You sign a three-year seller note with a $100,000 face value and a 5% coupon, and the cash that actually changes hands is $92,268.70. Come year-end, a deceptively simple question lands on your books: how much interest did you pay this year — the $5,000 coupon on the paper, or something else? Accounting's answer is something else. Your true interest expense is the market rate applied to what you actually owe at each moment, and the gap between that number and the coupon quietly shrinks the discount until the note reaches face value at maturity. That procedure is the effective interest method, and GAAP requires it whenever debt is issued for anything other than face amount. This guide walks through exactly how it works.
Why debt rarely changes hands at face value
A bond or note has two interest rates, and confusing them is the root of every error in this area. The stated rate (also called the coupon or contract rate) is printed on the paper: it sets the cash that changes hands each period — face value times stated rate. The market rate (also called the effective rate or yield) is what investors actually demand for lending to this borrower, for this long, at this moment. The market rate is set in the market, not in the contract.
The issue price is simply the present value of the note's future cash flows discounted at the market rate. When the two rates differ, the price must differ from face value to compensate:
- Stated rate below market rate → discount. A 5% coupon in an 8% market is a below-market promise, so buyers pay less than face value. The $7,731.30 gap in the opening example is the discount: the market's way of topping your 5% coupon up to an 8% yield.
- Stated rate above market rate → premium. A 7% coupon in a 5% market is a generous promise, so buyers pay more than face value. The extra cash you receive upfront is the premium, and it effectively prepays part of your interest.
- Stated rate equals market rate → par. Price equals face value, no discount or premium, and interest expense equals the coupon every period. This is the only case where the coupon is your interest expense.
Notice what the discount really is: additional interest cost you will recognize over the life of the debt, on top of the coupons. A premium is the mirror image — a reduction of interest cost spread over the debt's life. The effective interest method is the machinery that spreads it.
The one formula that runs the whole schedule
Each period, under the effective interest method:
Interest expense = carrying value at the start of the period × market rate at issuance
Three things about this formula trip people up, so note them now. First, the rate is the market rate at issuance, frozen on day one — it never updates when market rates move later. Second, the base is the carrying value (face value minus unamortized discount, or plus unamortized premium), which changes every period as amortization accrues. Third, for semiannual coupons, both the rate and the coupon are halved per period — a 6% annual market rate with semiannual payments means 3% per period, applied twice a year.
The amortization for the period is then just the plug:
- Discount: amortization = interest expense − cash coupon (expense exceeds cash; the difference increases carrying value toward face).
- Premium: amortization = cash coupon − interest expense (cash exceeds expense; the difference decreases carrying value toward face).
That is the entire method.
Worked example: a discount note, period by period
Take the opening example: $100,000 face, 5% annual coupon, three years, issued when the market demands 8%. The cash coupon is $5,000 a year. Discounting all three payments at 8% gives proceeds of $92,268.70, so the discount is $7,731.30 and the day-one carrying value is $92,268.70.
Year 1. Interest expense = $92,268.70 × 8% = $7,381.50. Cash paid is $5,000, so amortization = $7,381.50 − $5,000 = $2,381.50. Carrying value rises to $92,268.70 + $2,381.50 = $94,650.20. The journal entry debits interest expense for $7,381.50, credits cash for $5,000, and credits the discount account for $2,381.50.
Year 2. Interest expense = $94,650.20 × 8% = $7,572.02. Amortization = $7,572.02 − $5,000 = $2,572.02. Carrying value rises to $97,222.22.
Year 3. Interest expense = $97,222.22 × 8% = $7,777.78. Amortization = $7,777.78 − $5,000 = $2,777.78. Carrying value lands exactly on $100,000.00 — face value at maturity, as it must.
Check the totals: amortization of $2,381.50 + $2,572.02 + $2,777.78 = $7,731.30, exactly the original discount. And total interest expense over the three years is $22,731.30 — the $15,000 of coupons plus the $7,731.30 discount. The method never creates or destroys interest; it only decides when you recognize it.
Study the pattern, because it is the signature of the method: with a discount, interest expense rises each period ($7,381.50 → $7,572.02 → $7,777.78) as the growing carrying value earns the fixed 8%. With a premium the pattern reverses — expense falls each period as the shrinking carrying value earns the fixed rate. If your schedule shows flat expense on discounted debt, you are looking at straight-line amortization, not the effective interest method.
The premium mirror image
Flip the rates: $100,000 face, 7% coupon, three years, issued into a 5% market. The $7,000 annual coupon discounted at 5% prices the note at $105,446.50, so the premium is $5,446.50 and day-one carrying value is $105,446.50.
Year 1 expense = $105,446.50 × 5% = $5,272.33 against $7,000 cash, so amortization = $1,727.67 and carrying value falls to $103,718.83. Year 2 expense = $5,185.94, amortization $1,814.06, carrying value $101,904.77. Year 3 expense = $5,095.24, amortization $1,904.76, carrying value $100,000.01 — face value, give or take a rounding cent you absorb in the final entry.
Total expense is $15,553.51: the $21,000 of coupons minus the $5,446.50 premium. The premium was interest you collected upfront, and the method pays it back against expense a little each period. This is also why a premium note's interest expense is always below its coupon.
Practical note on rounding: carry amortization schedules to the cent, and let the final period absorb the leftover so carrying value lands exactly on face. A schedule whose last line leaves $0.01 of discount unamortized is the most common cosmetic defect in hand-built schedules, and auditors always flag it.
Why GAAP insists on this method
The authoritative rule is ASC 835-30, which requires amortizing any discount or premium "in such a way as to result in a constant rate of interest when applied to the amount outstanding at the beginning of any given period" — that constant-rate requirement is the interest method, as the FASB calls it. IFRS reaches the same place through IFRS 9's effective interest rate. The straight-line method — spreading the discount evenly — is permitted only when its result does not differ materially from the effective interest method, which in practice means tiny discounts, short terms, or books nobody audits.
The reason is economic honesty. Your outstanding obligation under discounted debt genuinely grows each period: you owe more tomorrow than today because unpaid economic interest accrues onto the balance. A constant 8% on a growing balance is what "8% debt" means. Straight-line amortization, by contrast, reports a declining effective yield on discount debt — the same dollar amortization against a rising balance — which understates early-period expense and overstates it later. For a three-year note the distortion is modest; for a 30-year bond issued deep in a discount, straight-line misstates interest by wide margins in both directions.
There is also comparability: two companies borrowing at the same market rate should report the same interest cost even if one negotiated a high coupon with a premium and the other a low coupon with a discount. The effective interest method guarantees exactly that: both report market rate times carrying value. Straight-line would show different costs for economically identical debt.
Where a small business actually meets this method
Few small businesses issue traded bonds, but the method shows up constantly in ordinary private-company life — anywhere cash received differs from the face amount owed:
- Seller-financed acquisitions. A retiring owner sells you the business for a $500,000 note at 3% when banks charge 8%. GAAP requires imputing interest at the market rate: you record the note (and the acquired assets) at present value, book the difference as a discount, and amortize it with the effective interest method. Booking the note at face would overstate both the assets and, eventually, understate interest.
- Below-market shareholder and related-party loans. A founder lends the company $200,000 interest-free for five years. Under ASC 835-30 the company records the note at present value using a market rate and amortizes the discount into interest expense — with the offset typically to additional paid-in capital, since the founder contributed the interest subsidy as equity. The common error is recording the loan at face and reporting zero interest, which understates both expense and the founder's capital contribution.
- Convertible notes with original issue discount. Startup bridge notes frequently carry OID — $100,000 of notes sold for $95,000, sometimes with warrants attached. The OID is a contra-liability amortized into interest expense via the effective interest method at a rate higher than the stated coupon, which is why bridge rounds often produce surprisingly large interest expense relative to their cash coupons.
- Bond investments you hold. The method is symmetric: a business holding another company's discounted bonds recognizes interest income as carrying value times the market rate at purchase, growing each period. Booking only the coupon understates income early and overstates the discount accretion later.
- Debt issuance costs. Points, legal fees, and underwriting spreads are netted against the loan's carrying value under GAAP (not parked as a separate asset), which lowers day-one carrying value and raises the effective rate. They amortize through the same machinery — another reason your book interest exceeds the coupon.
Government relief loans are the notable exception: FASB guidance confirmed borrowers do not impute market-rate interest on below-market government loans such as PPP and EIDL facilities, but recognize interest on the stated contractual terms. That carve-out exists because the subsidy is a government grant in substance, not a market transaction — it does not extend to private below-market notes.
Booking it in a plain-text ledger
The mechanics translate directly into double-entry bookkeeping. A discount account sits alongside the note as a contra-liability — a debit balance inside the liabilities section — so that note minus discount always equals carrying value. Here is the opening example's issuance plus its first year-end entry:
2026-01-01 open Assets:Cash:Checking USD
2026-01-01 open Liabilities:Notes-Payable USD
2026-01-01 open Liabilities:Notes-Discount USD
2026-01-01 open Expenses:Interest USD
2026-01-01 * "Seller note issued at a discount" "Face 100000, 5 pct coupon, 8 pct market, proceeds 92268.70"
Assets:Cash:Checking 92268.70 USD
Liabilities:Notes-Discount 7731.30 USD
Liabilities:Notes-Payable -100000.00 USD
2026-12-31 * "Year 1 coupon and discount amortization" "Expense 92268.70 x 8 pct = 7381.50; coupon 5000"
Expenses:Interest 7381.50 USD
Assets:Cash:Checking -5000.00 USD
Liabilities:Notes-Discount -2381.50 USDAfter the year-end entry, the discount account holds $5,349.80 ($7,731.30 − $2,381.50), so note minus discount is $94,650.20 — exactly the schedule's carrying value. Each subsequent year-end repeats the pattern with that year's computed expense and amortization. If you track debt in Beancount, this three-account structure keeps the schedule auditable: the discount balance at any date must equal the remaining unamortized discount on your amortization table, which gives you a built-in reconciliation check every close.
Five mistakes that break amortization schedules
Using today's market rate instead of the issuance rate. The effective rate freezes the day the debt is issued. Recomputing expense at current rates each period turns amortized-cost debt into something resembling fair-value accounting, and no auditor will accept it. The only thing that changes the rate is a troubled-debt restructuring or a formal modification accounted for as an extinguishment.
Forgetting to halve semiannual terms. A 6% bond paying semiannually uses 3% per period over twice as many periods. Applying the full annual rate each half-year roughly doubles reported expense. Every CPA exam harvests points from this error, and real-world spreadsheets reproduce it faithfully.
Booking the coupon as the expense. The single most common mistake: debiting interest expense for the cash paid and amortizing nothing. On discount debt this understates expense (and overstates profit) every period; on premium debt it overstates expense. If your books show interest expense exactly equal to debt-service cash on anything issued off-par, the schedule is missing.
Recording a below-market note at face. When no established price exists and the note carries no interest or an unreasonable rate, ASC 835-30 requires recording the exchange at fair value — the present value at a market rate — and treating the difference as discount or premium. Face-value booking overstates the asset acquired and buries the interest subsidy entirely.
Amortizing issuance costs straight into expense. Points and fees paid to originate a loan are not day-one expenses under GAAP; they reduce initial carrying value and amortize as additional interest over the loan's life through the effective interest method. Expensing them immediately overstates first-year costs and understates the loan's true yield in every later year.
Building your own schedule
You need four inputs: face value, stated rate and payment frequency, term, and the market rate at issuance. For traded bonds the market rate is observable; for private notes it is the rate the borrower would pay a willing lender for comparable debt — your bank's current term-loan quote for similar collateral and maturity is the usual starting point, documented in the file. Then build one row per period: start with carrying value, compute expense, subtract the coupon to get amortization, and roll the carrying value forward. Seed the first row with proceeds (present value at the market rate), and verify the last row lands on face value with the discount fully amortized.
Spreadsheet discipline matters more than spreadsheet sophistication. Put the effective rate in one named cell referenced by every row, never retyped; keep the coupon formula as face times stated rate per period; and add a check row confirming total amortization equals the original discount or premium. When the note is modified, prepaid, or refinanced mid-life, stop the old schedule at the event date, recognize any gain or loss on extinguishment, and start a fresh schedule only if new debt is recognized. Never splice new terms into old rows — that is how carrying values drift silently off face.
For debt with irregular payments — seasonal PIK toggles, contingent interest, payment deferrals — the same constant-yield principle applies, but the cash-flow estimates require judgment and usually a CPA's involvement. The method stays identical; only the inputs get harder.
