How the Calculations Work
Plain-English explanation of every formula
Transparency matters when you're making financial decisions. Here's exactly how every number in our calculators is computed, with no hidden assumptions.
Mortgage Monthly Payment
We use the standard amortisation formula used by banks worldwide:
Monthly Payment = Loan × Rate × (1 + Rate)^n ÷ ((1 + Rate)^n - 1)
Where Loan = property price minus deposit, Rate = annual rate divided by 12, and n = mortgage term in years multiplied by 12. The payment stays the same every month, but the split between interest and principal changes — early on, most goes to interest. Over time, more goes to principal.
Worked example
Property: 300,000. Deposit: 60,000. Loan: 240,000. Rate: 3.5% (monthly: 0.292%). Term: 25 years (300 months). Monthly payment: approximately 1,200. Over the full term, you'd pay roughly 120,000 in interest on top of the 240,000 principal.
Investment Compound Growth
Your investment grows monthly using compound interest. Each month: new balance = previous balance × (1 + monthly return) + monthly contribution. Monthly return = (annual return - fund fee) divided by 12.
This means your contributions and your existing balance both earn returns, and those returns earn further returns the following month. Over long periods, this compounding effect is dramatic — after 30 years, typically 60-70% of your portfolio is growth rather than contributions.
Property Net Value
Net value = equity + net rental profit accumulated over time.
Equity = current property value minus remaining mortgage balance. The property value grows by the annual appreciation rate. The mortgage balance decreases as you make payments.
Net rental profit = total rent received to date minus total interest paid minus total maintenance minus total property tax. Note: only the interest portion of mortgage payments counts as a cost — the principal portion builds equity, so it's not "lost" money.
Rental Income
Annual rent = monthly rent × 12 × (1 - vacancy rate). We apply a 2% annual rent increase by default to reflect inflation and market adjustments. The vacancy rate accounts for empty periods between tenants.
Property Costs
Annual maintenance = property value × maintenance percentage. This covers repairs, upkeep, and unexpected issues. Annual property tax = property value × tax rate. Both costs increase as the property appreciates.
Monthly Cashflow
This is the number that tells you whether the property puts money in your pocket or takes it out each month:
Monthly Cashflow = (Annual Rent - Mortgage Payments - Maintenance - Property Tax) ÷ 12
Positive means the property generates income. Negative means you pay the shortfall from your own pocket.
ROI (Return on Investment)
Investment ROI = (portfolio value - total cash invested) ÷ total cash invested × 100. This tells you the percentage return on every dollar you put in.
Property ROI = (net value - total amount spent) ÷ total amount spent × 100. "Total spent" includes deposit, all mortgage payments, maintenance, and property tax. It only goes up — rent income is reflected in the net value instead.
Total Spent (Property)
This tracks every dollar that has left your bank account for the property: deposit + all mortgage payments + all maintenance + all property tax. Rent received is not subtracted from this number — it's shown separately in the net value. This gives you an honest picture of the real cash commitment.
Inflation Adjustment (Advanced Calculator)
When enabled, all future values are deflated to today's purchasing power using 2.5% annual inflation. A value of 500,000 in year 20 would be shown as approximately 305,000 in today's money. This helps you compare future values more realistically.
Invest the Difference (Advanced Calculator)
If the property costs more per month than your planned investment contribution, the difference is added to the investment instead. This answers the question: "What if I put ALL the money I'd spend on property into the stock market instead?"
Rent vs Buy Calculator
The renter invests their savings at the expected return rate, plus invests the monthly difference between the buyer's total costs and their rent. The buyer builds equity through mortgage payments and property appreciation. The renter's rent increases annually. The comparison shows total wealth at each point.
What We Don't Model
No calculator can capture everything. Key things we exclude: personal income tax and capital gains tax (these vary enormously by country, income level, and holding period), stamp duty and closing costs (one-off costs that vary by location), transaction costs when selling, insurance costs, letting agent fees, the time and stress of property management, market crashes and recovery periods, and individual mortgage approval criteria.
We recommend using our calculator as a starting point, then consulting a qualified financial advisor for decisions involving significant amounts of money.