Buy-to-Let Mortgage Stress Test Calculator

BTL mortgage stress test calculator checks if rental income covers stressed interest at lender DSCR (typically 1.25-1.45x). lenders apply stress rate (often 5.5%+) regardless of actual product rate. 200k loan at 5.5% stress = 917/month interest. With 1.25 DSCR: required monthly rent = 1,146. Below this: mortgage rejected.

Example: 400k property, 100k deposit, 300k loan. Stress rate 5.5% = 1,375/month interest. 1.25 DSCR = 1,719 required monthly rent. 1.45 DSCR (limited company) = 1,994. Property rent 1,800: passes individual (1.25), fails limited company (1.45). Important to know which DSCR your lender uses.

BTL stress test rules: the relevant financial regulator (Prudential Regulation Authority) requires 1.25x at 5.5% stress (or 5-year fix if rate higher). Limited company SPVs typically need 1.45x. Higher LTV (75-80%): stricter requirements. New requirements for portfolio landlords (4+ properties): full portfolio stress test. Many BTL applications fail stress test - have alternative funding plan or accept lower LTV.

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 toward extreme values 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 investors run this

Most people's intuition for compounding is wrong — not because the math is hard, but because linear thinking doesn't account for curves. Running numbers through a calculator like this one is the cheapest way to recalibrate that intuition before making an irreversible decision about contribution rate, asset mix, or retirement age.

What this doesn't capture

Steady-rate math ignores real-world volatility. Actual returns are lumpy; sequence-of-returns risk matters most in drawdown; fees and taxes drag on compound growth; and behaviour changes in drawdowns can reduce outcomes below the projection. The number represents one scenario rather than a forecast.