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 a property price of 400,000 and a 100,000 deposit, stressed at 5.5% against a 1.25 coverage requirement, the result is PASSES. The actual mortgage rate is recorded for comparison and does not enter the test: lenders assess the rent against the stressed rate, not the rate actually being paid.

Which inputs matter most

Four inputs drive the result. Property Price and Deposit set the loan, and the loan scales the required rent directly, so a larger deposit lowers the bar in proportion. Stress Rate % and Lender DSCR Multiplier scale it too: at the defaults, moving the multiplier from 1.25 to 1.45 lifts the required rent from 1,719 to 1,994, which turns a passing 1,800 into a shortfall. Expected Monthly Rent is the figure being tested against that bar. Actual Mortgage Rate % does not enter the calculation at all, since the point of a stress test is to set aside the rate currently on offer.

What's happening under the hood

The loan is the property price minus the deposit. The monthly stressed interest is that loan multiplied by the stress rate and divided by 12, which treats the mortgage as interest-only, the usual basis for this kind of test. The required rent is the stressed interest multiplied by the coverage ratio, and the test passes when the expected rent reaches it. At the defaults: a 300,000 loan, 1,375 of monthly stressed interest, 1,719 required against 1,800 expected, so it passes with 81 a month to spare.