutes interest, but instead constitutes part of the principal for purposes of calculating the maximum lawful rate or amount of interest on the loan.