Amortizing Bonds: How They Work, Risks, and Tax Treatment

Amortizing bonds return your principal gradually over the life of the bond instead of in one lump sum at maturity. Each scheduled payment blends interest with a slice of principal, so the outstanding balance shrinks with every payment and hits zero on the final date. That cash-flow pattern separates them from “bullet” bonds, and it drives two things every buyer needs to understand: the specific risks the structure creates, and the federal tax rules that apply to any premium, discount, or original issue discount attached to the bond.

How Each Payment Splits Between Interest and Principal

On a fixed-rate amortizing bond, every periodic payment has two parts: interest on the balance still outstanding, and a reduction of that balance. Early payments are mostly interest because the balance is still large. As principal gets chipped away, less interest accrues in the next period, so a bigger share of the same fixed payment goes to principal. By the last payment, almost all of it is retiring debt.

The level payment comes from PMT = PV × i ÷ (1 − (1 + i)^−N), where PV is the original principal, i is the periodic interest rate, and N is the number of payments. The formula produces the same dollar amount every period, with the final payment sometimes adjusted a few cents to zero out rounding.

A $10,000 bond at 5% annual interest paid monthly works out to about $188.71 per month. In month one, $41.67 is interest (5% ÷ 12 × $10,000) and $147.04 reduces the balance to $9,852.96. Month two’s interest drops to $41.05 on the smaller balance, and $147.66 goes to principal. That rebalancing continues to the final payment.

What an Amortization Schedule Tells You

The amortization schedule is a payment-by-payment table showing the date, total payment, interest portion, principal portion, and remaining balance. For a fixed-rate bond, the total column stays constant while the interest and principal columns move in opposite directions.

Watch the remaining-balance column closely. It tells you how much of your capital is still tied up in the bond at any point, and it feeds the next period’s interest calculation. If you sell before maturity or need to compute your tax basis after amortizing a premium, that number is where you start.

Common Types

Mortgage-Backed Securities

Mortgage-backed securities are the largest category. An issuer pools thousands of residential or commercial mortgages, and as homeowners make monthly payments, the cash flows pass through to bondholders. Each underlying mortgage amortizes on its own, so the MBS inherits that declining-balance profile. Pooling spreads default risk across many borrowers, but it creates a separate problem covered below: prepayment risk.

Asset-Backed Securities

Asset-backed securities apply the same model to auto loans, equipment leases, credit card receivables, and similar debt. Auto loan ABS typically have shorter terms than mortgage pools, so principal returns faster and the question of where to reinvest it comes up sooner.

Risks the Structure Creates

Reinvestment Risk

Because principal comes back with every payment, you constantly face the question of where to put that money next. If rates have fallen since you bought the bond, each returned dollar gets reinvested at a lower yield. That drag hits amortizing bonds harder than bullet bonds, which lock your principal in until maturity. The steady drip of returned capital is useful when you need the cash flow and costly when rates decline.

Prepayment Risk

Prepayment risk applies to bonds backed by loans that borrowers can pay off early. When mortgage rates drop, homeowners refinance, and MBS investors get a wave of unscheduled principal back at exactly the wrong time to reinvest. When rates rise, prepayments slow, extending the bond’s effective life and locking you into a below-market yield longer than expected.¹ Fixed-income professionals call this two-sided trap negative convexity: the bond underperforms a comparable bullet bond in both rising and falling rate environments, and earns a yield premium in periods of stability.

Prepayments also cap price appreciation. When rates fall, a bullet bond’s price rises freely because its cash flows are locked in. An MBS price rises less, because the market knows those favorable cash flows will likely be cut short by refinancing. MBS investors collect a higher coupon than a comparable Treasury but give up some of the price gains when rates move in their favor.¹

Bond Premium: Taxable vs. Tax-Exempt

If you paid more than face value for a bond, the excess is a bond premium. Federal law gives you a specific framework for handling it, and the framework differs by bond type.

Taxable Bonds: An Irrevocable Election

For a taxable bond bought at a premium, you can elect to amortize the premium over the bond’s remaining life. The amortized amount each year offsets your interest income from the bond rather than being claimed as a separate deduction.² Without the election, you would report the full coupon as interest income even though part of every payment effectively returns the premium you overpaid.

Premium amortization uses the constant yield method. Each period, multiply the bond’s adjusted purchase price by its yield to maturity, then subtract the coupon payment. The difference is the amortization for that period. Early on, when the adjusted price is highest, the amount is smaller because the yield calculation produces a number close to the coupon. As the price adjusts downward, the gap widens and more premium gets absorbed per period.

The election is irrevocable. Once you elect to amortize premium on any taxable bond, the choice applies to every taxable bond you already hold and every one you buy afterward. You cannot pick and choose. Revocation requires IRS permission, and the IRS can impose conditions on the revocation.² For most investors holding a mix of premium and par bonds the election still makes sense, but the all-or-nothing scope deserves thought before you file.

Tax-Exempt Bonds: Amortization Is Mandatory

For tax-exempt bonds bought at a premium, amortization is not optional. You must reduce basis by the allocable premium each period, even though you cannot take a deduction for it because the interest is already tax-exempt.² The premium offsets the qualified stated interest recognized as tax-exempt income each accrual period.³

If the premium allocable to a period exceeds the interest for that period, the excess is a nondeductible loss. You cannot use it to offset other income, and basis still decreases by the full premium amount, which affects gain or loss if you sell before maturity.³

Basis Drops as Premium Amortizes

Whether the bond is taxable or tax-exempt, amortizing premium reduces your cost basis. Federal law requires the adjustment: for taxable bonds, basis drops by the amount deducted or offset against interest; for tax-exempt bonds, it drops by the premium amount disallowed as a deduction.⁴ If you paid $1,050 for a $1,000 bond and amortized $30 of premium before selling, your adjusted basis is $1,020, and any gain or loss is measured from that figure.

Original Issue Discount

OID is the mirror image of a premium. When a bond is originally issued at a price below its face value, the gap between the issue price and the redemption price is OID. You must include OID in gross income as it accrues each year, even without receiving cash for it. Accrual follows a constant yield method: multiply the adjusted issue price at the start of each period by the yield to maturity, then subtract any stated interest paid during the period. The excess is the OID reported as income.⁵

A de minimis exception applies. If total OID is less than one-quarter of 1% of the stated redemption price multiplied by the number of full years from issuance to maturity, you can treat the OID as zero and skip the accrual rules.⁶ On a 10-year, $1,000-face bond, that threshold is $25. A bond issued at $980 has a $20 discount, below the threshold, and is not treated as OID.

If you hold a debt instrument with $10 or more in OID for the year, the issuer or broker reports it on Form 1099-OID. Box 1 shows the OID amount for the period you held the instrument; Box 8 shows OID on Treasury obligations.⁶ Report the taxable amount on the interest line of Form 1040 or 1040-SR.

Market Discount and Why It Matters for Amortizing Bonds

Market discount is different from OID. It arises when you buy an already-issued bond on the secondary market for less than its face value, or, for an OID bond, less than its adjusted issue price. The discount is the gap between the redemption price at maturity and your purchase price.⁷

The same de minimis structure applies. If market discount is less than one-quarter of 1% of the stated redemption price multiplied by the number of complete years remaining to maturity after acquisition, the discount is treated as zero.⁷ Gain on a de minimis market discount bond is capital gain.

Above the threshold, the treatment shifts. Gain on disposition of a market discount bond is ordinary income up to the accrued market discount, and only gain beyond that qualifies as capital gain. This matters especially for amortizing bonds: partial principal payments on a market discount bond are included in gross income as ordinary income to the extent they do not exceed the accrued market discount.⁸ Because amortizing bonds deliver principal payments throughout their life, you can owe ordinary income tax on those payments as they arrive rather than deferring the discount to maturity.

Selling Before Maturity

If you sell an amortizing bond before it matures, gain or loss depends on sale proceeds versus adjusted basis. For a premium bond where you elected to amortize, basis has been declining with each period. For a discount bond, basis may have been rising if you elected to accrue market discount currently. Comparing sale price to original purchase price will give you the wrong number.

Say you bought a taxable bond for $1,050 (face $1,000) and amortized $35 of premium over three years. Adjusted basis is $1,015. If you sell for $1,025, you have a $10 capital gain, not the $25 loss you might assume by comparing $1,025 to $1,050. Tax-exempt bonds get the same basis reduction because amortization there is mandatory. Skipping the basis adjustment is one of the most common errors bondholders make, and it produces either overpaid tax or underreported gain.

Reporting on Your Tax Return

Brokers report bond premium amortization on Form 1099-INT. Box 11 shows amortizable premium on taxable covered securities, Box 12 shows premium on Treasury obligations, and Box 13 shows premium on tax-exempt bonds.⁹ Some brokers report a net interest figure that already reflects the premium offset, in which case the premium box is left blank.

For taxable bonds, subtract the amortizable premium from interest income on Schedule B of Form 1040. IRS Publication 550 walks through the mechanics: list interest income, subtotal, then enter the amortizable bond premium as a line labeled “ABP Adjustment” and subtract it. If the amortizable premium for a period exceeds the stated interest for that period, you can include the excess as an itemized deduction, but only to the extent cumulative interest inclusions on the bond exceed cumulative premium deductions. Any amount that does not clear that hurdle carries forward.¹⁰

OID is reported on the interest line of Form 1040 or 1040-SR using figures from Form 1099-OID, with each payer and amount listed on Schedule B.⁶ Keep your amortization schedules, adjusted basis calculations, and every 1099. Those records are what stand between clean reporting and an IRS adjustment later.

1
Federal Reserve Bank of Kansas City. The Prepayment Risk of Mortgage-Backed Securities
2
Office of the Law Revision Counsel. 26 USC 171 – Amortizable Bond Premium
3
eCFR. 26 CFR 1.171-2 – Amortization of Bond Premium
4
Office of the Law Revision Counsel. 26 USC 1016 – Adjustments to Basis
5
Office of the Law Revision Counsel. 26 USC 1272 – Current Inclusion in Income of Original Issue Discount
6
Internal Revenue Service. Publication 1212 – Guide to Original Issue Discount (OID) Instruments
7
Office of the Law Revision Counsel. 26 USC 1278 – Definitions and Special Rules
8
Office of the Law Revision Counsel. 26 USC 1276 – Disposition Gain Representing Accrued Market Discount Treated as Ordinary Income
9
Internal Revenue Service. Instructions for Forms 1099-INT and 1099-OID
10
Internal Revenue Service. Publication 550 – Investment Income and Expenses