Simple Interest

Simple interest is the method of calculating interest charges based solely on the outstanding principal balance of a loan — without compounding on previously accrued but unpaid interest. It is the standard interest calculation method for residential mortgage notes in the United States and the basis for nearly every amortization schedule a note investor will encounter.

How Simple Interest Is Calculated

The formula for simple interest on a mortgage is straightforward:

Daily interest = (Outstanding principal balance x Annual interest rate) / 365

For example, a loan with a $50,000 UPB at a 6% annual rate accrues:

($50,000 x 0.06) / 365 = $8.22 per day

Over a 30-day month, that equals approximately $246.58 in interest. When the borrower makes a monthly payment, the interest portion is satisfied first, and the remainder reduces the principal balance. As the principal decreases over time, the interest portion of each payment shrinks and the principal portion grows — the defining characteristic of an amortizing loan.

Simple Interest vs. Compound Interest

The distinction between simple and compound interest is critical for understanding how mortgage debt behaves:

For note investors, the simple interest structure means that even on a deeply delinquent non-performing loan, the debt grows at a predictable, linear rate. A loan that has been delinquent for three years has accumulated a calculable amount of accrued interest — not a compounding spiral.

Why Simple Interest Matters in Workouts

When negotiating resolutions on non-performing notes, simple interest math directly affects your strategy:

• Accrued interest as leverage. On a long-delinquent loan, accrued simple interest can add up to a significant amount on top of the UPB. The total legal payoff — principal plus accrued interest, late fees, and advances — may far exceed the original loan balance. This creates room to offer a discounted payoff that feels like a substantial concession to the borrower while still generating a strong return for you.
• Loan modification structuring. When modifying a non-performing loan, you can choose to capitalize some or all of the accrued interest into the new principal balance, forgive it entirely, or defer it as a non-interest-bearing balance. Each approach changes the borrower's monthly payment and your projected return differently.
• Principal reduction clarity. Because interest only accrues on principal, reducing the principal balance through a modification immediately reduces the daily interest charge — making the loan more affordable for the borrower and improving re-performance probability.

Daily vs. Monthly Accrual

Most residential mortgages use daily simple interest accrual, which means the exact interest charge depends on how many days elapse between payments. This has practical implications:

• Early payments save interest. A borrower who pays on the 1st of the month instead of the 15th saves 14 days of interest accrual.
• Late payments cost more. Every day past the due date adds another day of interest to the next payment's interest portion.
• Payoff statements are date-specific. When a borrower requests a payoff quote, the amount changes every day because interest continues to accrue until the funds are received. Payoff statements include a per-diem figure so the borrower can calculate the exact amount for any given wire date.

Understanding simple interest mechanics is foundational for pricing notes, structuring workouts, and projecting returns. Every amortization schedule, payoff calculation, and modification proposal you encounter in the note business is built on this formula.