Compound Interest

Compound interest is interest calculated on both the original principal and all previously accumulated interest, causing the outstanding balance to grow at an accelerating rate when payments are not applied. Unlike simple interest, which is computed only on the initial principal, compound interest treats unpaid interest as part of the new base for future calculations. For mortgage note investors, compounding is a critical concept that affects everything from evaluating non-performing loans with large accrued interest balances to calculating true yields on performing loans.

How Compound Interest Works

The mechanics of compounding depend on two variables: the interest rate and the compounding frequency. The more frequently interest compounds, the faster the balance grows.

The basic formula is:

A = P (1 + r/n)^(nt)

Where:

• A = final amount (principal + interest)
• P = original principal
• r = annual interest rate (decimal)
• n = number of compounding periods per year
• t = time in years

To illustrate the impact of compounding frequency on a $100,000 balance at 8% annual interest over 5 years with no payments made:

The difference between annual and daily compounding on this example is roughly $2,200 — meaningful, but not dramatic. Where compounding becomes a major factor is on longer timelines and higher interest rates, both of which are common in the note investing space.

Compound Interest and Mortgage Notes

Most residential mortgages in the United States use simple interest calculated on the outstanding principal balance. Each monthly payment covers the interest that accrued since the last payment, with the remainder applied to principal per the amortization schedule. As long as the borrower pays on time, there is no "interest on interest" because the interest is paid off each month.

However, compound interest becomes relevant in several note investing scenarios:

• Non-performing loans with accrued interest. When a borrower stops making payments, interest continues to accrue. Depending on the loan terms and state law, unpaid interest may be added to the principal balance, creating a compounding effect. A loan with a $75,000 UPB can balloon to $95,000 or more after two years of non-payment at a high interest rate.
• Default interest provisions. Some notes include a higher default interest rate that kicks in after a specified number of missed payments. Combined with compounding, default interest can significantly inflate the total amount owed.
• Loan modifications. When negotiating a modification with the borrower, the note holder must decide how to treat accrued interest. Options include capitalizing it (adding it to the principal, which effectively locks in the compounding effect), forgiving it, or deferring it to the end of the loan term.
• Yield calculations. When pricing a performing note, the effective yield depends on how interest compounds over the remaining term. Two notes with the same stated interest rate can deliver different returns depending on compounding frequency and amortization structure.

Compounding and the Time Value of Money

For note investors, compound interest is inseparable from the broader concept of time value of money. When you purchase a note at a discount, your effective yield is determined by the relationship between what you paid, the cash flows you receive, and the time over which those cash flows arrive. This is the basis for calculating internal rate of return (IRR) and cash-on-cash return.

Consider a simple example: you buy a performing note for $60,000 that pays $600 per month with 120 months remaining and a payoff balance of $72,000. Your return is not just the difference between purchase price and total payments — it is the compound rate at which your $60,000 investment grows through monthly cash flows over the holding period.

Practical Considerations for Note Investors

When evaluating notes, keep these compounding-related factors in mind:

• Scrutinize the accrued interest balance. On non-performing loans, the total payoff amount often far exceeds the original UPB. Understand what portion is principal, what is accrued interest, and whether the interest has been compounding. The total payoff figure matters for legal enforcement, but your bid should be based on realistic recovery — most workouts involve significant interest forgiveness.
• Check the note terms. Read the promissory note to determine the stated interest calculation method. Most residential notes specify simple interest on the declining principal balance, but some — particularly private or hard money loans — may specify compound interest explicitly.
• Use financial calculators. Pricing notes accurately requires computing present values and yields that account for compounding. A financial calculator or spreadsheet with TVM (time value of money) functions is essential, not optional.
• Understand state usury limits. Some states cap the total interest (including compounded amounts) that can be charged. Exceeding these limits can render portions of the debt unenforceable — a risk factor to evaluate during due diligence.

Compound interest is one of the most fundamental concepts in finance. In note investing, it works both for you and against you — it accelerates the growth of accrued balances on defaulted loans you hold, but it also drives the yield advantage you capture when purchasing notes at a discount. Understanding how it operates in the specific context of mortgage notes is essential for accurate pricing and sound investment decisions.