Housing
Early mortgage payments are almost entirely interest, and here is why
Amortisation is not a trick played on borrowers. It follows inevitably from charging interest on a balance that starts at its largest.
Everything here earned its place by changing an outcome. Nothing about mortgage amortisation is included to round the number up.
What matters most
- Interest is charged on the outstanding balance, which is highest at the start.
- The capital portion of a level payment grows every month.
- Overpayments early in the term remove far more total interest than late ones.
Where the split comes from
A repayment mortgage has a level monthly payment made up of interest on the current balance and whatever is left over, which reduces the balance. At the start the balance is at its maximum, so the interest portion is at its maximum and the capital portion is small.
As the balance falls, the interest due falls, and since the payment is fixed the capital portion rises to fill the gap. Nobody designed this to disadvantage borrowers; it is what happens when you charge interest on a declining balance with a level payment.
The curve accelerates
The relationship is not linear: the capital portion grows slowly at first and then increasingly quickly as the balance falls. On a long term at a meaningful rate, it can take a substantial share of the term before the capital portion exceeds the interest portion.
The arithmetic is straightforward: this is why a mortgage several years old can show a balance barely below the original advance. It also means the final years repay capital rapidly, which is the mirror image of the same arithmetic.
Why the term matters so much
Extending the term lowers the payment, but it lowers the capital portion by more, so the balance falls more slowly for longer. The total interest over the life of the loan rises substantially with term, even at an identical rate.
Lenders in several markets have extended standard terms to maintain affordability as prices rose, which raises lifetime cost. Comparing total interest across terms, not just monthly payments, is what makes that trade visible.
Overpayments are worth most early
A pound of capital repaid in year one avoids interest for the whole remaining term; the same pound in the final year avoids almost none. This is why early overpayments have a disproportionate effect on total interest and on the term. Most fixed-rate mortgages permit annual overpayments up to a stated percentage without charge, and exceeding that limit triggers a fee.
In numbers, ask whether the overpayment reduces the term or the monthly payment, because the interest saved differs considerably.
Interest-only is a different structure entirely
An interest-only mortgage pays no capital, so the balance at the end equals the balance at the start. That requires a credible plan to repay the capital, and historically some borrowers reached the end without one.
The monthly payment is lower and the total interest higher, because the balance never falls. Availability, regulation and required repayment plans differ substantially by country.
Check how interest is calculated
Daily interest calculation means an overpayment reduces interest from the day it is received. Annual interest calculation, still used in some markets, can mean overpayments do not take effect until a set date, which reduces their benefit.
Practically, the offer document states the method, and it materially changes whether overpaying monthly is worthwhile. Where interest is calculated annually, timing overpayments just before the calculation date is worth asking about.
Everything above, in order of what to do first
- Where the split comes from. A repayment mortgage has a level monthly payment made up of interest on the current balance and whatever is left over, which reduces the balance.
- The curve accelerates. The relationship is not linear: the capital portion grows slowly at first and then increasingly quickly as the balance falls.
- Why the term matters so much. Extending the term lowers the payment, but it lowers the capital portion by more, so the balance falls more slowly for longer.
- Overpayments are worth most early. A pound of capital repaid in year one avoids interest for the whole remaining term; the same pound in the final year avoids almost none.
- Interest-only is a different structure entirely. An interest-only mortgage pays no capital, so the balance at the end equals the balance at the start.
- Check how interest is calculated. Daily interest calculation means an overpayment reduces interest from the day it is received.
The takeaway
The split is arithmetic, not policy. Overpayments made early are worth several times the same amount made late.
Write the number down before you decide. It usually decides for you.
Questions readers ask
Why has my balance barely moved after five years?
Because interest is charged on the balance, which starts at its largest. Early payments are mostly interest by arithmetic, and the capital portion accelerates later.
Is it better to reduce the term or the payment when overpaying?
Reducing the term saves more interest overall; reducing the payment improves monthly cashflow. Lenders often apply one by default, so state which you want in writing.
