The effective interest rate method is the preferred technique for systematically amortizing a bond's discount or premium from the balance sheet into interest expense over the bond's life. It calculates each period's interest expense by applying a single, constant effective interest rate — the market rate that existed at issuance — to the bond's beginning-of-period carrying amount, rather than amortizing the same fixed dollar amount every period the way the simpler straight-line method does.
How the method works period by period
Each period, interest expense is calculated as the effective rate multiplied by the bond's carrying amount at the start of that period, while cash interest paid remains fixed, based on the bond's stated contractual rate applied to its face value. The difference between these two figures is the amount of discount or premium amortized that period, and that amortization amount adjusts the bond's carrying amount for the next period's calculation, so the carrying amount gradually moves toward the bond's face value as the bond approaches maturity.
A simple worked example
A $100,000 bond with a 5% contractual rate, issued at a discount to yield an effective 6% market rate, has an initial carrying amount below $100,000. In the first period, interest expense equals 6% of that lower carrying amount, while cash interest paid equals 5% of the $100,000 face value; the difference between the two is the discount amortized that period, which increases the carrying amount slightly for the next period's calculation. Because the carrying amount grows slightly larger each period, the dollar amount of discount amortized grows slightly larger too, period over period, until the bond reaches full face value exactly at maturity.
Why interest expense moves with the carrying amount
Unlike the straight-line method, which amortizes the same fixed dollar amount every period regardless of the bond's changing carrying amount, the effective interest rate method produces interest expense that rises or falls in step with the bond's carrying amount, which more accurately reflects the true economic cost of borrowing throughout the bond's life. This is why accounting standards require the effective interest rate method whenever the difference between it and the simpler straight-line method would be material to the financial statements.
Why the effective interest rate method matters for accurate reporting
Because the effective interest rate is fixed at the rate that existed when the bond was originally issued, applying it consistently to the bond's own carrying amount each period ensures that total interest expense over the bond's full life, combining cash interest paid and amortized discount or premium, accurately represents the genuine cost of that specific borrowing, exactly as the market priced it at the time of issuance, rather than drifting from that true economic cost through a simpler but less precise averaging method.
Effective interest rate method versus straight-line amortization
The simpler straight-line method of amortizing bond discount or premium divides the total discount or premium evenly across every remaining period, producing a constant dollar amortization amount each period regardless of the bond’s changing carrying amount. Accounting standards permit straight-line only when the difference between it and the effective interest rate method would not be material to the financial statements, since straight-line is easier to calculate but less theoretically precise about matching expense to the bond’s true, period-by-period economic cost.