How this instrument works
This instrument answers a narrower question than a full amortization table: not what the loan costs over its whole life, and not what the monthly payment is, but how the very first year of payments splits between interest and principal. That split is what a taxpayer needs for this year's mortgage-interest deduction, what a landlord needs to post as interest expense on Schedule E, or what a small business needs to separate on its books when a term loan's payment covers both interest and repayment in a single line.
The reason the split looks the way it does is that interest is recalculated every month against whatever balance is still outstanding, not against the original loan amount. Early in a 30-year term the balance has barely moved, so nearly every dollar of the payment is interest; only the sliver left over reduces principal. This surprises people who conflate two different numbers — the Annual interest rate, %, a fixed percentage set at closing, with the actual dollar amount charged in any given year, a figure that shrinks steadily as the balance falls even though the rate itself never changes.
The arithmetic assumes every payment lands on time at the scheduled amount, with no extra principal paid down, no points or origination fees folded in, and no rate change mid-year — true of a plain fixed-rate loan but not of every real contract. It reports a number, not a filing: the result here is not itself a completed tax return or bookkeeping entry, and a lender's own year-end statement, a Form 1098 or its business-loan equivalent, may round or accrue slightly differently and should be used as the figure of record once it arrives.
- Enter the original balance under Loan amount, $ — the amount borrowed, not what is owed today.
- Set the lender's quoted rate under Annual interest rate, % and the repayment length under Loan term, years.
- Read Monthly payment for the fixed installment the loan formula produces.
- Check Balance after 12 payments to see how little the debt has actually fallen after a full year.
- Compare Principal paid in year 1 against Interest paid in year 1 for the split most loan statements never show directly.
Worked example — a $250,000 loan at 6% over 30 years
Set Loan amount, $ to 250000, Annual interest rate, % to 6, and Loan term, years to 30. The payment formula returns a fixed Monthly payment of $1,498.88, the same figure for the first installment as the last. Running that payment forward twelve times puts Balance after 12 payments at $246,929.97 — a $250,000 debt that has fallen by only $3,070.03 across a full year of paying.
That $3,070.03 drop is exactly Principal paid in year 1. Subtracting it from the year's total payments, 12 × $1,498.88 = $17,986.52, leaves Interest paid in year 1 at $14,916.49 — roughly 83 cents of every dollar sent to the lender in year one. A homeowner itemizing deductions could use that $14,916.49 to estimate this year's mortgage deduction before a Form 1098 arrives to confirm the exact number.
Questions
Why is year-1 interest so much larger than year-1 principal?
Interest is charged each month on whatever balance is still outstanding, and in year one of a 30-year loan that balance has barely moved from the original amount. On the $250,000, 6%, 30-year example, $14,916.49 of the year's $17,986.52 in payments is interest — about 83% — because the debt itself only fell by $3,070.03 across the same twelve months.
Is Interest paid in year 1 the same number my lender reports on Form 1098?
It should land close but not necessarily identical. This figure assumes twelve payments land exactly on schedule at the stated rate; a real lender's statement follows its own accrual calendar, payment dates, and rounding, and any escrowed items are reported separately. Treat this as a planning estimate and use the lender's actual year-end statement for a tax filing.
How is the interest rate different from the interest paid?
Annual interest rate, % is a fixed percentage set when the loan is signed and does not change year to year on a fixed-rate loan. Interest paid in year 1 is a dollar amount computed by applying that rate to a shrinking balance, and it falls every year the loan runs even though the rate itself stays exactly the same.
Does this work for a business or rental-property loan, not just a mortgage?
Yes. Any loan repaid with equal monthly installments at a fixed rate — a commercial term loan, a rental property mortgage, a practice or equipment loan — amortizes the same way, so the same split between year-1 interest and year-1 principal applies whether the deduction lands on Schedule A, Schedule E, or a business return.
Why doesn't Balance after 12 payments equal the loan amount minus twelve payments?
Because each payment is not pure principal. A $1,498.88 payment on the example loan reduces the balance by far less than $1,498.88, since most of it covers that month's interest charge first; only what remains afterward chips away at the debt, which is why twelve payments of $1,498.88 remove just $3,070.03 from the balance rather than $17,986.52.
What happens to year-1 interest if I make an extra principal payment?
This sheet does not model extra payments — it assumes exactly twelve scheduled installments at the stated amount. An extra payment made partway through the year would lower the balance sooner than shown here, which lowers every subsequent month's charge and would make the real interest paid somewhat below this figure.
References
- IRS — Publication 936, Home Mortgage Interest Deduction
- Consumer Financial Protection Bureau — Owning a Home
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.