function calculateCashFlow() {
// 1. Get input values
var price = parseFloat(document.getElementById('purchasePrice').value);
var downPercent = parseFloat(document.getElementById('downPayment').value);
var interest = parseFloat(document.getElementById('interestRate').value);
var term = parseFloat(document.getElementById('loanTerm').value);
var rent = parseFloat(document.getElementById('monthlyRent').value);
var taxYear = parseFloat(document.getElementById('annualTaxes').value);
var insYear = parseFloat(document.getElementById('annualInsurance').value);
var hoaMonth = parseFloat(document.getElementById('monthlyHOA').value);
var vacancyPct = parseFloat(document.getElementById('vacancyRate').value);
var maintPct = parseFloat(document.getElementById('maintenanceRate').value);
// 2. Validation
if (isNaN(price) || isNaN(downPercent) || isNaN(interest) || isNaN(term) || isNaN(rent)) {
alert("Please fill in all required fields with valid numbers.");
return;
}
// Set defaults for optional fields if empty/NaN
if (isNaN(taxYear)) taxYear = 0;
if (isNaN(insYear)) insYear = 0;
if (isNaN(hoaMonth)) hoaMonth = 0;
if (isNaN(vacancyPct)) vacancyPct = 0;
if (isNaN(maintPct)) maintPct = 0;
// 3. Financial Calculations
var downPaymentAmount = price * (downPercent / 100);
var loanAmount = price – downPaymentAmount;
// Mortgage Calculation (Monthly P&I)
var monthlyRate = (interest / 100) / 12;
var numPayments = term * 12;
var monthlyPI = 0;
if (interest > 0) {
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) / (Math.pow(1 + monthlyRate, numPayments) – 1);
} else {
monthlyPI = loanAmount / numPayments;
}
// Operating Expenses
var monthlyTax = taxYear / 12;
var monthlyIns = insYear / 12;
var vacancyCost = rent * (vacancyPct / 100);
var maintCost = rent * (maintPct / 100);
var totalMonthlyExpenses = monthlyPI + monthlyTax + monthlyIns + hoaMonth + vacancyCost + maintCost;
var operatingExpensesNoMortgage = monthlyTax + monthlyIns + hoaMonth + vacancyCost + maintCost;
// Net Operating Income (NOI) = Annual Rent – Annual Operating Expenses (Excluding Mortgage)
var annualNOI = (rent * 12) – (operatingExpensesNoMortgage * 12);
// Cash Flow
var monthlyCashFlow = rent – totalMonthlyExpenses;
var annualCashFlow = monthlyCashFlow * 12;
// Investment Returns
// Cash on Cash Return = Annual Cash Flow / Total Cash Invested (Down Payment + usually closing costs, but we use DP here for simplicity)
var cashInvested = downPaymentAmount; // Could add closing costs field in V2
var cocReturn = 0;
if (cashInvested > 0) {
cocReturn = (annualCashFlow / cashInvested) * 100;
}
// Cap Rate = Annual NOI / Purchase Price
var capRate = (annualNOI / price) * 100;
// 4. Update DOM
document.getElementById('displayMortgage').innerText = "$" + monthlyPI.toFixed(2);
document.getElementById('displayExpenses').innerText = "$" + totalMonthlyExpenses.toFixed(2);
// NOI Display (Monthly)
document.getElementById('displayNOI').innerText = "$" + (annualNOI / 12).toFixed(2);
var cfElement = document.getElementById('displayCashFlow');
cfElement.innerText = "$" + monthlyCashFlow.toFixed(2);
// Color coding for cash flow
if (monthlyCashFlow >= 0) {
cfElement.className = "result-value positive-cf";
} else {
cfElement.className = "result-value negative-cf";
}
document.getElementById('displayCoC').innerText = cocReturn.toFixed(2) + "%";
document.getElementById('displayCapRate').innerText = capRate.toFixed(2) + "%";
// Show results
document.getElementById('resultContainer').style.display = "block";
}
Understanding Rental Property Analysis
Investing in real estate is one of the most reliable ways to build wealth, but it requires precise mathematical analysis. A Rental Property Cash Flow Calculator is an essential tool for investors to determine the viability of a potential deal before signing any contracts.
What is Cash Flow?
Cash flow represents the net amount of money moving into and out of your rental business. Positive cash flow means the property generates more income than it costs to operate, providing you with passive income. Negative cash flow implies the property requires monthly capital from your pocket to sustain, which is a risky position for most investors.
Our calculator computes cash flow using the formula: Cash Flow = Gross Rental Income – (Mortgage + Taxes + Insurance + HOA + Vacancy + Maintenance).
Key Metrics Explained
Net Operating Income (NOI): This is the annual income generated by the property after deducting all operating expenses but before deducting mortgage payments and taxes. It is a pure measure of the property's efficiency.
Cap Rate (Capitalization Rate): Calculated as NOI / Purchase Price, the Cap Rate helps compare the return on investment of different properties regardless of how they are financed. A higher Cap Rate generally indicates a better return, though often accompanied by higher risk.
Cash on Cash Return: This metric measures the annual return on the actual cash you invested (down payment). It is crucial for understanding how hard your money is working for you compared to other investment vehicles like stocks or bonds.
Why Factor in Vacancy and Maintenance?
Novice investors often make the mistake of calculating returns based solely on rent minus mortgage. However, real life includes vacancies (months without tenants) and maintenance (broken water heaters, roof repairs). Our calculator allows you to set percentage reserves for these inevitabilities, ensuring your profit projections remain realistic and safe.
How to Improve Your ROI
If the calculator shows a negative or low cash flow, consider negotiating a lower purchase price, increasing the down payment to lower the mortgage, or looking for ways to value-add to the property to justify higher rent. Analyzing multiple scenarios is the key to finding a winning investment property.