Amortization of bond discount is the process of systematically moving the balance in Discount on Bonds Payable into bond interest expense over the remaining life of the bond. A bond is issued at a discount when investors are only willing to pay less than its face value, typically because its stated coupon rate is below the market interest rate at the time of issuance, and that shortfall represents additional interest cost the issuer will effectively pay over the bond's life beyond the cash coupon payments alone.
Why the discount has to be amortized
If a company simply expensed the entire discount the moment the bond was issued, it would badly overstate interest expense in that first period and understate it in every period afterward. Amortizing the discount gradually recognizes that extra borrowing cost evenly, or in proportion to the bond's carrying value, across the time the bond is actually outstanding, keeping interest expense matched to the periods the debt was actually used.
How the amortization is recorded
Each period, the journal entry debits Bond Interest Expense and credits Discount on Bonds Payable, which reduces the contra-liability discount balance and, since the discount is subtracted from the face amount of Bonds Payable to determine the bond's carrying value, simultaneously increases the bond's reported carrying value toward its face amount. By the bond's maturity date, the full discount has been amortized away and the carrying value exactly equals the face amount due to be repaid.
Straight-line versus effective interest amortization
The straight-line method amortizes an equal dollar amount of discount each period, which is simple but produces an interest expense that doesn't track the bond's actual carrying value particularly well. The effective interest rate method instead applies a constant rate to the bond's beginning carrying value each period, which more accurately reflects the true cost of borrowing and is the method required under US GAAP whenever the difference between the two approaches is material.
Effect on reported interest expense
Because a discount means the company received less cash than it must eventually repay, amortizing that discount into interest expense means reported interest expense each period is higher than the cash coupon interest actually paid out — the gap is exactly the amortization for that period. This is one of the clearest everyday examples of accrual accounting diverging from cash flow, and analysts comparing a bond issuer's reported profitability to its actual cash interest burden need to account for it explicitly, usually by adding the cash interest paid and the change in the discount balance back together to reconcile the two figures.
A simple worked example
A company issuing $1,000,000 face value of bonds for $960,000 records a $40,000 discount. Amortizing that discount in equal amounts over a 10-year term under the straight-line method adds $4,000 to bond interest expense each year beyond the cash coupon paid, and by the end of year 10 the discount has been fully amortized, leaving the bond's carrying value exactly equal to its $1,000,000 face amount due at maturity.