Understanding Mortgage Amortisation: Where Your Payment Actually Goes

An amortising loan is designed so that a constant payment reduces the balance to exactly zero at the end of the term. Getting that to work requires the split between interest and principal to change every single month, and that changing split explains almost everything people find surprising about mortgages.

The mechanism

Each month:

1. Interest is charged on the current outstanding balance.
2. Your payment covers that interest first.
3. Whatever remains reduces the principal.
4. Next month the balance is smaller, so interest is smaller, so more of the same

payment goes to principal.

The payment is constant. The composition is not.

What that looks like in practice

A £300,000 loan at 6% over 30 years. The payment comes from the standard amortisation formula:

payment = P × i × (1+i)^n ÷ ((1+i)^n − 1)

where P is the principal, i the monthly rate (6% ÷ 12 = 0.005) and n the number of payments (360). That gives £1,798.65.

In month one, 83% of the payment is interest and £298.65 touches the debt. The 50/50 crossover — the first payment where principal exceeds interest — happens at month 223, in year 19 of 30. Total interest over the full term is £347,515, slightly more than the amount borrowed.

Two details worth noticing in that table. After five years of payments the balance has fallen by only £20,837 of £300,000, which is why selling early feels like you have made no progress. And the principal portion grows at a compounding rate itself: £298.65, then £400.83, then £729.27, then £1,326.84.

Why overpaying early is disproportionately effective

An overpayment reduces the principal immediately, which removes all future interest that principal would have generated for the entire remaining term.

On the loan above, a single extra £5,000 at month 12 cuts total interest from £347,515 to £325,146 — a saving of £22,369 — and clears the loan 15 months early. The same £5,000 at month 300 saves only £1,698, because there are barely five years of interest left for it to cancel.

An extra £100 every month from the start is worth more again: the loan finishes in 313 months instead of 360 (3 years 11 months early) and total interest falls to £294,168, saving £53,347. Double it to £200 a month and the loan ends at month 279, saving £91,174.

A "biweekly" schedule works by the same mechanism and nothing more. Paying half the monthly amount every two weeks means 26 half-payments a year, equal to 13 monthly payments instead of 12 — an extra £149.89 a month here. That finishes the loan at month 295 and saves £73,666. There is no special magic in the fortnightly timing; it is a disguised 8.3% overpayment.

Model any of these with the mortgage payoff calculator.

Three things to check before overpaying:

• Early repayment charges on fixed-rate deals, often a percentage of the

balance repaid.

• How the lender applies it. Insist it reduces the balance rather than being

held as a credit toward future scheduled payments.

• Recast versus term reduction. Reducing the term keeps the payment and saves

the most interest. A recast keeps the term and lowers the payment, which saves less but improves monthly cash flow. Lenders usually default to one; ask.

Term length is a real trade-off

Five years shorter costs £134.25 more a month and saves £67,644. Halving the term to 15 years costs £732.92 more a month and saves £191,832. Whether that is worth it depends entirely on what else that money would do — this is a real decision, not an obvious one. Note the shape of the trade: payments rise slowly while interest falls fast, because a shorter term attacks the balance that interest is charged on.

Common mistakes

• Treating total interest as wasted. It is the price of occupying the house

for 30 years rather than renting. Compare it with rent, not with zero.

• Assuming a lower rate always wins. A lower rate with a long term can cost

more in total than a higher rate over a short one. Run both.

• Forgetting that interest accrues daily on some products. A payment date

moved earlier in the month saves a little interest on daily-accrual loans and nothing at all on monthly-accrual ones.

• Comparing a 30-year schedule you will not keep. Most borrowers move or

refinance within ten years, so the first-decade numbers matter more than the lifetime total.

Reading a schedule properly

Look for three things rather than scanning every row:

• The crossover month where principal first exceeds interest.
• The balance at year 5, since most people move or refinance before then.
• Total interest, which is the actual price of borrowing.

What amortisation does not include

Your real monthly outgoing includes property tax, insurance, and often PMI — lenders bundle these as PITI. Amortisation covers only principal and interest. The mortgage calculator handles the rest as separate components.

It also assumes a fixed rate. On a variable or tracker product the payment is recalculated whenever the rate changes, using the balance and remaining term at that moment — so the schedule is really a snapshot that is correct until the next reset. The amortization calculator is the right place to re-run it after a rate change, and the loan calculator applies the identical formula to car and personal loans, which amortise exactly the same way over shorter terms.