How Loan Amortisation Works, and Why Early Payments Are Mostly Interest

If you have ever looked at a loan statement a year in and wondered why the balance has barely moved, this is why.

A fixed payment, a changing split

On a standard amortising loan - most mortgages, car loans and personal loans - your monthly payment stays constant for the whole term. What changes is how that payment is divided between interest and principal.

Each month:

Interest is calculated on the current outstanding balance
Whatever is left of your payment reduces the principal
Next month, the balance is smaller, so the interest portion is smaller, so more of the same payment goes to principal

At the start, the balance is at its largest, so interest takes the biggest bite. By the end, the balance is small and almost the entire payment reduces principal. The shift is gradual and then accelerates.

The numbers on a real example

Take a $20,000 loan at 6.5% over five years. The monthly payment works out to about $391.

Month 1:

Interest: $20,000 times (6.5% divided by 12) = about $108
Principal: $391 minus $108 = about $283

Month 60:

Interest: about $2
Principal: about $389

Same payment. In the first month, 28% of it went to the lender as interest. In the last, under 1% did.

Over the full term you repay about $23,470 on a $20,000 loan - roughly $3,470 in interest. You can generate the full schedule for any loan with our loan calculator.

Why longer terms cost so much more

Stretching the same loan from five years to seven drops the monthly payment, which is why lenders offer it. It also means the balance stays high for longer, and interest is charged on that balance every month it remains.

The monthly payment falls by roughly $80. The total interest rises by around $1,500. That is the trade, and it is worth making deliberately rather than by default.

Why overpayments are worth most early

An extra payment made against principal removes that amount from the balance for every remaining month of the loan. The earlier it lands, the more months it affects.

An extra $1,000 in month 1 of the loan above saves considerably more interest than the same $1,000 in month 50, because in month 1 it avoids interest on 59 further months.

Two practical notes:

Tell the lender the overpayment is for principal. Otherwise some will treat it as an advance on next month's payment, which does not have the same effect
Check for prepayment penalties before making large overpayments. They are less common on personal loans than they once were, but they exist

Where this does not apply

Not every debt amortises. Credit cards do not - there is no fixed term, and the minimum payment is recalculated from the balance each month, which is why card balances can persist for years. Interest-only loans do not either, since the principal does not reduce at all during the interest-only period.

The distinction matters: with an amortising loan, doing nothing still clears the debt on schedule. With revolving credit, doing the minimum can leave you in essentially the same place years later.

What to take from this

Early in a loan, most of your payment services interest rather than reducing debt
Longer terms reduce the monthly payment and increase the total cost
Overpayments are worth far more early than late
Always check how the lender applies extra payments

Sources

Current as of August 2026. Figures are illustrative examples calculated with the tools on this site, not offers.