Buy-to-let mortgage calculator
Work out your monthly payments and check whether the rent passes a lender's stress test before you apply.
Key facts
- Most buy-to-let mortgages are interest-only, with a deposit of about 25% or more.
- Lenders test the rent against a stressed rate, often about 5% to 5.5%, not the rate you pay.
- Rent usually needs to cover 125% of that interest (basic-rate taxpayers and companies) or 145% (higher-rate taxpayers).
- Every lender sets its own figures, so use this as a guide before speaking to a broker.
How the rent test works
The lender multiplies your loan by its stress rate to get a notional yearly interest bill. The rent must cover that by 125% or 145%.
| £150,000 loan at a 5.5% stress rate | Basic rate (125%) | Higher rate (145%) |
|---|---|---|
| Stressed interest a month | £687.50 | £687.50 |
| Minimum rent a month | £859 | £997 |
Higher-rate taxpayers face the tougher test because Section 24 limits their mortgage interest relief, so less rent is left after tax.
Other costs to budget for
Arrangement fees (often a flat fee or a percentage of the loan), valuation and legal fees, and Stamp Duty including the 5% surcharge. Then check the deal still works after tax with the landlord tax calculator.
Frequently asked questions
How much deposit do I need for a buy-to-let?
Usually at least 25% of the price. Bigger deposits (35% to 40%) often get better rates.
Why is the lender's test rate higher than my rate?
Lenders check the rent would still cover the mortgage if rates rose. Five-year fixes are often tested at a lower rate than two-year fixes.
Can I top up with my salary if the rent is too low?
Some lenders allow 'top slicing', using personal income to make up a shortfall. Ask a mortgage broker.
Sources
- GOV.UK: Tax relief for residential landlords, how it's worked out
- GOV.UK: Stamp Duty Land Tax residential property rates
Sources last checked 10 November 2026. See all our sources and how we keep pages up to date.