Buy-to-Let Mortgage Calculator UK: Work Out Your Monthly Payments

Knowing your monthly mortgage payment before you commit to a buy-to-let purchase is not optional — it is the foundation of every sensible investment decision. A difference of half a percentage point on a £150,000 mortgage changes your monthly cost by £40 to £60, which over a 25-year term amounts to £12,000 to £18,000 in additional interest. Getting the numbers right at the outset determines whether a property generates positive cash flow or quietly drains your savings.

Our free buy-to-let mortgage calculator lets you model monthly payments for any UK investment property in seconds. Enter your property price, deposit, interest rate, and mortgage term, then toggle between interest-only and capital repayment to see exactly how each structure affects your cash flow, total interest paid, and equity position over time.

This guide walks through everything you need to understand behind those numbers: how the calculator works, what inputs to use, a fully worked example comparing repayment types, the rental yield you need to satisfy lender stress tests, common mistakes that catch new investors out, and the steps to take once you have your figures. Whether you are buying your first investment property or refinancing an existing portfolio, this is the reference you need.

How a Buy-to-Let Mortgage Calculator Works

A buy-to-let mortgage calculator is a straightforward financial tool that applies standard mortgage mathematics to your specific numbers. At its core, it takes the amount you are borrowing, the interest rate the lender charges, and the length of time over which you will repay, then calculates what you owe each month. The calculation differs depending on whether you choose an interest-only or a capital repayment mortgage — and that distinction has a profound effect on both your monthly outgoings and your long-term financial position.

For an interest-only mortgage, the calculation is simple: multiply the loan amount by the annual interest rate and divide by twelve. You pay only the interest each month, and the original loan balance remains unchanged throughout the term. For a capital repayment mortgage, the calculator uses a standard amortisation formula that ensures each monthly payment covers both interest and a portion of the principal, so the loan is fully repaid by the end of the term.

The repayment type is the single most consequential choice you make. Interest-only keeps your monthly costs low and maximises cash flow — ideal if you plan to sell the property or refinance at the end of the term. Capital repayment costs more each month but steadily builds equity, so you own the property outright when the mortgage ends. Most buy-to-let investors choose interest-only for cash flow reasons, but you should model both options in the calculator to understand the trade-off for your specific situation.

Key Inputs Explained

Each input you enter into a buy-to-let mortgage calculator has a direct impact on the result. Getting them right means your projections will closely match reality. Getting them wrong — even slightly — can lead to a nasty surprise when your mortgage offer arrives.

Worked Example: £200k Property at 75% LTV

Let us walk through a concrete example that illustrates exactly how a buy-to-let mortgage calculator produces its figures. We will use a £200,000 property — a realistic price point for a two-bedroom flat in many UK cities outside London — with a 25% deposit of £50,000, giving a mortgage of £150,000 at an interest rate of 5.5% over a 25-year term.

Interest-Only Calculation

The interest-only calculation is straightforward: £150,000 multiplied by 5.5% gives £8,250 per year in interest, divided by 12 months equals £687.50 per month. This figure stays constant for as long as the interest rate does not change. At the end of the 25-year term, you still owe the full £150,000 and must either sell the property, refinance, or repay from other funds.

Capital Repayment Calculation

The repayment calculation uses the standard amortisation formula. With the same £150,000 mortgage at 5.5% over 25 years, the monthly payment is approximately £920 per month. In the early years, most of each payment goes towards interest — in month one, roughly £688 is interest and only £232 reduces the loan balance. By year 15, the split is roughly even. By the final years, almost the entire payment goes towards principal.

This is exactly the kind of comparison our buy-to-let mortgage calculator produces instantly. Change any input — try a 4.8% rate, a 30-year term, or an 80% LTV — and the results update immediately, letting you model dozens of scenarios in minutes.

What Rental Yield Do You Need?

Calculating your mortgage payment is only half the equation. The other half is whether the rental income covers that payment — and by enough margin to satisfy your lender. This is where the Interest Coverage Ratio (ICR) comes in. Most buy-to-let lenders require the monthly rent to be at least 125% to 145% of the monthly mortgage interest payment, calculated at a stress-test rate of 5.5% (even if your actual rate is lower).

The ICR requirement exists to protect both you and the lender against rate increases and void periods. A 125% ICR means the rent must be 25% higher than the interest payment; 145% is typical for higher-rate taxpayers because Section 24 restricts how much mortgage interest they can offset against rental income.

These figures assume 75% LTV and a 5.5% stress-test rate, which is the standard used by most mainstream lenders in 2026. If your actual mortgage rate is lower than 5.5%, your real monthly payment will be lower — but the lender still uses 5.5% to assess whether you qualify. This means a property can generate positive cash flow in practice but still fail the ICR test on paper.

Use the rental yield figure alongside the mortgage calculator to assess viability. If you know the achievable rent for a property, divide it by the minimum rent from the table above. If the result is 1.0 or higher, the property should pass the ICR test. If it is below 1.0, you will either need a larger deposit (to reduce the mortgage amount) or a property with higher rental yield.

How to Use Our Free Calculator

Our buy-to-let mortgage calculator is designed to give you accurate monthly payment figures in under 30 seconds. Here is how to get the most out of it:

1. Enter the property price. Use the asking price, your intended offer, or the current market value if you are refinancing. The calculator accepts any value from £10,000 upwards.
2. Set your deposit amount or LTV. For a first buy-to-let purchase, 25% deposit (75% LTV) is the standard starting point. If you have more equity available, try 60% or 65% LTV to see how it reduces your rate and monthly payment.
3. Enter the interest rate. Use your quoted rate if you have a mortgage agreement in principle. Otherwise, use 5.5% as a reasonable mid-range estimate for 2026 BTL rates. For stress testing, try 6.5% or 7% to see your worst-case payment.
4. Choose the mortgage term. 25 years is standard. Try 30 years on a repayment mortgage to see how a longer term reduces monthly payments (at the cost of more total interest).
5. Toggle between interest-only and repayment. The calculator shows results for both modes. Compare them side by side to understand the monthly cost difference and the long-term implications for equity and total interest paid.
6. Review your results. The calculator displays your monthly payment, total interest over the term, and total cost. Use these figures to assess whether the property's rental income provides sufficient coverage.

The calculator handles both interest-only and repayment mortgages using standard UK mortgage mathematics. Results are indicative — your actual mortgage offer may differ based on lender-specific criteria, fees, and product features — but they provide an accurate baseline for investment analysis and comparison.

Common Mistakes When Calculating BTL Costs

A mortgage calculator gives you the borrowing cost, but it does not capture the full picture of owning a buy-to-let property. The most common mistake new investors make is treating the mortgage payment as their only cost, then being surprised when the actual return is far lower than they projected. Here are the costs and factors that catch landlords out:

1. Forgetting the stamp duty surcharge. Since April 2025, the additional stamp duty rate on second properties in England and Northern Ireland is 5% (up from 3%). On a £200,000 purchase, that is £11,500 in stamp duty alone — a significant upfront cost that directly affects your return on invested capital.
2. Ignoring void periods. No property is occupied 365 days a year, every year. Budget for at least one month of void per year (8.3% vacancy rate) when calculating your expected rental income. In areas with oversupply, two months is more realistic.
3. Not stress-testing interest rates. Your mortgage might be fixed at 5% today, but what happens when the fix ends in two or five years? Model your cash flow at 6%, 6.5%, and 7% to understand your exposure if rates rise before you refinance.
4. Overlooking letting agent and management fees. If you use a letting agent, budget 8% to 12% of rent for management fees, plus a tenant-find fee of 50% to 100% of one month's rent for each new tenancy. These fees significantly erode your net yield.
5. Forgetting landlord insurance. Buildings insurance, landlord liability insurance, and rent guarantee insurance typically cost £200 to £600 per year depending on the property. This is a non-negotiable expense that many calculators do not include.
6. Ignoring the Section 24 impact. Since April 2020, mortgage interest is no longer deductible from rental income for individual landlords. Instead, you receive a 20% tax credit. For higher-rate taxpayers, this effectively increases your tax bill by 20% of your mortgage interest. On a £150,000 mortgage at 5.5%, that is £1,650 per year in additional tax. Read our detailed guide on Section 24 mortgage interest restrictions.
7. Not accounting for maintenance and repairs. Budget 10% to 15% of annual rent for ongoing maintenance, repairs, and periodic refurbishment. A boiler replacement (£2,500-£4,000), new bathroom (£3,000-£6,000), or rewiring (£3,000-£5,000) can eliminate an entire year's profit if you have not budgeted for it.

The stamp duty surcharge deserves particular attention because it has a compounding effect on your investment return. On a £200,000 property with a £50,000 deposit, the £11,500 stamp duty increases your total cash outlay to £61,500 — a 23% increase in the capital you need upfront. Many investors calculate their yield based on the deposit alone, which overstates the return. Always include stamp duty, legal fees (£1,000-£2,000), survey costs (£400-£800), and mortgage arrangement fees (£999-£1,999) in your total investment calculation.

Section 24 is the other major factor that calculator results alone do not capture. If you are a higher-rate taxpayer, the restriction means you pay income tax on your full rental income (including the portion that goes to mortgage interest), then receive a basic-rate tax credit. The net effect is that your effective tax rate on rental income is higher than it would be with full mortgage interest relief. For a detailed breakdown of how this works and strategies to mitigate it, see our Section 24 guide.

Next Steps After Calculating Your Mortgage

Once you have your monthly payment figure from the calculator, that number becomes the starting point for a series of decisions. Here is the path most successful BTL investors follow:

The gap between projected and actual returns is where most buy-to-let disappointments occur. The investors who succeed are those who model conservatively, account for every cost, and track their real figures against their projections. Start with the mortgage calculator to get your baseline, then build your full investment case around it.

Frequently Asked Questions