How to Calculate Interest on a Promissory Note

Interest is where a promissory note stops being intuitive. Two notes for the same amount at the same rate can produce very different totals depending on how the interest is calculated, and the way amortized payments divide each one between interest and principal catches borrowers off guard the first time they see it. None of the math is hard once you know which method applies, but you have to know which method applies, because simple interest, compound interest, and an amortized schedule are three different things. Here is how each one works, with numbers.

Simple Interest: The Straightforward Method

Simple interest is calculated only on the original principal, never on the interest that accrues. The formula is principal times the annual rate times the time in years. Say you lend 10,000 dollars at 6 percent simple interest for one year. The interest is 10,000 times 0.06 times 1, which is 600 dollars, so the borrower repays 10,600 dollars. Over two years at the same simple rate, the interest is 10,000 times 0.06 times 2, which is 1,200 dollars. Notice that the interest is always figured on the original 10,000, regardless of how much time passes. Simple interest is common in private promissory notes precisely because it is easy to understand and easy to verify, which reduces disputes.

Compound Interest: Interest on Interest

Compound interest is calculated on the principal plus the accumulated interest, so the balance grows faster over time. With interest compounding annually, the first year on that same 10,000 dollars at 6 percent adds 600 dollars, bringing the balance to 10,600. The second year, the 6 percent is figured on 10,600, not the original 10,000, so it adds 636 dollars, bringing the balance to 11,236. Over the two years, compound interest produced 1,236 dollars versus the 1,200 from simple interest, and that gap widens the longer the loan runs and the more frequently it compounds. Compounding can occur annually, monthly, or daily, and more frequent compounding means more interest. Because compounding can significantly increase the cost, a promissory note should state clearly whether interest is simple or compound, and if compound, how often it compounds.

Amortized Payments: How Most Installment Loans Work

Most installment promissory notes use amortization, where the borrower makes equal periodic payments that gradually pay off both principal and interest over the loan term. The twist that surprises people is how each payment is split. Early in the loan, most of each payment goes toward interest because the balance is still high, and only a little goes to principal. As the balance shrinks, the interest portion of each payment falls and the principal portion grows, so later payments pay down the balance much faster. The total payment stays the same each period, but its composition shifts over time. This is why paying extra early in a loan saves so much interest, since it attacks the principal while the interest charges are still at their highest.

A Worked Amortization Example

Suppose a 12,000 dollar loan at 6 percent annual interest is repaid over 12 monthly payments. The monthly rate is 6 percent divided by 12, which is 0.5 percent. The fixed monthly payment works out to roughly 1,033 dollars. In the first month, the interest portion is 0.5 percent of 12,000, which is 60 dollars, so about 973 dollars of that first payment goes to principal. The next month interest is figured on the reduced balance, so slightly less goes to interest and slightly more to principal, and this continues until the final payment clears the loan. Working an amortization schedule by hand for every payment is tedious, which is why our loan payoff calculator does it for you, showing the payment, the interest, and the remaining balance for each period.

Notice how short the loan term keeps the interest small in that example. Stretch the same 12,000 dollars at 6 percent over five years instead of one and the total interest climbs sharply, because the balance stays high for much longer and the 6 percent is charged against it every month along the way. This is the lever borrowers most underestimate: the rate gets the attention, but the term quietly decides how much that rate costs in dollars. A lower rate over a long term can cost more than a higher rate paid off quickly, which is why looking at the total interest, not just the rate, is what tells you the real price of the loan.

State the Method Clearly in the Note

Whatever method you use, the promissory note has to say so explicitly, because the same rate produces different totals under different methods. State the interest rate, whether it is simple or compound, and if compound how often it compounds, or if amortized, the payment amount and schedule. Confirm the rate is within your state usury limit before you write it down, since exceeding the legal cap can void the interest or trigger penalties. Spelling out exactly how interest is calculated is what prevents a borrower from later disputing the total owed, and it lets both sides see the real cost of the loan from the start. A clear promissory note states the interest method as plainly as it states the principal.