Buy to Let Mortgage Calculator UK — Yield, Cash Flow & ICR Stress Test

Buy to Let Mortgage Calculator: A Complete UK Guide

Whether you are buying your first rental flat in Manchester or adding a fourth property to your portfolio in Bristol, a buy to let mortgage calculator is the fastest way to sense-check a deal before you speak to a broker. This free BTL mortgage calculator models repayment mortgage payments, gross rental yield, monthly cash flow after the mortgage, and the interest coverage ratio (ICR) stress test that UK lenders apply — giving you a realistic picture in seconds rather than guesswork on a spreadsheet.

What Does a Buy to Let Calculator Show?

A proper buy to let mortgage calculator UK tool goes beyond a simple payment estimate. It connects the property price, your deposit, the mortgage rate and term, and the expected monthly rent to show whether the investment stacks up financially. The gross rental yield tells you the headline return before costs. Monthly cash flow shows what is left in your pocket after the mortgage payment — the figure that determines whether you can sustain the property during void periods or when maintenance bills arrive. The ICR stress test mirrors how lenders assess affordability: they stress your loan at a higher notional rate (typically your pay rate plus 2%, or 5.5% minimum) and check that rent covers the stressed interest by at least 125%.

Understanding Buy to Let Mortgage Repayments

Most landlords choose between interest-only and repayment buy-to-let mortgages. This calculator models a repayment mortgage, where each monthly payment includes both interest and capital. Repayment deals build equity over time and are often required for higher loan-to-value ratios or certain lender products. Interest-only mortgages have lower monthly payments — only the interest is paid each month — but the full loan must be repaid at the end of the term, typically by selling the property or remortgaging. Use the mortgage payment figure from this calculator as your baseline cost when modelling cash flow.

Gross Rental Yield Explained

Gross rental yield is calculated as annual rent divided by property purchase price, expressed as a percentage. A £250,000 property let at £1,200 per month generates £14,400 in annual rent, producing a gross yield of 5.76%. In 2026, many UK investors target gross yields of 5% to 7% depending on region — higher in the North and Midlands, lower in London and the South East where capital growth has historically compensated for thinner yields. Gross yield is useful for comparing properties quickly, but it ignores mortgage costs, void periods, management fees, insurance, maintenance, and tax.

Monthly Cash Flow: The Number That Matters

Positive monthly cash flow means rental income exceeds the mortgage payment. Negative cash flow does not automatically make a deal bad — many landlords accept a short-term deficit for capital growth — but you must be able to fund the shortfall from other income. When using this buy to let calculator, also budget for letting agent fees (typically 8–12% of rent), landlord insurance, annual safety certificates, maintenance (allow 1% of property value per year), and void periods (four to six weeks per year is prudent). Section 24 tax rules mean individual landlords only receive a 20% tax credit on mortgage interest, which can significantly reduce net returns for higher-rate taxpayers.

The 125% ICR Stress Test

UK buy-to-let lenders do not assess affordability on your salary in the same way as residential mortgages. Instead, they apply an interest coverage ratio stress test. The lender calculates a stressed monthly interest payment using a notional rate — usually 5.5% or your product rate plus 2%, whichever is higher — and divides your monthly rent by that figure. The result must meet or exceed 125% for basic-rate taxpayers and limited company borrowers, or 145% for higher-rate taxpayers borrowing in their personal name. If your ICR falls short, you may need a larger deposit, a property with higher rent, or a lender with more flexible criteria.

UK Stamp Duty on Buy-to-Let Properties

When purchasing an additional property in England or Northern Ireland, you pay the standard Stamp Duty Land Tax (SDLT) rates plus a 3% surcharge on each band. For a £250,000 buy-to-let, the surcharge alone adds thousands to your upfront costs. Scotland charges the Additional Dwelling Supplement (ADS) on second homes, and Wales applies the higher residential rates under Land Transaction Tax (LTT). These costs are not included in this calculator but must be factored into your total cash investment. First-time buyers purchasing a buy-to-let do not receive first-time buyer relief — the property is treated as an additional dwelling. Always obtain a stamp duty quote before committing to a purchase, as the surcharge can materially affect your return on investment.

How to Use This BTL Mortgage Calculator

Enter the property purchase price, adjust the deposit slider to match your planned loan-to-value, set the mortgage interest rate and term, and input your expected monthly rent. Results update instantly. Compare scenarios by changing the deposit percentage or rent to see how yield, cash flow, and ICR respond. If you are comparing multiple properties, run each through the calculator with local rent estimates from comparable listings on Rightmove or Zoopla.

Limitations and Next Steps

This buy to let mortgage repayment calculator provides indicative figures only. Actual lender criteria, product rates, and stress test parameters vary between institutions. It does not account for tax, running costs, or stamp duty. For a borrowing limit based purely on rental income, use our buy-to-let eligibility affordability calculator. When you are ready to proceed, speak to a specialist buy-to-let mortgage broker who can source products matched to your tax status, portfolio size, and property type.

Tips before you apply