Buy To Let Cash Flow Sensitivity Calculator
Stress-test your rental cash flow against higher mortgage rates, rent falls and void periods, and see your Interest Coverage Ratio under each scenario.
Property & Mortgage Inputs
Scenario Analysis
| Scenario | Monthly Cash Flow | ICR |
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Calculation Breakdown
Monthly interestEnter your figures to see each calculation step.
What Is Cash Flow Sensitivity Testing?
Cash flow sensitivity testing checks how a rental property's monthly surplus holds up if things go wrong at the same time: a higher mortgage rate, a lower rent, or extra time between tenants. It goes beyond a single "does it work today" number.
How Does The Interest Coverage Ratio Work?
ICR compares rental income to the mortgage interest payment. Lenders commonly look for 125% to 145% cover, often tested at a stress rate well above the actual pay rate, to confirm the property could withstand higher borrowing costs.
Who Should Use This?
Landlords buying a rental property, reviewing an existing portfolio or approaching the end of a fixed-rate deal should stress-test the cash flow before committing. If refinancing is the next step, take the sustainable rent and rate scenarios into the Buy To Let Remortgage Calculator to compare payment, LTV, coverage and equity-release headroom.
Common Mistakes
A common mistake is only checking cash flow at today's rate and rent, without allowing for a rate rise, a void period, or a rent reduction in a softer market. Another is ignoring the tax relief restriction on mortgage interest for higher-rate taxpayers.
What Happens Next?
If the worst-case scenario shows a healthy buffer, the numbers are relatively resilient. If it shows a shortfall, consider a larger deposit, a lower purchase price, fixing the rate for longer, or building a larger cash reserve.
Important Considerations
This tool approximates interest cost and does not fully model capital repayment, selling costs, capital gains tax, or a specific lender's underwriting criteria. Treat it as a planning aid, not a lending decision.
Frequently Asked Questions
What is a buy-to-let cash flow sensitivity test?
It stress-tests a rental property's monthly cash flow against realistic shocks, such as a higher mortgage rate, a rent fall, or extra void periods, so you can see how much buffer you actually have.
What is the Interest Coverage Ratio (ICR)?
ICR compares rental income to the mortgage interest payment. Lenders commonly look for an ICR of around 125% to 145%, often tested at a stress rate above the actual pay rate.
Why do lenders use a stress rate above my actual rate?
To check the property could still cover the mortgage if rates rise, rather than only just covering it at today's rate.
How is monthly mortgage interest calculated here?
As mortgage balance multiplied by the annual interest rate, divided by 12. This approximates the interest-only cost, which is the relevant cash flow risk for a rate shock.
Does this calculator include mortgage capital repayment?
No. It focuses on the interest cost. Repayment mortgages have a higher total monthly cost because of the capital portion, which this tool notes but does not fully amortise.
Why does my tax band matter?
Since April 2020, individual landlords cannot deduct mortgage interest from rental income before tax. They get a basic-rate tax credit instead, which raises the effective borrowing cost for higher-rate taxpayers.
What counts as a realistic void period?
Many landlords budget 2 to 4 weeks a year for void periods between tenancies, though this varies by area and property type.
What is the combined worst case scenario?
It applies a rate rise, a rent fall and extra void weeks together, to show the cash flow position if several risks happen at the same time.
Is a negative cash flow always a problem?
A short-term negative scenario may be manageable with a cash buffer, but a consistently negative base case suggests the numbers may not work at current rents and rates.
Does this replace a mortgage lender's affordability check?
No. It gives an illustrative stress test only. Lenders apply their own specific underwriting criteria and stress rates.
Should I include letting agent fees if I self-manage?
No, set the agent fee to 0% if you self-manage, but consider adding your own time cost separately when judging the investment.
What maintenance percentage should I use?
Many landlords budget 10% to 15% of rent for maintenance and repairs, though older properties may need a higher allowance.
Can I use this for a portfolio of properties?
This tool models one property at a time. For a portfolio, run each property separately and add the results together.
Is this financial or tax advice?
No. This is an illustrative planning tool only, not financial, tax or lending advice.
Where can I check the official rules?
GOV.UK publishes guidance on tax relief changes for landlords, and the Bank of England publishes buy-to-let underwriting standards.
Sources
- GOV.UK: changes to tax relief for residential landlords
- Bank of England: buy-to-let underwriting standards
- NRLA: buy-to-let mortgages
Last updated: 2026-07-22. This page gives an estimate only and is not legal, tax, financial, or employment advice.