Buy to Let Mortgage Calculator
This buy to let mortgage calculator works out borrowing the way lenders do: the rent must cover the interest at a stressed rate by 125% or 145%. On £1,200 a month rent, a two-year fix stress-tested at 6.9% and 125% cover supports about £166,957, so a £250,000 purchase needs a deposit of about £83,000.
- Basic-rate / company ICR125%
- Higher-rate ICRoften 145%
- PRA minimum stress rate5.5%
Updated November 2026 · Uses PRA stress-test rules
Calculate your buy-to-let borrowing
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How buy-to-let affordability is worked out
Buy-to-let lenders mainly lend on the rent, not your salary. They use an interest cover ratio (ICR): the monthly rent must be a set percentage of the mortgage interest calculated at a higher stress rate.
Maximum loan = annual rent ÷ (ICR × stress rate). Example: £14,400 a year ÷ (1.25 × 6.9%) = £166,957. The loan is then capped by the maximum loan to value, usually 75% for the best rates.
Stress rates and the PRA rules
Prudential Regulation Authority rules from 2017 require lenders to test affordability at a minimum of 5.5%, or allow for a rise of at least 2 percentage points over the first five years if that gives a higher rate. Mortgages fixed for five years or more are exempt from the 5.5% floor, so lenders often stress them at or near the pay rate, which can increase borrowing substantially.
| Borrower type | Typical ICR |
|---|---|
| Basic-rate taxpayer | 125% |
| Limited company | 125% |
| Higher or additional-rate taxpayer | 145% (some lenders 160%) |
| HMO or multi-unit | often higher, up to 175% |
Rent needed for a buy-to-let mortgage
| Loan | Rent at 125% and 5.5% | Rent at 145% and 5.5% | Rent at 125% and 6.9% |
|---|---|---|---|
| £150,000 | £859 | £997 | £1,078 |
| £200,000 | £1,146 | £1,329 | £1,438 |
| £250,000 | £1,432 | £1,661 | £1,797 |
| £300,000 | £1,719 | £1,994 | £2,156 |
Monthly rent needed = loan × ICR × stress rate ÷ 12.
Other buy-to-let costs to plan for
- A deposit of at least 25% for most mainstream deals, more for the lowest rates.
- Stamp Duty with the higher rates for additional properties, plus legal and valuation fees.
- Arrangement fees, often 1% to 3% of the loan, especially on low-rate deals.
- Income tax on rent: individual landlords get only a 20% tax credit on mortgage interest. Check returns with our rental yield calculator.
Frequently asked questions
How much can I borrow for a buy-to-let?
Usually the lower of the rent test and the LTV limit. On £1,200 a month rent at 125% ICR and a 6.9% stress rate, about £166,957.
What is the 125% rule for buy-to-let?
The rent must be at least 125% of the monthly mortgage interest calculated at the lender’s stress rate. Higher-rate taxpayers are often tested at 145%.
What stress rate do buy-to-let lenders use?
At least 5.5% for products fixed under five years, or the pay rate plus about 2 points if higher. Lenders commonly use 5.5% to 8%, with lower rates on 5-year fixes.
Is a 5-year fix better for buy-to-let borrowing?
Often yes for affordability, because fixes of five years or more can be stress-tested at the pay rate, increasing the maximum loan.
How much deposit do I need for a buy-to-let?
Usually at least 25% of the price, but if the rent does not support a 75% loan you will need more.
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.