Buy to Let Mortgage Calculator — How Much Can You Borrow?

A buy to let mortgage calculator works out how much you can borrow to purchase a property you'll rent out, rather than live in. The rules, rates and affordability tests all differ from a residential mortgage.

Instead of using your salary, lenders assess a buy to let on the expected rental income. They apply an Interest Coverage Ratio (ICR) stress test — under the PRA's SS13/16 framework — to check the rent comfortably covers the mortgage, even if rates rise.

This buy to let mortgage calculator applies that test for you, alongside the typical 75% loan-to-value cap, and shows your rental yield and deposit so you can size up a deal before you view it. Don't forget the extra costs when budgeting: a 5% stamp duty surcharge applies to most buy to let purchases in England and Northern Ireland on top of standard SDLT, and if the property is a rental home, the Renters' Rights Act (in force since 2025-26) changes some of your obligations as a landlord too.

The Interest Coverage Ratio (ICR) is the margin by which your rent must cover the mortgage interest at a stressed rate. It's the single most important buy to let affordability test.

The thresholds are 125% for basic-rate taxpayers and limited companies, 145% for higher-rate taxpayers, and up to 165% for additional-rate taxpayers with some lenders — occasionally up to 175% for HMOs and multi-unit blocks (MUFBs), which lenders treat as higher-risk lets. Lenders stress at around 5.5% for 2-year fixed products, or your pay rate plus 2%, whichever is higher. Choose a 5-year fixed product instead and many lenders will stress at a softer rate — often just the pay rate, or pay rate plus 1% — since the PRA's SS13/16 rules allow relaxed stress testing on longer fixes. That difference alone can unlock significantly more borrowing.

The maths on an interest-only basis is: maximum loan = annual rent ÷ (ICR × stress rate). If the rent falls short, you either need a bigger deposit, a higher-yielding property, or — as covered below — a lender who offers top-slicing. Check your personal affordability too with the mortgage affordability calculator.

If your rental income doesn't quite clear the ICR threshold, that isn't necessarily the end of the deal. Some lenders — including Paragon, Precise and Aldermore — offer top-slicing, which lets your surplus personal income "top up" the rental coverage to help the application pass.

This is particularly useful for higher-rate taxpayers facing the stricter 145% ICR, or for lower-yielding properties in expensive areas where the rent-to-price ratio is tight. Not every lender offers it, and those that do will still want to see the rental income doing most of the work — but it's a genuine option worth raising with a specialist buy to let broker if your numbers are close but not quite there.

Gross rental yield is your annual rent divided by the property price, shown as a percentage. On a £220,000 home let for £1,200 a month, that's £14,400 ÷ £220,000 = about 6.5%.

In 2026, gross yields are typically 4–6% in London and the South East, 6–10% in the Midlands and North, and 10%+ for specialist lets like HMOs. Net yield — after management, insurance, voids and maintenance — is usually 2–3% lower, so always budget for costs.

Most buy to let lenders want a minimum 25% deposit (75% LTV). A 40% deposit (60% LTV) unlocks the sharpest rates and makes the stress test easier to pass because you're borrowing less.

Some specialist lenders accept 15–20% down, but at a rate premium. As you build equity, a house equity calculator helps you see how much you could release for the next deposit, and a remortgage calculator shows the cost of refinancing.

Most buy to let mortgages are interest-only — you pay just the interest each month, which maximises cashflow. The full loan is repaid at the end of the term, usually by selling the property or from savings.

Repayment mortgages clear the balance over the term but have higher monthly payments, cutting cashflow. Many landlords choose interest-only for cashflow and rely on capital growth. Compare the true monthly cost with our residential mortgage calculator.

Say you're a higher-rate taxpayer buying a £220,000 property that rents for £1,200 a month, at a 5.5% stress rate and 145% ICR.

The ICR test allows a maximum loan of about £180,000 (£14,400 ÷ (1.45 × 0.055)). But the 75% LTV cap limits you to £165,000, so that's your maximum — needing a £55,000 deposit. The gross yield is about 6.5%, and the interest-only payment at a 5.2% pay rate is roughly £715 a month. Remember to budget separately for the 5% stamp duty surcharge on top of this.

Switching to a limited company (125% ICR) would raise the ICR-based limit to around £209,000, though the 75% LTV cap would still apply. If the rent had fallen just short of passing, a lender offering top-slicing might still approve the deal using your surplus personal income. Try your own numbers in the buy to let mortgage calculator above.

Official sources & further reading: check the stamp duty surcharge on GOV.UK, read landlord guidance at MoneyHelper, and track the Bank of England base rate. Browse every tool on the mortgage calculators homepage.