Loan Amortization Schedule: How Each Payment Splits Into Interest and Principal

You have been paying your business loan for a year, twelve on-time payments, thousands of dollars out the door — and the balance has barely moved. That is not a billing error. On a typical five-year equipment loan, the first year of payments can send more than half of every dollar to interest. An amortization schedule is the table that shows you exactly where each dollar goes, payment by payment, so you can plan around that pattern instead of being blindsided by it.

This guide walks through how to read and build one: the three formulas behind every row, why early payments are interest-heavy, what extra principal payments actually save you, and how to record each payment correctly in your books.

What an Amortization Schedule Actually Shows​

An amortization schedule is a row-per-payment table covering the life of a fixed-rate loan. Each row contains the same five columns:

• Payment number and date — which installment this is
• Total payment — the fixed amount you pay, identical every period
• Interest portion — the cost of borrowing for that period
• Principal portion — the amount that actually reduces what you owe
• Remaining balance — what you still owe after the payment

The total payment never changes, but the split inside it does. Early rows are dominated by interest; late rows are almost entirely principal. The schedule makes that drift visible, which is why lenders, accountants, and auditors all want one on file: it is the shared reference for how much of each payment is deductible interest and how much is debt repayment.

Schedules like this apply to any fully amortizing loan with fixed payments — term loans, equipment financing, SBA loans, commercial mortgages, and auto loans all work the same way.

The Three Formulas Behind Every Row​

You only need three calculations, repeated once per row. All the spreadsheet templates in the world are just these three formulas copied down a column.

1. The fixed payment​

The monthly payment comes from the present-value annuity formula:

M = P × r × (1 + r)^n / ((1 + r)^n − 1)

• P is the original loan amount
• r is the monthly interest rate (annual rate divided by 12)
• n is the total number of payments

In a spreadsheet, the same result comes from =PMT(rate/12, years*12, -loan_amount). The negative sign on the loan amount keeps the payment positive.

One detail that trips people up: this formula uses the loan's note rate — the contractual rate printed on the promissory note — not the APR. The APR folds in fees and closing costs for disclosure purposes, but your payment is calculated from the note rate alone.

2. The interest portion​

Each period's interest is simply the period rate applied to the balance you still owe:

Interest = current balance × r

Nothing else enters the calculation. This single line is the reason extra principal payments are so powerful, as you will see below: every dollar of principal you retire early is a dollar that stops earning interest for the lender, permanently.

3. The principal portion and new balance​

Whatever is left of the payment after interest goes to principal, and the balance drops by that amount:

Principal = M − Interest New balance = current balance − Principal

Then the next row starts from the new balance. That is the entire machine.

Building the Table: A Worked Example​

Suppose you borrow $25,000 for workshop equipment at 8 percent annual interest, repaid over 5 years (60 monthly payments).

First, convert the rate: r = 0.08 / 12 = 0.006667. Plug into the payment formula and you get a fixed payment of about $506.91 per month.

Now build the first few rows:

Check the arithmetic on row 1: interest is $25,000 × 0.006667 = $166.67. Principal is $506.91 − $166.67 = $340.24. The new balance is $25,000 − $340.24 = $24,659.76. Row 2 repeats the same steps from the new balance.

To build the full 60-row schedule in a spreadsheet, enter the loan amount, rate, term, and start date in an input block, compute the payment once with PMT, then fill the interest, principal, and balance formulas down. Add columns for the payment date and, if you want to model prepayments, an extra-principal column whose value is subtracted from the balance each row.

Fast-forward to the end of this loan and the split has inverted. Around payment 55, roughly $480 of the $506.91 goes to principal and under $30 to interest. Same payment, mirror-image composition. Over the full five years you pay about $5,415 in interest on top of the $25,000 — a total cost of borrowing the schedule makes concrete before you sign.

Why Early Payments Are Interest-Heavy​

Borrowers often read the early rows as a trick. It is not; it is arithmetic. Interest is always charged on the outstanding balance, and the balance is largest at the start. With a fixed payment, a large interest charge leaves only a small remainder for principal. As the balance shrinks, the interest charge shrinks with it, and the fixed payment redirects the difference to principal automatically.

Three practical consequences follow from this shape:

• Refinancing early in the loan saves the most. The interest you avoid by refinancing is concentrated in the rows you have not paid yet. Refinancing a five-year loan in year one rewrites mostly-interest rows; refinancing in year four rewrites mostly-principal rows and saves far less.
• Selling a financed asset early can leave you underwater. If the equipment depreciates faster than the principal-heavy rows arrive, the sale proceeds may not cover the remaining balance.
• Extra payments hit hardest at the beginning. A dollar of extra principal in month 6 eliminates interest on that dollar for the remaining 54 months. The same dollar in month 54 saves only a few months of interest.

What Extra Principal Payments Do to Total Interest​

Because interest is computed on the current balance, any extra amount applied to principal shortens the schedule from the far end: each extra dollar retires rows of future interest, and the loan ends early. Lenders do not re-run the payment formula when you prepay — the monthly amount stays the same, but the balance hits zero sooner, so the last several rows simply never happen.

A concrete example shows the scale. Take a $300,000 thirty-year loan at 6 percent: the monthly payment is about $1,799, and paying exactly on schedule costs roughly $348,000 in interest over thirty years. Add one extra full payment per year — the equivalent of what a biweekly schedule produces, since 26 half-payments equal 13 monthly payments — and the loan retires roughly five years early, saving on the order of sixty thousand dollars in interest.

Smaller amounts still matter. An extra $100 a month on that same loan saves tens of thousands in interest and cuts more than five years off the term. The mechanism is always the same: extra principal shrinks the base that future interest accrues on, and the savings compound over every remaining period.

Before prepaying, check two things. First, confirm your loan has no prepayment penalty — some commercial and SBA loans charge for early payoff, and the penalty can exceed the interest you would save. Second, confirm the lender applies the extra amount to principal rather than treating it as an early next payment; many servicers require you to mark the extra amount explicitly, and an unmarked overpayment may just sit as a credit against next month's bill without reducing the balance.

Recording Each Payment in Your Books​

Here is where the schedule earns its keep in daily bookkeeping. Every loan payment is two different economic events bundled into one bank transfer: interest is an expense on your income statement, while principal is balance-sheet debt repayment. Recording them correctly is what keeps your profit, your loan balance, and your tax deduction all accurate at once.

The most common bookkeeping mistake with loans is recording the entire payment as an expense. That overstates your costs by the principal amount, understates your profit, and leaves a loan balance on the books that never decreases. The mirror-image mistake — posting the whole payment against the loan — means interest is never expensed, so profit is overstated and the interest deduction disappears. The schedule prevents both by giving you the exact split for every payment.

In a plain-text ledger, each payment records all three legs explicitly:

2026-11-01 * "First National Bank" "Equipment loan payment #1"
  Liabilities:Loans:Equipment   340.24 USD
  Expenses:Interest:Loans       166.67 USD
  Assets:Checking              -506.91 USD

The liability leg shrinks the loan balance by the principal portion, the expense leg captures the deductible interest, and the checking leg records the cash out. The two amounts come straight from that month's schedule row, so the schedule doubles as your data-entry source document.

After posting, run one reconciliation check: compare the loan balance in your books against the principal balance the lender prints on the statement. The two should match to the cent. If your books show more, interest was posted as principal somewhere or a payment was missed; if they show less, principal was posted as interest or a payment was recorded twice. For a fuller treatment of the surrounding workflow, the guides on recording purchases and payments walk through the same post-and-reconcile rhythm for other transaction types.

Mistakes That Break the Schedule​

A schedule is only as reliable as its inputs. Watch for these five failure modes:

1. Using the APR instead of the note rate. Your payment follows the contractual rate. Building the schedule from the APR produces a payment that matches nothing the lender bills.
2. Forgetting that extra payments need instructions. As noted above, an unmarked overpayment may not reduce the balance at all. Verify on the next statement that the balance dropped by the full extra amount.
3. Booking the whole payment as interest at tax time. A year of $506.91 payments is $6,083 out the door, but only about $1,900 of the first year's total is interest. Handing your preparer the cash total instead of the schedule's interest column overstates the deduction and understates the remaining loan.
4. Mixing escrow into the split. If your payment bundles taxes or insurance with principal and interest, strip the escrow portion out before applying the schedule — only the principal-and-interest part follows the table.
5. Letting the schedule go stale after a rate change. The table assumes a fixed rate. If your loan adjusts, rebuild the schedule from the current balance at the new rate rather than continuing the old rows.

Your Next Step​

Pull your largest outstanding loan — the term loan, the equipment note, the commercial mortgage — and build its schedule this week, even a rough one in a spreadsheet. Find which row you are on, read off how much of your next payment is interest, and confirm your books split the last payment the same way the schedule says. That single checkup ties your cash flow forecast, your monthly bookkeeping, and your interest deduction to one consistent source of truth, and it takes less time than reconciling one bank statement.