How this buy to let mortgage calculator helps
A buy to let mortgage is different from a residential loan. Lenders care not just about your income, but also whether the rent can comfortably cover the mortgage under a stress test. This calculator helps you quickly estimate the key numbers investors usually check before making an offer.
In one place, you can estimate your loan size, monthly payment, rental yield, loan-to-value (LTV), and interest coverage ratio (ICR). You can also see whether a deal appears to pass a typical lender affordability rule.
What the calculator is estimating
1) Loan amount and LTV
Your loan amount is simply property value minus deposit. LTV is loan divided by property value. Lower LTV often means better rates and less risk, but requires more cash up front.
2) Monthly mortgage payment
You can switch between interest-only and repayment:
- Interest-only: lower monthly payments, but you still owe the full loan principal at the end of the term.
- Repayment: higher monthly payments, but the loan balance reduces over time and is fully repaid by term end (assuming fixed rate and no changes).
3) Rental yield and monthly cash flow
Gross yield gives a quick property-level view (annual rent ÷ property value). Cash flow is more practical because it subtracts financing and running costs. If cash flow is negative, you may be subsidizing the property each month.
4) ICR stress test
Many buy to let lenders use a stress test where rent must exceed a multiple of stressed interest payments. A common benchmark is 125% or 145% ICR, depending on borrower profile and tax status. This calculator shows:
- Required rent under your selected stress rate and ICR
- Your actual ICR based on expected rent
- An estimated maximum loan supported by rent
How to use the outputs in real decisions
Start with realistic rent, not best-case rent. If you are uncertain, use the lower end of local comparables. Next, increase your monthly non-mortgage costs to include management, maintenance, and a buffer for void periods. If the deal still works with conservative assumptions, it is usually more resilient.
You can then test scenarios:
- What happens if rates rise by 1%?
- What if rent is 5% lower than expected?
- Would a larger deposit unlock a better rate and stronger cash flow?
Worked example (using the default values)
Suppose the property is £250,000 with a £62,500 deposit. The loan is £187,500 (75% LTV). At 5.5% interest-only, your monthly interest cost is much lower than repayment, which can improve short-term cash flow. But remember, with interest-only, the full principal still needs a repayment strategy later.
If monthly rent is £1,250, the calculator also checks whether that rent clears your chosen stress rule (for example, 125% at 5.5% stress). If it fails, you might need a larger deposit, a lower purchase price, stronger rent, or a different lender criteria set.
Important limitations
- This tool does not replace lender underwriting or broker advice.
- Tax treatment varies by ownership structure and personal circumstances.
- Rates, fees, valuation outcomes, and lender policy can change quickly.
- Real returns depend on maintenance, tenancy quality, voids, and exit costs.
Practical checklist before you buy
- Confirm realistic rent with at least 2 to 3 local letting agents.
- Check lender ICR rules for your exact borrower type.
- Model a higher interest rate stress case.
- Keep a cash reserve for repairs and vacancies.
- Review legal, licensing, and compliance obligations for your area.
Use this buy to let mortgage calculator as a first-pass filter. It is ideal for quickly comparing opportunities and avoiding deals that look good only under optimistic assumptions.