How Mortgage Amortization Works

A repayment mortgage charges the same monthly amount for the whole term, which makes it easy to assume the loan reduces at a steady rate too. It doesn't. Amortization — the schedule by which a loan balance falls to zero — front-loads interest and back-loads capital repayment, and understanding why changes how you should think about overpaying, refinancing and comparing terms.

How the split is calculated

Each month, interest is charged on whatever the outstanding balance happens to be. Whatever is left of the fixed payment after that interest is deducted reduces the balance itself. Because the balance falls a little every month, next month's interest charge is a little smaller, which leaves a little more of the payment to reduce capital — a slow, compounding acceleration that only becomes obvious over years, not months.

M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ] where r = annual rate ÷ 12, n = years × 12

M is solved so the balance reaches exactly zero on the final scheduled payment — that is the entire purpose of the formula. It is the same calculation the site's mortgage calculator runs, including the year-by-year table and balance chart.

Worked example: £250,000 over 25 years at 4.5%

The monthly payment is £1,389.58, fixed for the full term. Total repaid over 300 months is £416,874, of which £166,874 — about 67p in every £1 borrowed — is interest rather than capital.

In the first month, two-thirds of the payment is interest. The split reaches 50/50 only around month 115 of the 300 — a little over a third of the way through the term — and by the midpoint (month 150) about 57% of each payment is already reducing the balance. This is the mechanical reason mortgage advice so often says the first few years barely touch the amount you actually borrowed — it's arithmetic, not a lender being unfair.

Why a lender's quote can differ from a calculator

• Day-count conventions vary — some lenders accrue interest daily rather than monthly, which shifts the figures by small amounts.
• Fees (arrangement, valuation, legal) are often excluded from a simple amortization calculation but appear in a lender's total cost of borrowing figure.
• A real mortgage frequently fixes for an initial period (2, 5 years) then reverts to a variable rate — a single-rate amortization schedule cannot model that switch.
• Insurance, ground rent and property taxes are routinely bundled into a lender's monthly figure but are not part of the capital-and-interest calculation itself.

Why overpaying early is disproportionately effective

Because interest is charged on the outstanding balance every month, any amount that reduces the balance early removes interest that would otherwise have been charged on it for every remaining month of the term. A £5,000 overpayment in year 1 of the example above removes roughly £5,000 of principal that would otherwise have accrued interest for up to 24 more years; the same £5,000 overpaid in year 24 only saves the interest for the handful of months left. This is also why shortening a term (e.g. 30 years to 25) usually raises the monthly payment only modestly while cutting total interest substantially — the loan simply spends less total time accruing interest on a large balance.

Comparing terms honestly

A longer term lowers the monthly payment but raises total interest substantially, because the same money is borrowed for longer. Run both ends of the term range you're considering through the calculator before assuming the lower monthly figure is the better deal — it depends entirely on whether affordability or total cost is the binding constraint.

Limitations

• This describes a single fixed-rate repayment mortgage. Variable, tracker, offset and interest-only products amortize differently or not at all.
• None of this is a lender quotation or a decision in principle — a real quote depends on affordability assessment, credit history and the lender's own stress testing, none of which a formula captures.