Buy-to-Let Mortgage Stress Test Calculator

A buy-to-let stress test checks whether rental income would still cover the mortgage interest if the lender assesses affordability at a higher stress rate than the actual product rate. Lenders typically require the rent to cover that stressed interest by a set multiple, the debt service coverage ratio (DSCR), commonly in the 1.25 to 1.45 range. For example, a 200,000 loan stressed at 5.5% works out to about 917 a month in interest; at a 1.25 DSCR the required monthly rent is about 1,146. Rent below that level would typically fall short of the lender's test.

Worked example: a 400,000 property with a 100,000 deposit gives a 300,000 loan. Stressed at 5.5%, that is about 1,375 a month in interest. At a 1.25 DSCR the required rent is about 1,719; at 1.45 (often applied to company structures) it is about 1,994. A property renting at 1,800 would pass the 1.25 test and fall short of the 1.45 one. DSCR requirements vary by lender and structure.

How stress tests are applied varies by lender and country. A common pattern is a minimum stress rate (often around 5.5%, or the product rate plus a margin) combined with a DSCR requirement; higher loan-to-value lending tends to face stricter requirements. Some lenders apply a fuller review to borrowers who already hold several mortgaged properties. Many applications fall short of the stress test; common responses include a larger deposit, a lower loan-to-value product, or a different lender.

Quick example

With property price of 400,000 and deposit of 100,000 (plus actual mortgage rate of 5% and stress rate of 5.5%), the result is PASSES. Change any figure and watch the output shift — it's often more useful to see the pattern than to memorise the formula.

Which inputs matter most

You enter Property Price, Deposit, Actual Mortgage Rate %, Stress Rate %, and Expected Monthly Rent. Not every input has equal weight. Adjusting one input at a time shows which ones move the result most.

What's happening under the hood

Stress monthly interest = loan × stress rate / 12. Required rent = stress interest × DSCR. The formula is listed in full below. If the number looks off, you can retrace the calculation by hand — that's the point of showing the working.

Why run this

Running the numbers makes the trade-offs concrete. Small changes in the inputs can move the result more than intuition suggests, which is hard to judge without working it out.

What this doesn't capture

This is a simplified model that holds its assumptions constant. Real outcomes vary with market conditions, costs, taxes, and timing, so the figure is best read as one scenario rather than a forecast.