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

The split, year by year

The shift from interest to principal is gradual and back-loaded. Here is roughly how a $250,000 loan at 6.5% over 30 years divides, at a fixed payment of about $1,580 a month.

Two features stand out. In year one, roughly 85% of everything paid is interest. And the halfway point in time is nowhere near the halfway point in balance: after 15 years of a 30-year loan, well over half the original balance is still outstanding.

That asymmetry is the whole reason early overpayments are worth so much more than late ones, and the reason selling or refinancing early leaves you with less equity than the years elapsed would suggest.

Why the split moves at all

Each month the interest portion is calculated on the remaining balance, not the original amount. Since the payment is fixed, whatever is not consumed by interest goes to principal.

Month one on the loan above:

• Interest: $250,000 times 6.5% divided by 12, which is about $1,354
• Principal: $1,580 minus $1,354, which is about $226

Month two starts from a balance of $249,774, so the interest is a fraction lower and the principal a fraction higher. Repeat 360 times and the composition inverts completely. Nothing about the schedule is arbitrary; it falls out of applying a rate to a shrinking balance while holding the payment constant.

What changing the term does

The monthly payment and the total interest move in opposite directions, and the effect is much larger than most people expect.

Halving the term from 30 to 15 raises the payment by about 38% and cuts the total interest by more than half. The 30-year loan is not cheaper; it is smaller per month and much larger in total.

This is the trade to understand before choosing a term. A longer term buys monthly affordability and sells you the total. Neither answer is universally right, but the number that gets advertised is always the monthly one.

Amortisation and equity

Because principal repayment is back-loaded, the equity you build in the early years comes mostly from your deposit and from any change in the asset's value, not from your payments.

On the loan above, five years of payments totalling about $94,800 reduce the balance by roughly $15,400. The other $79,400 was interest. If you sell at year five, that is the position you are selling from.

This is worth knowing before treating a mortgage as forced saving. It becomes forced saving eventually, but the mechanism barely operates in the first several years.

Where the standard schedule does not apply

Not every loan amortises the way described here:

• Interest-only loans pay no principal at all during an initial period, so the balance does not move until the period ends and payments jump
• Balloon loans amortise on a long schedule but require the remaining balance in a lump sum at a fixed date
• Credit cards do not amortise. There is no fixed payment and no end date, which is why a balance can persist indefinitely. See how credit card interest is calculated
• Adjustable-rate loans re-amortise when the rate changes, producing a new payment for the remaining term
• Loans with precomputed interest, sometimes used for smaller consumer loans, calculate the total interest up front, which changes what early repayment saves you

Reading your own schedule

Every amortising loan has a schedule, and your servicer can provide it. It is worth pulling once, because it answers questions that are otherwise guesswork:

1. What proportion of my current payment is actually reducing the balance?
2. What will my balance be at the point I expect to sell or refinance?
3. How much total interest will I pay if I run the loan to term?
4. What does one extra payment, applied to principal, remove from the total?

Our loan calculator produces the monthly payment, the total interest, and the month-by-month split for any amount, rate and term, which lets you answer the first three directly. The fourth is covered in how extra loan payments work.

Two loans, same rate, different timing

A useful way to feel the schedule is to compare a new loan with one halfway through its term, at the same rate and payment.

The payment has not changed, but it is nearly three times more productive. This is why refinancing into a fresh 30-year term late in a loan can be a poor trade even at a lower rate: you reset to the front of the schedule, where the payment barely touches the balance, and start the interest-heavy years again.

If you refinance, compare the total remaining interest on both options rather than the monthly payment. A lower rate over a longer remaining term frequently costs more in total.

Sources

• CFPB: What is amortisation?
• CFPB: Understanding loan options

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