Buy to Let Mortgage Calculator UK

A buy-to-let mortgage calculator works out how much a lender might advance against a rental property, based on the rent it earns rather than your personal income alone. Enter the property value, the rent, an interest rate and a deposit, and the calculator checks the deal against a lender's rental cover test to give you an estimate of the maximum loan.

This guide walks through the inputs a calculator needs, how the rental cover test actually works, and two worked examples so you can sanity-check any figure a calculator or broker gives you.

Try our free buy to let mortgage calculator for an interactive estimate of max loan, LTV and monthly payments.

What this calculator does

Unlike a residential mortgage, where affordability is mostly about your salary, a buy-to-let (BTL) mortgage is assessed on whether the rent covers the mortgage interest by a healthy margin. A calculator takes your inputs, applies a stressed interest rate, and tells you:

• The maximum loan the rent can support
• Whether your target loan-to-value (LTV) passes that test
• The gap (if any) between what you want to borrow and what the rent justifies

It is a screening tool, not a mortgage offer. Every lender has its own stress rate, its own minimum ICR and its own view of your income tax position, so two calculators can give different answers for the same property. Use the result to shortlist realistic deals before you speak to a broker or lender.

The reason BTL lending works this way comes down to risk. A residential lender is largely betting on your salary continuing; a BTL lender is betting on the property continuing to let at a sustainable rent even if interest rates rise. That is why the test looks at rent against a stressed rate rather than at your income against the actual monthly payment. It also explains why two landlords buying an identical property, one as a basic-rate taxpayer and one as a higher-rate taxpayer, can be offered different maximum loan amounts on the same rent.

Inputs you need (rent, rate, LTV, stress rate)

To get a useful answer, you need five figures:

1. Property value - the purchase price or current valuation.
2. Monthly rent - the achievable market rent, not an optimistic guess. Letting agents can usually give you a realistic figure for the area.
3. Deposit or target LTV - most BTL lenders want at least 25% deposit (75% LTV), though some will go to 80% or 85% for the right borrower and property.
4. Pay rate - the actual interest rate on the product you are considering.
5. Stress rate - the notional, higher rate the lender uses to test affordability. This is not the rate you will actually pay; it is a buffer against future rate rises.

If you are buying through a limited company (an SPV), the calculator also needs to know that, because company borrowing is usually tested against a lower ICR threshold than personal-name borrowing. See our guide on how much deposit you need for a buy-to-let mortgage for how deposit size interacts with rate and risk.

A sixth input matters even though it rarely appears on a simple calculator: product fees. BTL products often carry an arrangement fee of one to three per cent of the loan, sometimes added to the loan itself rather than paid upfront. A cheaper rate with a large fee can cost more over a two or five-year fix than a slightly higher rate with a low fee, so once a calculator gives you a shortlist of viable loan amounts, ask any broker or lender for the true cost over the full fixed term, not just the headline rate.

How ICR / rental cover works for BTL

The rental cover test is usually called the interest coverage ratio (ICR), sometimes "rental cover" or the "stress test". It compares the rent to the stressed monthly interest, not to the full mortgage payment.

The Prudential Regulation Authority's guidance to lenders (in force since 2017) sets the general shape of the test:

• 125% ICR for basic-rate taxpayers and most limited company borrowers.
• 145% ICR for higher and additional-rate taxpayers borrowing in their personal name, because Section 24 restricts personal landlords to a 20% tax credit on mortgage interest rather than full relief.
• A stress rate of around 5.5% is common for five-year fixed products, with some lenders using a higher rate for two-year fixes.

In plain terms, if the stressed monthly interest on a loan is £1,000, a basic-rate taxpayer needs at least £1,250 a month in rent to pass; a higher-rate taxpayer needs £1,450. Our guide to the interest coverage ratio for buy-to-let covers this in more depth, including how lenders decide which threshold applies to you.

Worked examples

Example 1: Personal name, basic-rate taxpayer

At £1,100 a month, the rent comfortably clears the £945 threshold, so this loan amount should pass the lender's test.

Example 2: Limited company (SPV), same property

Using the same figures but a 125% ICR (typical for company borrowing regardless of the director's personal tax rate) and a slightly higher pay rate often seen on SPV products, the calculation follows the same method: stress the loan at the lender's floor rate, then multiply by the ICR percentage. Because company ICR is usually 125% rather than 145%, an SPV structure can sometimes support a larger loan on the same rent, which is one reason many portfolio landlords buy through a company. It is not automatically the right structure for everyone; see our guide on rental yield to check the numbers still work once you account for corporation tax and the extra running costs of a company.

Working backwards from your rent

If you already know the rent and want to find the maximum loan, rearrange the formula: maximum stressed interest = rent / ICR, then maximum loan = (maximum stressed interest x 12) / stress rate. This is the calculation most online tools automate, but it is simple enough to do on paper if you want to check a broker's figure.

Example 3: When the test fails

Take the same £220,000 property and £1,100 monthly rent, but this time the buyer is a higher-rate taxpayer borrowing £180,000 in their personal name (a lower deposit, higher loan).

Here the required rent (£1,196) is higher than the actual rent (£1,100), so the loan fails the test at that amount. The buyer has three realistic options: increase the deposit to reduce the loan, find a property with higher achievable rent, or restructure the purchase through a limited company where the lower 125% ICR might bring the required rent down to a level the property clears. This is exactly the kind of shortfall a calculator is useful for catching before you are committed to a purchase.

Limitations and next steps in Property HQ

A calculator cannot see your full picture. It will not know about:

• Your other income, debts or existing mortgages, which some lenders "top-slice" against a shortfall in rental cover.
• Whether the property is a house in multiple occupation (HMO) or new-build flat, both of which attract different lending criteria.
• Valuation risk - a surveyor may value the property below the asking price, which changes your effective LTV.
• Product fees, which can be a flat amount or a percentage of the loan and materially affect the true cost of borrowing.
• Whether you already own several buy-to-let properties. Lenders assessing a "portfolio landlord" (four or more mortgaged properties) usually want a background portfolio review, showing your full portfolio still stacks up on rental cover, not just the new property in isolation.
• Early repayment charges on any existing fix you might be porting or replacing, which can eat into the saving from a lower rate.

Treat any calculator result as a starting point for a conversation with a broker, not a guarantee of what you will be offered. If you are managing several buy-to-let mortgages, Property HQ tracks fixed-rate end dates, stress-tests rental cover across your whole portfolio and flags when a remortgage is worth exploring, so you are not repeating this calculation manually every time a fix comes up for renewal.

Disclaimer

This guide is general information for UK landlords, not financial or mortgage advice. Lending criteria vary by lender and change over time - speak to a mortgage broker or check the lender's own affordability calculator before making a decision.