function calculateRentalKPIs() {
// Get input values
var price = parseFloat(document.getElementById('purchasePrice').value);
var down = parseFloat(document.getElementById('downPayment').value);
var rate = parseFloat(document.getElementById('interestRate').value);
var term = parseFloat(document.getElementById('loanTerm').value);
var rent = parseFloat(document.getElementById('monthlyRent').value);
var ops = parseFloat(document.getElementById('monthlyExpenses').value);
// Validation to prevent NaN errors
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(term) || isNaN(rent) || isNaN(ops)) {
alert("Please fill in all fields with valid numbers.");
return;
}
// 1. Calculate Mortgage Payment
var loanAmount = price – down;
var monthlyRate = (rate / 100) / 12;
var totalPayments = term * 12;
var mortgagePayment = 0;
if (loanAmount > 0) {
if (rate === 0) {
mortgagePayment = loanAmount / totalPayments;
} else {
mortgagePayment = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, totalPayments)) / (Math.pow(1 + monthlyRate, totalPayments) – 1);
}
}
// 2. Calculate Totals
var totalMonthlyExpenses = mortgagePayment + ops;
var monthlyCashFlow = rent – totalMonthlyExpenses;
var annualCashFlow = monthlyCashFlow * 12;
// 3. Calculate Cash on Cash Return
// Formula: Annual Cash Flow / Total Cash Invested (Down Payment)
var cocReturn = 0;
if (down > 0) {
cocReturn = (annualCashFlow / down) * 100;
}
// 4. Update UI
document.getElementById('resMortgage').innerText = "$" + mortgagePayment.toFixed(2);
document.getElementById('resTotalExpenses').innerText = "$" + totalMonthlyExpenses.toFixed(2);
var cashFlowEl = document.getElementById('resCashFlow');
cashFlowEl.innerText = "$" + monthlyCashFlow.toFixed(2);
// Color coding for cash flow
if (monthlyCashFlow >= 0) {
cashFlowEl.className = "result-value highlight-positive";
} else {
cashFlowEl.className = "result-value highlight-negative";
}
var annualFlowEl = document.getElementById('resAnnualFlow');
annualFlowEl.innerText = "$" + annualCashFlow.toFixed(2);
if (annualCashFlow >= 0) {
annualFlowEl.className = "result-value highlight-positive";
} else {
annualFlowEl.className = "result-value highlight-negative";
}
var cocEl = document.getElementById('resCOC');
cocEl.innerText = cocReturn.toFixed(2) + "%";
if (cocReturn >= 0) {
cocEl.className = "result-value highlight-positive";
} else {
cocEl.className = "result-value highlight-negative";
}
// Show result box
document.getElementById('resultBox').style.display = "block";
}
How to Calculate Rental Property Cash Flow
Investing in real estate is one of the most reliable ways to build wealth, but simply buying a property doesn't guarantee profit. The key to successful real estate investing is positive cash flow. Our Rental Property Cash Flow Calculator helps you determine if a potential investment will put money in your pocket every month or become a financial liability.
Understanding the Key Metrics
To accurately assess a rental property, you need to look beyond just the rental income. Here are the critical components used in this calculation:
Net Monthly Cash Flow: This is the amount of money left over after all expenses have been paid. It is calculated as Total Income – Total Expenses. A positive number means profit; a negative number means you are losing money every month.
Cash on Cash Return (CoC): This percentage tells you how hard your money is working for you. It compares your annual pre-tax cash flow to the total amount of cash you actually invested (primarily your down payment). A CoC return of 8-12% is generally considered a solid investment in many markets.
How the Calculation Works
This tool uses a standard amortization formula to estimate your mortgage payments based on the loan term and interest rate. It then adds your operating expenses (taxes, insurance, HOA fees, maintenance reserves) to find your total monthly outflow. Finally, it subtracts this total from your projected rental income to give you a clear picture of the property's financial performance.
Why Cash Flow Matters
Relying solely on property appreciation is risky. Positive cash flow ensures that the property pays for itself and provides a buffer against vacancies or unexpected repairs. Experienced investors prioritize cash flow because it provides immediate income and financial freedom, rather than waiting years for a potential payout upon sale.