Simple vs. Compound Interest

Simple interest is calculated only on the principal balance, while compound interest is calculated on the principal plus any previously accrued interest — interest on interest. The distinction sounds technical, but it affects how a note's balance behaves over time and, occasionally, how a note is valued. The good news for most note holders: standard amortizing mortgage notes use simple interest, so the mechanics are straightforward.

How each works

• Simple interest: Each period's interest is principal × periodic rate. As payments reduce the principal, the interest portion shrinks. A typical amortizing mortgage note works this way — each payment covers the simple interest accrued on the current balance, with the remainder reducing principal. No interest is charged on unpaid interest as long as payments are made on schedule.
• Compound interest: Interest is periodically added to the balance, and future interest is charged on that larger amount. Compounding accelerates growth. It is common in savings and some consumer debt, but is generally not how a standard mortgage note accrues when paid as agreed.

Why most mortgage notes are simple-interest

In a normal amortizing note, the borrower pays the accrued interest each month before it can be added to principal, so there is nothing to compound. The familiar front-loaded amortization pattern — mostly interest early, mostly principal later — is a simple-interest result, not compounding. This is why a borrower who pays on time never owes "interest on interest."

Where compounding can sneak in

Compounding effects can appear at the edges:

• Missed payments / default. If a borrower stops paying, unpaid interest may, under some note terms, be added to the balance, after which interest accrues on the larger sum — an effectively compounding outcome on a non-performing note. Accrued interest on a defaulted loan can therefore balloon.
• Negative amortization. Rare in owner-financed notes (and restricted for consumer loans under Dodd-Frank), this is where the payment does not even cover the interest, so the shortfall is added to principal and compounds. Buyers view neg-am structures cautiously.
• Daily vs. monthly accrual. Some notes accrue interest daily; this changes the precise payoff figure slightly but is still simple interest on the balance.

Why it matters when you sell

A note buyer reads the note to confirm how interest is calculated, because it affects the payment stream and the payoff math. For the vast majority of clean, performing, simple-interest amortizing notes, this is a non-issue. But if your note has unusual accrual terms, a compounding default provision, or any negative-amortization feature, disclose it — it changes how the buyer models cash flow and value. Clear, standard simple-interest terms make a note easier to underwrite and sell.