Buy-to-Let Mortgage Calculator UK

Buy-to-Let Mortgage Calculator, stress-test, compare and plan

Financing is the lever that makes a buy-to-let property investment work or not. The free August Buy-to-Let Mortgage Calculator helps you size the loan, understand the monthly payment on interest-only or repayment products, and check lender-style coverage tests before you apply. Model rates, terms and fees in minutes. Ready to try August?

Interest-only vs repayment: compare side by side

Most mortgage widgets show a monthly payment and stop there. Ours goes further. It lets you:

• Compare interest-only vs capital-and-interest on the same inputs.

• Include product/arrangement fees paid upfront or added to the loan and amortise them sensibly across the fixed period to see the true effective cost.

• Run ICR/DSCR stress tests similar to those many UK lenders use, so you know if the rent supports the borrowing.

Key buy-to-let mortgage concepts for UK landlords

• Loan-to-Value (LTV) - The loan as a percentage of the property value. Common bands are 60%, 70%, 75% and 80%. Lower LTV often means cheaper rates and easier coverage.

• Interest-only (IO) vs Repayment (C&I) - IO maximises monthly cash flow but you must have a repayment plan for the capital. C&I reduces the balance every month, building equity but raising the payment.

• Product/arrangement fee - Often a fixed amount or a percentage of the loan. Paying it upfront or adding it to the loan changes both cash in and the effective rate.

• Stress rate & ICR - Many lenders test rent against a stressed interest rate (for example, pay rate plus a margin) and require an Interest Coverage Ratio (ICR), typically between 125% and 145%, varying by borrower type and tax band.

• Early Repayment Charges (ERCs) - Apply during the incentive period. Useful to include in your downside planning if you may refinance or sell.

How to calculate ICR and pass the BTL stress test

Monthly repayment (C&I) - Calculated using the standard amortisation formula over the term.
Monthly interest-only payment - Loan × Rate ÷ 12.

ICR (interest cover):
ICR = Annual Rent ÷ (Annual Interest at the lender’s stress rate)
If ICR is below the lender’s threshold, you may need lower LTV, higher rent, a different product, or (where permitted) top-slicing.

The calculator shows both payment and coverage so you can tune LTV, rent assumptions and product choices with confidence. Also see how Section 24 tax affects your buy-to-let returns.

Basic rate vs higher rate ICR

The ICR threshold lenders apply depends on your tax position. Basic rate taxpayers and limited companies typically need to demonstrate 125% ICR at the stressed rate. Higher rate taxpayers face a 145% threshold because Section 24 means their after-tax profit from the same rent and mortgage combination is lower, lenders account for the reduced buffer by requiring a larger gross income margin.

This distinction matters significantly when modelling a deal. A property that passes the stress test for a basic-rate taxpayer at 75% LTV may fail for a higher-rate taxpayer at the same LTV. The calculator lets you enter your ICR target directly so you can model the correct threshold for your own tax position.

How much rent do you need to pass the BTL stress test?

Most lenders calculate minimum rent as: (Loan × Stress rate ÷ 12) × ICR threshold. For example, on a £187,500 loan at a 7% stress rate with 125% ICR: monthly stressed interest = £187,500 × 7% ÷ 12 = £1,093.75; minimum rent = £1,093.75 × 1.25 = £1,367/month. If your target rent is below that figure, you either need lower LTV, a longer fixed term (which some lenders stress at the pay rate rather than a margin above it), or a property with stronger rental evidence. The calculator shows this threshold automatically once you enter rent and toggle the ICR test.

Worked UK example

• Purchase price £250,000; value also £250,000

• LTV 75% ⇒ Loan £187,500

• Product fee 2% (£3,750) added to the loan ⇒ £191,250 balance for IO example

• Interest rate 5.25% (fixed)

• Term 25 years

• Monthly rent £1,350

Interest-only payment: £191,250 × 5.25% ÷ 12 = £836.72/month
If on C&I: payment ≈ £1,133/month at 5.25% over 25y (illustrative).

ICR check (stress rate 7.00%):
Annual stressed interest = £191,250 × 7.00% = £13,387.50
ICR = £1,350 × 12 ÷ £13,387.50 = 121%
If your target is 125%–145%, options include reducing LTV, increasing rent with evidence, switching product, or adding personal income where a lender allows top-slicing.

In the calculator, switch the stress rate and LTV to see how fast ICR improves.

How mortgage fees affect your true monthly cost

Fees matter. A lower rate with a high fee can be more expensive over a short fixed period. The calculator lets you:

• Pay the fee upfront (this means lower monthly interest, but higher cash in).

• Add the fee to the loan (this means higher monthly interest, but less cash in).

• Amortise the fee across the fixed term to view a like-for-like effective cost per month, which makes product comparisons fair.

Inside August, tag the fee to the property so your ROI and cash-on-cash reflect the real cost of capital.

Portfolio landlord mortgage: what lenders check

If you hold four or more mortgaged properties, lenders may underwrite you as a portfolio landlord. Expect extra attention on your global ICR, experience, and document hygiene. August helps by keeping rent tracking via bank feeds, document storage, compliance checklist, smart reminders, maintenance requests, property insights and a AI property assistant all with your tenants on the app too.

From mortgage model to live portfolio management

• Plan vs Actual - Export your mortgage plan to August; we track live payments from bank feeds.

• Compliance guardrails - Gas, EICR, CO alarms, licensing and landlord insurance reminders reduce avoidable lender issues at renewal.

• Evidence pack - Store ASTs, inventories, rent schedules and service-charge notices. Create clean lender or broker packs at renewal without digging through email.

Underwriting assumptions: how to stress-test conservatively

Use conservative rent (evidence-based), include a void allowance, budget for service charge/ground rent on leaseholds, and remember Section 24 rules if you are an individual (interest relief given as a basic-rate credit). Companies may be taxed differently. Speak to a qualified adviser for your position.

How to use the August Buy-to-Let Mortgage Calculator

1. Enter value/purchase price and choose your LTV or loan amount.

2. Select product type (interest-only or repayment), rate, term and product fee (pay now or add to loan).

3. Add monthly rent and toggle stress rate & ICR to check coverage.

4. Compare products like-for-like using the effective monthly cost view.

The rate and term you choose have a significant effect on your monthly payments and your overall return. Our guide to how buy-to-let mortgages work in 2026 explains interest coverage ratio stress tests, the difference between interest-only and repayment products, and what lenders look for. To understand how your mortgage interest cost sits within your wider tax position, read our article on how rental income is taxed in the UK.

Disclaimer

This calculator provides estimates for information purposes only and does not constitute financial advice. Actual buy to let mortgage offers, rates, and repayments may vary. Always seek advice from a qualified mortgage adviser before making financial decisions.