Mortgage interest: monthly interest vs total interest (worked examples)
Mortgage interest guide (UK): monthly vs total interest, and how rate, term and overpayments change cost. Includes worked examples.
Summary
When people ask “how much interest will I pay?”, they can mean two different things:
- monthly interest: the interest part of this month’s payment (or this month’s interest charge)
- total interest: the accumulated interest cost over a long period (for example, over two years or over the full term)
Monthly interest helps you understand your payment breakdown and cash flow. Total interest helps you compare the long-run cost of different terms, rates and overpayment plans.
Use the calculator to run your own scenarios:
- Calculator: /mortgage-interest/
Key terms (quick definitions)
- APRC: a standardised “total cost of credit” percentage (not the same as total interest).
- SVR: the lender’s standard variable rate after an initial deal ends (often used as a placeholder rate).
How it works
Monthly interest (simple intuition)
Ignoring daily vs monthly conventions, a simple way to estimate interest for a month is:
[ \text{monthly interest} ≈ \text{balance} × \text{annual rate} \div 12 ]
On an interest-only mortgage, your monthly payment is mostly (or entirely) this interest amount. On a repayment mortgage, your payment covers interest plus some of the capital.
Total interest (why term length matters)
Total interest isn’t just “monthly interest × months” because the balance changes over time on repayment mortgages. But two truths are usually reliable:
- a higher rate usually increases total interest
- a longer term often increases total interest (because you’re borrowing for longer)
Why “total interest” needs a time horizon
When comparing deals, define the horizon you care about:
- two years (if you plan to remortgage at the end of a two-year fix)
- five years
- full term (if you expect to keep the mortgage for decades)
Different horizons can flip which option is cheaper.
Worked examples
These examples are illustrative.
Example 1: Monthly interest on a large balance
- Balance: £250,000
- Rate: 5% per year
Estimated monthly interest:
[ £250,000 × 0.05 \div 12 ≈ £1,041.67 ]
Final result: at this balance/rate, the interest component is roughly £1,042/month. On a repayment mortgage, your payment would be higher than that (because it also repays capital).
Example 2: Same balance, higher rate
Keep the same balance (£250,000) but assume 6%:
[ £250,000 × 0.06 \div 12 = £1,250 ]
Final result: the monthly interest estimate increases by about £208/month. That difference compounds over time.
Example 3: Overpayment reduces future interest
- Balance: £200,000
- Rate: 6%
- One-off overpayment: £5,000
The “next month” interest saving on the overpayment amount (roughly) is:
[ £5,000 × 0.06 \div 12 = £25 ]
Final result: the immediate monthly interest estimate drops by about £25. The larger benefit is that the balance stays lower for many months, reducing total interest over time.
Common mistakes
- Treating APRC as the same thing as “total interest paid”.
- Comparing deals without choosing a time horizon (two years vs full term).
- Forgetting that SVR/reversion rate assumptions can dominate the long-run cost.
- Using interest-only “monthly interest” as if it were a repayment payment.
- Ignoring fees (they can change the total cost even when rates look similar).
- Not stress testing the rate (a higher rate can change both affordability and total cost).
What to do next
- Run your scenario: /mortgage-interest/
- If you’re comparing monthly payment vs total interest, use: /mortgage-repayment/
- If you want to model overpayments: /mortgage-overpayment/
- Glossary: /glossary/aprc/ and /glossary/svr/