An interest only mortgage keeps your monthly payments lower by charging only the interest each month — the original loan never reduces. In the UK in 2025, interest only is still available for both residential and buy-to-let borrowers, but the criteria are meaningfully tighter than they were before 2008 and the financial implications of choosing interest only over repayment are substantial. This calculator shows you both options side by side so the comparison is clear before you commit.
Interest Only vs Repayment Mortgage Calculator
How interest only mortgage payments are calculated
The interest only formula is the simplest calculation in UK mortgage lending. Unlike the amortisation formula used for repayment mortgages, no capital reduction is involved — the entire monthly payment covers only the interest accrued.
Example: £250,000 at 4.5%
= (£250,000 × 0.045) ÷ 12
= £11,250 ÷ 12
= £937.50 per month
What makes this calculation straightforward also makes it deceptively simple. Because the payment never changes regardless of how many years have passed — the balance stays at £250,000 throughout — the total cost only becomes clear when you add up 25 years of monthly payments and then add the outstanding capital: £937.50 × 300 months = £281,250 in interest, plus the original £250,000 capital still owed = £531,250 total.
Compare that to a repayment mortgage on the same loan at the same rate over the same term: £1,389/month × 300 payments = £416,700 total — which includes both the interest and clearing the full £250,000 balance. The repayment mortgage costs £114,550 less in total, despite having a higher monthly payment. This is the central arithmetic of the interest only decision.
Interest only vs repayment — the full cost table
| Loan amount | IO monthly (4.5%) | Repayment monthly (4.5%) | Monthly saving (IO) | Extra total cost (IO) |
|---|---|---|---|---|
| £150,000 | £562 | £833 | £271/mo | +£68,700 |
| £200,000 | £750 | £1,111 | £361/mo | +£91,600 |
| £250,000 | £938 | £1,389 | £451/mo | +£114,550 |
| £300,000 | £1,125 | £1,667 | £542/mo | +£137,400 |
| £400,000 | £1,500 | £2,222 | £722/mo | +£183,200 |
| £500,000 | £1,875 | £2,778 | £903/mo | +£229,100 |
Based on 4.5% interest rate over 25 years. "Extra total cost" = additional interest paid on IO vs repayment, not including the capital still owed at the end of the IO term.
Who interest only genuinely suits — and who it does not
- Buy-to-let landlords maximising monthly cash flow with a clear exit strategy
- High-net-worth borrowers with substantial investment portfolios as repayment vehicles
- Short-term owners who plan to sell the property before the term ends
- Older borrowers who will downsize and use equity to repay the capital
- Those using the IO saving to pay down higher-interest debt before switching to repayment
- Borrowers on a pension or endowment specifically designed to clear the capital
- First-time buyers with no separate repayment vehicle or savings plan
- Anyone choosing IO primarily because repayment feels unaffordable
- Borrowers relying on property price growth alone as the repayment plan
- Those without a written, credible plan to repay the capital at term end
- Borrowers who would not review the repayment vehicle annually
- Anyone who does not fully understand that the full capital is due at the end
Who can get an interest only mortgage in the UK in 2025?
After the widespread misuse of interest only mortgages in the pre-2008 era — when many were sold without adequate repayment plans — the FCA introduced much stricter affordability rules. Today's criteria are considerably more demanding for residential borrowers.
Residential interest only
- Repayment vehicle required — most lenders require documentary evidence of a credible plan to repay the capital at the end of the term. Accepted vehicles include ISA or investment portfolios, endowment policies, pension lump sums, and planned property downsizing (with equity evidence).
- Minimum income — most lenders set a minimum gross income of £75,000–£100,000 for sole applicants or a combined £100,000+ for joint applications on residential interest only deals.
- Minimum equity or LTV cap — many lenders will only offer residential interest only up to 75% or 80% LTV, requiring a 20–25% deposit or substantial existing equity.
- Regular review requirement — lenders may require periodic confirmation that the repayment vehicle remains on track to clear the capital.
Buy-to-let interest only
Interest only is far more accessible for buy-to-let mortgages. The expected rental income covering the mortgage payment and the anticipated property sale at the end of the term are accepted as the repayment strategy. Most BTL lenders offer interest only up to 75% LTV as standard, provided the rental income covers the mortgage at a stress-tested rate of 125–145% coverage. See our BTL investment guide for a full breakdown of how IO affects buy-to-let returns.
Real UK interest only examples — the numbers in practice
Derek owns a two-bed flat in Manchester with a £185,000 buy-to-let mortgage on an interest only basis at 5.1%. Monthly interest payment: £786. The property rents for £1,100/month, generating gross monthly profit of £314 before costs.
If Derek switched to a repayment mortgage over 20 years at the same rate, his monthly payment would jump to £1,240 — turning the investment from profitable to loss-making. His exit strategy is to sell the property in 15 years when he retires, using the capital gain to repay the balance and fund part of his retirement.
He uses our rental yield calculator to monitor whether the investment remains viable each year as rates and rents change.
In 2007, Claire took out a £220,000 interest only residential mortgage at 5.5% over 25 years. Her monthly payment was always £1,008 — lower than the repayment alternative of £1,516 at the time. She chose IO because the monthly payment was more comfortable.
In 2032 the term ends and she still owes the original £220,000 in full. Her investment ISA — the original repayment vehicle — is worth approximately £110,000 after poor performance in the early years. She faces a £110,000 shortfall with no obvious way to make it up other than selling the property and downsizing significantly.
James earns £180,000 and is purchasing a £900,000 property in Richmond with a £650,000 interest only mortgage at 4.2% over 15 years. His monthly IO payment: £2,275. An equivalent repayment mortgage would cost £4,837/month.
James has a £400,000 S&S ISA and investment portfolio that he expects to grow to approximately £750,000 over 15 years at an assumed 6% annual return. He also has a defined benefit pension that will provide a significant lump sum. His broker documents both as the repayment vehicle at application, and the lender is satisfied.
The monthly saving of £2,562 versus repayment is redirected into his investment portfolio — compounding the return and building the repayment vehicle simultaneously.
Part-and-part mortgages — a middle ground worth knowing
If full interest only does not suit you but repayment payments feel too high, a part-and-part mortgage splits your loan between the two structures. Part of the balance runs on repayment terms (reducing the capital each month) while the remainder runs on interest only (keeping that portion's payment lower). You still need a repayment plan for the interest only portion, but the overall capital risk is significantly reduced.
For example, on a £300,000 mortgage you might arrange £200,000 on repayment and £100,000 on interest only. Here is how the monthly payment compares across the three options at 4.5% over 25 years:
The part-and-part option saves £181/month versus full repayment while leaving only £100,000 outstanding at the end — a far more manageable capital obligation than the full £300,000 on pure IO. Many lenders offer this arrangement and it suits borrowers who have a partial repayment vehicle (such as a modest pension lump sum) but not enough to clear the full balance.
The risks of interest only that borrowers underestimate
The most significant risk is arriving at the end of your term with the full capital sum due and no reliable means to repay it. Lenders can and do pursue repossession in this scenario. The FCA has repeatedly flagged that hundreds of thousands of UK borrowers on legacy interest only deals face exactly this situation.
A second risk is negative equity. If property values fall, your outstanding balance can exceed the property's value — trapping you in a mortgage you cannot exit by selling without realising a cash loss.
A third risk is rate exposure at renewal. Because the balance never reduces on IO, you always refinance the same original loan amount. Rate rises therefore hit IO borrowers harder over a longer period than equivalent repayment borrowers whose balance has been gradually reducing throughout.
Switching from interest only to repayment
If you are currently on an interest only residential mortgage and concerned about the end-of-term capital, the earlier you act the more manageable the switch becomes. Options include:
- Switch to full repayment — your lender can usually process this at any time. Your monthly payment will increase, but you begin reducing the balance immediately. The increase is more affordable the more years remain on the term.
- Switch to part-and-part — split the mortgage so a portion runs on repayment. This raises payments less dramatically while still reducing the end-of-term capital risk.
- Make regular overpayments on the IO balance — not all IO products permit this, but many allow a percentage of the balance to be overpaid annually. Check your mortgage terms and use these payments to voluntarily reduce the outstanding capital.
- Remortgage to a repayment deal — when your fixed rate expires, remortgage to a repayment basis with a new lender if your current lender cannot offer competitive terms.
If you are unsure which option is most appropriate for your specific situation, speak to an FCA-regulated mortgage adviser. The sooner you address an underfunded IO repayment vehicle, the lower the cost of the solution.
Common mistakes with interest only mortgages
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Treating the monthly saving as profit rather than deferred debt
The £451/month saving on interest only versus repayment on a £250k mortgage is not free money — it is a £250,000 obligation being deferred to the end of the term. That saving only has financial value if it is systematically invested in a vehicle that will produce enough to repay the capital. Most people who choose IO for the monthly payment saving do not make this investment consistently.
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Relying on property price growth as the only repayment strategy
UK property prices have historically risen over the long term — but not in every region, not in every decade, and not necessarily at the pace required to cover your specific outstanding balance after transaction costs. Lenders do not accept expected capital appreciation as a standalone repayment vehicle, and for good reason. It is a bet, not a plan.
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Setting up a repayment vehicle and never reviewing it
A pension or investment ISA set up as the repayment vehicle in 2010 may be significantly below its projected target in 2025 — through poor performance, missed contributions, or changing market conditions. An IO mortgage with a repayment vehicle needs annual review. If the vehicle is underperforming, the options available at year 5 or year 10 are far less disruptive than at year 24.
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Not understanding that BTL interest only works differently from residential
BTL landlords choosing IO are in a fundamentally different position from residential IO borrowers — the investment is a business asset, not their home, and the exit plan (property sale) is explicit and documented. The emotional and financial risk profile is entirely different. Conflating the two leads to residential borrowers mistakenly thinking IO is appropriate for them because it works well in a BTL context.
Frequently asked questions
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How do you calculate interest only mortgage payments in the UK?The formula is simple: (loan amount × annual interest rate) ÷ 12. On a £250,000 mortgage at 4.5%, that is (£250,000 × 0.045) ÷ 12 = £937.50 per month. The payment stays the same throughout the term because the balance never reduces. Use the calculator above to model any loan amount and rate instantly.
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Who can get an interest only mortgage in the UK in 2025?Residential interest only is available but requires a documented repayment vehicle (investment portfolio, pension, endowment, or planned downsizing), a minimum income of approximately £75,000–£100,000 for sole applicants, and typically a maximum LTV of 75–80%. Buy-to-let interest only is much more accessible — most BTL lenders offer it up to 75% LTV as standard, using the expected property sale as the repayment plan.
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Is interest only cheaper than a repayment mortgage?Month to month, yes. On a £250,000 mortgage at 4.5% over 25 years, interest only costs £938/month versus £1,389 on repayment. But interest only is significantly more expensive overall: you pay £281,250 in interest over 25 years, still owe the £250,000 capital, and must repay it separately — a true total cost of £531,250. The repayment mortgage total is £416,700, which clears the debt entirely. Interest only costs £114,550 more in total.
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What happens at the end of an interest only mortgage?The full original loan amount becomes due as a lump sum. If you cannot repay it — through your repayment vehicle, a property sale, or refinancing — the lender can take possession of your home. This is why having a documented, funded repayment strategy and reviewing it annually throughout the term is not optional — it is the central risk management requirement of every interest only mortgage.
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Can I switch from interest only to repayment during my mortgage term?Yes. Most UK lenders allow you to switch from interest only to a repayment basis at any time — either at a renewal point or mid-term by contacting your lender. Your monthly payment will increase, but you begin reducing the outstanding balance immediately. The earlier you switch, the more manageable the payment increase, as there are more years remaining over which the capital reduction is spread. Part-and-part arrangements are also possible with many lenders.
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