function calculateRental() {
// Get Inputs
var price = parseFloat(document.getElementById('purchasePrice').value);
var downPercent = parseFloat(document.getElementById('downPayment').value);
var interest = parseFloat(document.getElementById('interestRate').value);
var years = parseFloat(document.getElementById('loanTerm').value);
var closing = parseFloat(document.getElementById('closingCosts').value);
var rent = parseFloat(document.getElementById('monthlyRent').value);
var otherInc = parseFloat(document.getElementById('otherIncome').value);
var tax = parseFloat(document.getElementById('annualTax').value);
var insurance = parseFloat(document.getElementById('annualInsurance').value);
var hoa = parseFloat(document.getElementById('monthlyHOA').value);
var maintPercent = parseFloat(document.getElementById('maintenance').value);
var vacancyPercent = parseFloat(document.getElementById('vacancyRate').value);
var mgmtPercent = parseFloat(document.getElementById('managementFee').value);
// Validation
if (isNaN(price) || isNaN(downPercent) || isNaN(interest) || isNaN(years)) {
alert("Please enter valid numbers for the purchase details.");
return;
}
// 1. Mortgage Calculation
var downAmount = price * (downPercent / 100);
var loanAmount = price – downAmount;
var monthlyRate = (interest / 100) / 12;
var numPayments = years * 12;
var monthlyMortgage = 0;
if (interest > 0) {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) / (Math.pow(1 + monthlyRate, numPayments) – 1);
} else {
monthlyMortgage = loanAmount / numPayments;
}
// 2. Income Calculation
var totalMonthlyIncome = rent + otherInc;
// 3. Variable Expenses
var vacancyCost = totalMonthlyIncome * (vacancyPercent / 100);
var maintenanceCost = totalMonthlyIncome * (maintPercent / 100);
var managementCost = totalMonthlyIncome * (mgmtPercent / 100);
// 4. Fixed Monthly Expenses
var monthlyTax = tax / 12;
var monthlyInsurance = insurance / 12;
// Total Operating Expenses (excluding Mortgage)
var operatingExpenses = monthlyTax + monthlyInsurance + hoa + vacancyCost + maintenanceCost + managementCost;
// Total Expenses (including Mortgage)
var totalExpenses = operatingExpenses + monthlyMortgage;
// 5. Returns
var cashFlow = totalMonthlyIncome – totalExpenses;
var annualCashFlow = cashFlow * 12;
var noi = (totalMonthlyIncome – operatingExpenses) * 12;
var totalInitialInvestment = downAmount + closing;
var cocReturn = 0;
if (totalInitialInvestment > 0) {
cocReturn = (annualCashFlow / totalInitialInvestment) * 100;
}
var capRate = (noi / price) * 100;
// Display Results
document.getElementById('results').style.display = 'block';
// Format Currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
document.getElementById('resMortgage').innerText = formatter.format(monthlyMortgage);
document.getElementById('resExpenses').innerText = formatter.format(totalExpenses);
document.getElementById('resNOI').innerText = formatter.format(noi / 12) + " /mo";
document.getElementById('resCoC').innerText = cocReturn.toFixed(2) + "%";
document.getElementById('resCap').innerText = capRate.toFixed(2) + "%";
var cfElement = document.getElementById('resCashFlow');
cfElement.innerText = formatter.format(cashFlow);
if (cashFlow >= 0) {
cfElement.className = "result-value result-highlight";
} else {
cfElement.className = "result-value result-highlight negative";
}
}
Understanding 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 a profit. The most critical metric for any buy-and-hold investor is Cash Flow. This calculator helps you determine if a potential rental property will put money in your pocket every month or drain your savings.
How the Calculation Works
To accurately assess a rental property, we break down the financials into three main categories:
Gross Income: This includes the monthly rent and any other ancillary income (laundry, parking fees, storage).
Operating Expenses: These are the costs to keep the property running, excluding the loan. This includes taxes, insurance, HOA fees, maintenance reserves, vacancy buffers, and property management fees.
Debt Service: This is your principal and interest payment to the lender.
The formula for Cash Flow is simple: Income – (Operating Expenses + Debt Service) = Cash Flow.
Key Metrics Explained
Beyond simple cash flow, this calculator provides two advanced metrics to help you compare investments:
Cash on Cash Return (CoC): This measures the annual return on the actual cash you invested (down payment + closing costs). A CoC of 8-12% is generally considered a solid return in many markets.
Cap Rate (Capitalization Rate): This metric calculates the natural rate of return of the property assuming you bought it in cash. It is calculated by dividing the Net Operating Income (NOI) by the purchase price. It helps compare the quality of the deal regardless of financing.
Why Use a Cash Flow Calculator?
Many new investors make the mistake of only subtracting the mortgage from the rent. They forget to account for "phantom" costs like vacancy (months where the unit sits empty) and maintenance (saving for a new roof or water heater). By inputting realistic percentages for maintenance (typically 5-10%) and vacancy (typically 5-8%), this tool provides a realistic view of the property's long-term profitability.