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Search this question and you get calculators. Useful if you already know your numbers, less useful if you are standing in a flat wondering whether it will work.
So here is the arithmetic, and some tables you can read straight off. As a rough guide, a lender wants monthly rent of somewhere between about £570 and £960 for every £100,000 you borrow. That is a wide range on purpose: the bottom is a basic rate taxpayer or a company borrowing on a five year fix tested at the regulatory floor, and the top is an additional rate taxpayer on a product stressed hard. Everything below explains where the range comes from and where in it you are likely to sit.
Rent needed is the loan, times the stress rate, divided by twelve, times the coverage ratio.
Expect to need roughly £570 to £960 of monthly rent per £100,000 borrowed.
The regulatory floor assumes a minimum borrower rate of 5.5%.
125% coverage is common for basic rate and company borrowing, 145% for higher rate.
Lenders use the lower of your actual rent and the valuer assessment.
The calculation lenders actually use
Three numbers decide it.
The loan amount, which is what you are borrowing, not what the property costs. The stress rate, which is a notional interest rate the lender tests you against rather than the rate you will pay. And the coverage ratio, which is the margin the rent has to clear above that stressed interest.
The sum is: monthly rent needed equals the loan, times the stress rate, divided by twelve, times the coverage ratio.
On the stress rate, the Prudential Regulation Authority sets a floor. Its supervisory statement on buy to let underwriting says firms should assume a minimum borrower interest rate of 5.5% (paragraph 2.14). Paragraph 2.13 separately says firms should have regard to a set of factors in deciding the rate to test against, including a minimum increase of two percentage points in buy to let interest rates. Whichever approach produces the higher figure is generally what you get tested on.
Paragraph 2.12 disapplies the requirement to consider likely rate movements over the next five years where the rate is fixed or capped for five years or more. Many lenders respond by stressing those products at or near the pay rate, which is why a five year fix often supports a bigger loan. That is how the market has chosen to apply the rule rather than something the rule states, and the 5.5% floor still sits underneath.
On the coverage ratio, 125% is common for basic rate taxpayers and for limited company borrowing, and 145% is common for higher rate taxpayers. Some lenders go above 145%, with 160% and 165% both appearing for additional rate taxpayers. On a joint application the higher tax band usually sets the ratio for both of you.
Rent needed at a 5.5% stress rate
Illustrative only. These are worked examples using the regulatory minimum assumed rate, not a quote, and no lender is obliged to use these figures.
| Loan | Stressed monthly interest | Rent needed at 125% | Rent needed at 145% |
|---|---|---|---|
| £100,000 | £458 | £573 | £665 |
| £150,000 | £688 | £859 | £997 |
| £200,000 | £917 | £1,146 | £1,329 |
| £250,000 | £1,146 | £1,432 | £1,661 |
| £300,000 | £1,375 | £1,719 | £1,994 |
Rent needed at a 7% stress rate
Also illustrative. Lenders often stress two year fixes and variable products well above the 5.5% floor, and 7% is a realistic figure to sanity check yourself against rather than a number any particular lender uses. If you are an additional rate taxpayer, add roughly another 10% to 15% on top of the 145% column, because some lenders apply 160% or 165%.
| Loan | Stressed monthly interest | Rent needed at 125% | Rent needed at 145% |
|---|---|---|---|
| £100,000 | £583 | £729 | £846 |
| £150,000 | £875 | £1,094 | £1,269 |
| £200,000 | £1,167 | £1,458 | £1,692 |
| £250,000 | £1,458 | £1,823 | £2,115 |
| £300,000 | £1,750 | £2,188 | £2,538 |
Figures are for illustration only, based on the assumptions and sources shown, and are not a quote or a guarantee of what you can borrow or what a lender will accept. Lender criteria and rates change often. Read the full important information.
Two things jump out of those tables.
The gap between the two stress rates is large. On a £250,000 loan at 145%, the difference between being tested at 5.5% and at 7% is over £450 a month of rent. That is why which product you choose changes what you can borrow, not just what you pay.
And the gap between 125% and 145% is roughly 16% more rent for the same loan. That single difference between a basic rate and a higher rate taxpayer is often what decides whether a case works.
Speak to an expert
If you want the real number for a specific property rather than a table, that is a five minute conversation. I am a whole of market broker and I handle buy to let cases in personal names and limited companies.
Working it backwards from the rent you can actually get
Most landlords have the opposite problem. You know roughly what the property will let for and you want to know what you can borrow against it.
Turn the sum around: maximum loan equals monthly rent, divided by the coverage ratio, times twelve, divided by the stress rate.
A property letting at £1,200 a month, at 145% coverage and a 7% stress rate, supports roughly £141,900. The same rent at 125% coverage and a 5.5% stress rate supports roughly £209,500. Same property, same rent, and nearly £68,000 difference in what a lender will advance, purely from the tax position and the product.
Those are illustrative figures on the assumptions given. The maximum mortgage calculator and the rent calculator will let you run your own numbers.
What to do when the rent falls short
This is the common situation, and in my experience the usual cause is not the mortgage at all. It is that the property has been valued optimistically as a rental. People work from the asking price and a hopeful rent figure rather than what comparable properties in that street actually let for.
So the first move is always to get the rent expectation right. A letting agent realistic figure, not the best case one.
If the rent is genuinely what it is and the numbers still do not reach, the levers are: borrow less, which reduces the interest and therefore the rent required; consider a five year fix, for the reason set out above; look at whether personal income can bridge a modest gap, which is top slicing; or, if you are remortgaging rather than buying, look at a product transfer with your existing lender, which usually avoids a fresh rental assessment.
Which of those fits depends on your tax position, your other borrowing and what you want the property to do. Working that out is what an advice appointment is for.
Why the answer differs between lenders
Every lender sets its own stress rates and its own coverage bands within the regulatory framework, and they revise them regularly. Two lenders can look at the same property and the same rent and reach maximum loans thousands of pounds apart. That is the main practical argument for looking across the market rather than going to one bank, and it is the reason a calculator can only ever give you a range.
If you want the real number for a specific property, I am a mortgage broker in Winchester and I look at buy to let cases across the whole market. More on the buy to let mortgage hub, and if your current product is ending, the buy to let remortgage page.
Source: Bank of England, SS13/16 underwriting standards for buy-to-let mortgage contracts. Paragraph references are to the version currently in force; a January 2026 update takes effect 1 January 2027.
And if your worry is the income side rather than the rent, my guide to minimum income for a buy to let covers what lenders genuinely require.
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Every figure in this article is illustrative and calculated on the stated assumptions. It is not a quote, not a lending decision and not advice on your circumstances. Lender stress rates and coverage ratios vary and change. The Financial Conduct Authority does not regulate most buy to let mortgages.
