Buy-to-Let Mortgage Calculator
Monthly cost, total interest, and the Interest Cover Ratio lenders use to underwrite — at both pay rate and stressed rate.
How the lender's affordability test works
UK buy-to-let lenders don't underwrite a BTL like a residential mortgage. They size the loan based on whether the rent will cover the interest payments at a stressed pay rate, plus a margin called the Interest Cover Ratio.
The standard formula is: (Monthly rent ÷ Stressed monthly interest) × 100. Most basic-rate or limited-company landlords need 125%. Higher and additional-rate individuals typically need 145%. Stress rates are usually pay rate plus 2%, with a floor around 5.5%, although 5-year fixes often soften this.
If your stressed ICR falls below your lender's target, you'll need a bigger deposit, a stronger rent (often via HMO or holiday let), or a different lender — common BTL specialists like TMW, Paragon and Kent Reliance have different thresholds and top-slicing allowances.
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