What Is an Amortized Bond?

An amortized bond is one in which the principal (face value) on the debt is paid down regularly, along with its interest expense over the life of the bond. A fixed-rate residential mortgage is one common example because the monthly payment remains constant over its life of, say, 30 years. However, each payment represents a slightly different percentage mix of interest versus principal. An amortized bond is different from a balloon or bullet loan, where there is a large portion of the principal that must be repaid only at its maturity.

Understanding Amortized Bonds

The principal paid off over the life of an amortized loan or bond is divvied up according to an amortization schedule, typically through calculating equal payments all along the way. This means that in the early years of a loan, the interest portion of the debt service will be larger than the principal portion. As the loan matures, however, the portion of each payment that goes towards interest will become lesser and the payment to principal will be larger. The calculations for an amortizing loan are similar to that of an annuity using the time value of money, and can be carried out quickly using an amortization calculator.

Amortization of debt affects two fundamental risks of bond investing. First, it greatly reduces the credit risk of the loan or bond because the principal of the loan is repaid over time, rather than all at once upon maturity, when the risk of default is the greatest. Second, amortization reduces the duration of the bond, lowering the debt’s sensitivity to interest rate risk, as compared with other non-amortized debt with the same maturity and coupon rate. This is because as time passes, there are smaller interest payments, so the weighted-average maturity (WAM) of the cash flows associated with the bond is lower.

Example of Amortizing a Bond

30-year fixed-rate mortgages are amortized so that each monthly payment goes towards interest and principal. Say you purchase a home with a $400,000 30-year fixed-rate mortgage with a 5% interest rate. The monthly payment is $2,147.29, or $25,767.48 per year.

At the end of year one, you have made 12 payments, most of the payments have been towards interest, and only $3,406 of the principal is paid off, leaving a loan balance of $396,593. The next year, the monthly payment amount remains the same, but the principal paid grows to $6,075. Now fast forward to year 29 when $24,566 (almost all of the $25,767.48 annual payments) will go towards principal. Free mortgage calculators or amortization calculators are easily found online to help with these calculations quickly.

Straight-Line vs. Effective-Interest Method of Amortization

Treating a bond as an amortized asset is an accounting method used by companies that issue bonds. It allows issuers to treat the bond discount as an asset over the life of the bond until its maturity date. A bond is sold at a discount when a company sells it for less than its face value and sold at a premium when the price received is greater than face value.

If a bond is issued at a discount—that is, offered for sale below its par or face value—the discount must be treated either as an expense or it can be amortized as an asset. In this way, an amortized bond is used specifically for tax purposes because the amortized bond discount is treated as part of a company’s interest expense on its income statement. The interest expense, a non-operating cost, reduces a company’s earnings before tax (EBT) and, therefore, the amount of its tax burden.

Effective-interest and straight-line amortization are the two options for amortizing bond premiums or discounts. The easiest way to account for an amortized bond is to use the straight-line method of amortization. Under this method of accounting, the bond discount that is amortized each year is equal over the life of the bond.

Companies may also issue amortized bonds and use the effective-interest method. Rather than assigning an equal amount of amortization for each period, effective-interest computes different amounts to be applied to interest expense during each period. Under this second type of accounting, the bond discount amortized is based on the difference between the bond’s interest income and its interest payable. Effective-interest method requires a financial calculator or spreadsheet software to derive.

What Is an Amortized Loan?

An amortized loan is a type of loan with scheduled, periodic payments that are applied to both the loan’s principal amount and the interest accrued. An amortized loan payment first pays off the relevant interest expense for the period, after which the remainder of the payment is put toward reducing the principal amount. Common amortized loans include auto loans, home loans, and personal loans from a bank for small projects or debt consolidation.

How an Amortized Loan Works

The interest on an amortized loan is calculated based on the most recent ending balance of the loan; the interest amount owed decreases as payments are made. This is because any payment in excess of the interest amount reduces the principal, which in turn, reduces the balance on which the interest is calculated. As the interest portion of an amortized loan decreases, the principal portion of the payment increases. Therefore, interest and principal have an inverse relationship within the payments over the life of the amortized loan.

An amortized loan is the result of a series of calculations. First, the current balance of the loan is multiplied by the interest rate attributable to the current period to find the interest due for the period. (Annual interest rates may be divided by 12 to find a monthly rate.) Subtracting the interest due for the period from the total monthly payment results in the dollar amount of principal paid in the period.

The amount of principal paid in the period is applied to the outstanding balance of the loan. Therefore, the current balance of the loan, minus the amount of principal paid in the period, results in the new outstanding balance of the loan. This new outstanding balance is used to calculate the interest for the next period.

Amortized Loans vs. Balloon Loans vs. Revolving Debt (Credit Cards)

While amortized loans, balloon loans, and revolving debt–specifically credit cards–are similar, they have important distinctions that consumers should be aware of before signing up for one.

Amortized Loans

Amortized loans are generally paid off over an extended period of time, with equal amounts paid for each payment period. However, there is always the option to pay more, and thus, further reduce the principal owed.

Balloon Loans

Balloon loans typically have a relatively short term, and only a portion of the loan’s principal balance is amortized over that term. At the end of the term, the remaining balance is due as a final repayment, which is generally large (at least double the amount of previous payments).

Revolving Debt (Credit Cards) 

Credit cards are the most well-known type of revolving debt. With revolving debt, you borrow against an established credit limit. As long as you haven’t reached your credit limit, you can keep borrowing. Credit cards are different than amortized loans because they don’t have set payment amounts or a fixed loan amount.

Example of an Amortization Loan Table

The calculations of an amortized loan may be displayed in an amortization table. The table lists relevant balances and dollar amounts for each period. In the example below, each period is a row in the table. The columns include the payment date, principal portion of the payment, interest portion of the payment, total interest paid to date, and ending outstanding balance. The following table excerpt is for the first year of a 30-year mortgage in the amount of $165,000 with an annual interest rate of 4.5%