Buy-to-Let Mortgage Calculator
Find the maximum buy-to-let mortgage based on monthly rental income and the lender's rental coverage ratio (ICR) stress test.
How Buy-to-Let Mortgage Calculator works
How BTL mortgage lending works
Unlike residential mortgages, buy-to-let mortgage amounts are primarily driven by rental income rather than the borrower's personal income. Lenders apply a rental coverage ratio (ICR): the annual rent must cover the stress-tested interest by a minimum factor.
Rental coverage ratio (ICR)
Most lenders require rent to cover 125% of the interest payment at a stressed rate (typically 5–6%). For higher-rate and additional-rate taxpayers, many lenders require 145% to reflect the reduced tax relief on mortgage interest. Formula: Max Loan = Annual Rent ÷ (Stress Rate × ICR).
Stress rate
Lenders use a notional stress rate (typically 5–5.5% in 2025) rather than the actual product rate to test affordability. This ensures the loan remains serviceable if rates rise. The stress rate varies by lender.
LTV limits for BTL
Most BTL lenders cap LTV at 75–80%. This typically means a minimum 25% deposit on a BTL purchase. Some specialist lenders offer 80–85% LTV for lower-risk properties and borrowers.
Frequently asked questions
How is a buy-to-let mortgage calculated?
BTL lenders calculate the maximum loan from rental income: Max Loan = Annual Rent ÷ (Stress Rate × Coverage Ratio). Example: £1,500/month rent, 5.5% stress, 145% ICR → £18,000 ÷ (0.055 × 1.45) = £225,564 max loan.
What is the rental coverage ratio?
The ICR (Interest Coverage Ratio) is the minimum rent-to-interest multiple required. 125% means rent must be 25% above the stressed interest payment. Higher-rate taxpayers usually need 145% because their Section 24 mortgage interest tax relief is limited to the basic rate.
Do I need income to get a BTL mortgage?
Most BTL lenders require a minimum personal income of £25,000 per year in addition to the rental income calculation. Some specialist lenders offer "portfolio landlord" products with different criteria for experienced investors.
What deposit do I need for a buy-to-let mortgage?
Typically 25% (75% LTV). Some lenders accept 20% deposits. Lower deposits are rare for BTL and come with higher rates and stricter criteria.
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