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Rental Cashflow Calculator for Landlords
Understand and improve your rental cashflow
Healthy cash flow is what keeps a buy-to-let portfolio resilient. The August Rental Cashflow Calculator helps you move beyond headline yield and into the practical, month-by-month reality. Rent in and mortgages, voids and costs out. Use it to test scenarios before you buy.
What cashflow actually measures
Rental cashflow is the money left over after you pay all ongoing costs. It is usually calculated monthly for clarity, then annualised for comparisons. Unlike yield, cashflow includes finance costs and timing. It is the single measure most landlords use to judge day-to-day viability.
At its simplest:
Monthly cashflow = Gross rent − (Voids allowance + Management/letting + Maintenance + Insurance + Service charge/ground rent + Utilities you cover + Compliance + Mortgage interest/principal + Other costs)
In August, you can save these assumptions once, then map them to live bank transactions so the planned cashflow line sits beside the actual.
How to calculate cashflow with a UK example
Purchase price £250,000
Deposit 25% (£62,500) + buying costs £7,500 ⇒ cash in £70,000
Mortgage interest rate 5.25% (interest-only) on £187,500 ⇒ £820.31/month
Market rent £1,350/month
Typical monthly allowances:
Voids 6% of rent ⇒ £81
Management 10% of collected rent ⇒ £135
Service charge/ground rent £120
Insurance £20
Maintenance/safety checks reserve £60
Calculation:
Operating costs (ex-finance) = £81 + £135 + £120 + £20 + £60 = £416
Net operating cash before finance = £1,350 − £416 = £934
Mortgage interest = £820.31
Monthly cashflow = £934 − £820.31 = £113.69 (≈ £1,364/year)
If the loan were capital-and-interest (~£1,100/month at the same rate and 25-year term), monthly cashflow would be negative (~£166). The calculator lets you flip between IO and C&I to see the true impact on cash.
August tags bank transactions automatically so your payment dashboard shows actual rent numbers.
Stress tests lenders and you care about
Most buy-to-let lenders use Interest Coverage Ratio (ICR) and minimum stress rates. It is sensible to check both.
ICR = Annual rent ÷ (Annual mortgage interest at stress rate)
Many lenders want 125%–145% coverage (higher for basic-rate vs higher-rate taxpayers).Debt Service Coverage Ratio (DSCR) extends the concept to all debt service (capital + interest) where applicable.
Use the calculator’s stress toggle to model, for example, rate +2% and confirm whether the rent still covers finance and core costs comfortably. In August, pin your chosen stress test to the property record so renewal decisions are grounded in numbers, not guesswork.
Inputs that make or break cashflow
Cashflow is sensitive to a handful of assumptions:
Voids & arrears: Conservative landlords model 1–2 months every few years, or 5–8% per annum.
Management & compliance: Fees, inventories, check-in/out, gas/electrical, alarms, licensing.
Leasehold charges: Service charge and ground rent can dwarf other lines in blocks.
Repairs & capex: Routine maintenance plus a sinking fund for boilers, kitchens, roofs and externals.
Finance: Rate, product fees, IO vs C&I, ERCs at exit or remortgage.
The calculator makes these explicit. August then tracks the actuals. Consider schedule preventive works, adjust renewal rent, or improve tenant retention to cut voids.
Cash flow vs yield vs ROI
Yield is a helpful filter. Cashflow keeps you solvent. Cash-on-cash return shows how hard your invested cash is working (annual pre-tax cash flow ÷ total cash in). For decision-making, use all three. Model the deal in the calculator. August will give you the live cash position.
UK tax and finance, practical thoughts
For individual landlords, mortgage interest attracts a basic-rate tax credit rather than full deduction (often called “Section 24”). Companies may treat interest differently. Keep clean records of all allowable costs. August stores invoices in the documents feature, saving time, and reducing errors.
Portfolio view, from single unit to strategy
Once you have more than one property, consistency matters. Save a base case for each asset in the calculator, then inside August compare cashflow, highlight weak performers, and plan targeted actions (e.g., energy upgrades to reduce bills, insurance rebids, or service-charge challenges).
How to use the August Rental Cashflow Calculator
Enter monthly rent and choose a conservative rent for underwriting.
Add mortgage details. Balance, rate, product type (IO or C&I), and term.
Include voids as a percentage of rent and set management, service charge/ground rent, insurance, maintenance, utilities you pay, and compliance.
Toggle stress tests (higher rate, lower rent) and save versions such as “Base”, “Downside”, “Best case”.
Then use August to monitor actuals with live bank feeds and invoices uploaded to the app.
Why pair the calculator with August?
Live cash tracking: Bank feeds and rules categorise income across tenancies automatically.
Compliance prevents surprises: Reminders for gas, EICR, alarms, deposit protection and licensing help avoid fines and last-minute costs.
Downloads reports: Share payment reports with lenders or accountants in a click.
Start with the calculator, then keep the numbers honest in August.
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Disclaimer
This calculator provides cashflow estimates for guidance only and does not constitute legal, tax, or financial advice. Actual results may vary based on interest rates, costs, void periods, and your individual circumstances. Always consult a qualified adviser before making investment decisions.
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