How Loan Amortization Actually Works (With a Real Example)

You make your first payment on a 30-year mortgage — a real amount, every month, on time — and a year later, the balance has barely moved. It feels like something's wrong. It isn't. That's exactly how amortization is designed to work, and understanding why makes every other decision about extra payments, refinancing, and loan terms make a lot more sense.

A fixed-rate loan uses a standard formula to calculate a single monthly payment that stays constant for the entire term: M = P × r × (1+r)ⁿ ÷ ((1+r)ⁿ − 1), where P is the loan amount, r is the monthly interest rate, and n is the total number of payments. Every lender calculating a standard amortizing loan uses this same formula — it's not proprietary or negotiable, just math.

What that formula guarantees is a constant total payment. What it doesn't guarantee — and what surprises people — is a constant split between interest and principal inside that payment.

Interest for any given month is calculated on the remaining balance, not on the original loan amount. Early in a loan, the balance is at its highest, so the interest portion of that fixed payment is at its highest too — which means whatever's left over for principal is at its lowest.

As the balance shrinks month by month, the interest charged shrinks with it, and — since the total payment stays fixed — the principal portion grows to fill the difference. Over the life of the loan, that ratio flips almost completely: early payments are mostly interest, late payments are mostly principal.

“The payment never changes — but what it's actually made of shifts every single month, by design.”

On a $300,000 loan at 6% over 30 years, the fixed monthly payment is roughly $1,799. In month one, about $1,500 of that goes to interest and only about $299 reduces the balance. By year 15, the split is close to even. By the final years of the loan, the vast majority of each payment finally goes to principal — the balance drops fast at the very end, not the beginning.

This is exactly why an extra payment made early in a loan saves dramatically more total interest than the same extra payment made later: paying down principal early reduces the balance interest gets charged on for the entire remaining term, compounding the savings. The same extra dollar amount applied in year 25 barely moves total interest, because there's so little term left for the reduction to compound over.

Open Tools Library's Loan / EMI Calculator computes the monthly payment, the complete year-by-year or month-by-month amortization schedule, and — specifically — exactly how many months and how much total interest a fixed extra payment would save, so the effect described above is a real number for your actual loan, not just a general principle.