How UK Mortgage Repayments Work

A UK mortgage is a long-term loan secured against your property, repaid in monthly instalments over a term (typically 25 years, though up to 40 is now common). On a standard repayment mortgage, each monthly payment covers part interest, part capital. In the early years most of the payment is interest because the balance is still high; over time the capital portion grows as the balance falls. By the final year, almost all of the payment is going to capital. This calculator breaks down exactly how that split changes year by year.

Why overpayments save disproportionate interest in early years

Every extra pound you pay early in the term goes straight to capital, reducing the balance for the next month's interest calculation, which compounds. £100 per month extra on a £150,000 mortgage at 6% over 30 years saves more than £45,000 in interest and clears the loan ~6 years and 9 months early. The same £100/month started in year 20 saves a fraction of that. If you have spare cash and your lender allows penalty-free overpayments (most do up to 10% of the balance per year), it's usually one of the best risk-free returns available.

Repayment vs interest-only

Repayment fully pays off the loan over the term. Interest-only only pays the interest each month — the capital is still owed at the end and you need a separate repayment vehicle (savings, investments, sale of the property, or another mortgage) to clear it. The FCA tightened interest-only criteria in 2014 after scrutiny of legacy interest-only lending where many borrowers had no realistic repayment plan; most residential lenders now require an explicit repayment strategy before granting an interest-only loan, and tighter LTV caps. Buy-to-let mortgages remain commonly interest-only because the property itself is the exit plan.

Fixed vs variable rates

Most UK borrowers take a fixed-rate deal — 2, 3, 5 or 10 years — then remortgage when it ends. Trackers move with the Bank of England base rate plus a margin. Standard Variable Rates (SVRs) are the lender's default once your deal ends and are usually much more expensive than market deals — set a reminder six months before your fix ends and start shopping around then. This calculator uses a single rate for the whole term to keep the maths transparent; in reality, your effective rate will change at each remortgage, so re-run the calculator with your new rate when you change products.

Early Repayment Charges (ERCs)

Most lenders allow penalty-free overpayments up to 10% of the outstanding balance per calendar year while you're in a fixed or tracker deal. Above that, an Early Repayment Charge applies — typically 5% in year 1 sliding to 1% in year 5, though terms vary widely. Once on SVR, ERCs usually disappear and unlimited overpayment is allowed. Always check your mortgage offer document before making a large overpayment.

Pairs with the Stamp Duty Calculator

For a complete picture of the cost of buying a home, run your purchase price through our Stamp Duty Calculator to see SDLT (England), LBTT (Scotland) or LTT (Wales) due, plus first-time buyer relief and the 5% / 8% surcharges for additional properties. Add Stamp Duty to your deposit and conveyancing fees to know what you actually need at completion.