function calculateRentalCashFlow() {
// 1. Get Inputs
var price = parseFloat(document.getElementById('purchasePrice').value) || 0;
var downPercent = parseFloat(document.getElementById('downPaymentPercent').value) || 0;
var interestRate = parseFloat(document.getElementById('interestRate').value) || 0;
var loanTermYears = parseFloat(document.getElementById('loanTerm').value) || 0;
var closingCosts = parseFloat(document.getElementById('closingCosts').value) || 0;
var rent = parseFloat(document.getElementById('monthlyRent').value) || 0;
var vacancyRate = parseFloat(document.getElementById('vacancyRate').value) || 0;
var yearlyTax = parseFloat(document.getElementById('propertyTax').value) || 0;
var yearlyIns = parseFloat(document.getElementById('insurance').value) || 0;
var monthlyHOA = parseFloat(document.getElementById('hoaFees').value) || 0;
var maintRate = parseFloat(document.getElementById('maintenanceRate').value) || 0;
// 2. Calculations
// Loan Calculation
var downPayment = price * (downPercent / 100);
var loanAmount = price – downPayment;
var totalCashInvested = downPayment + closingCosts;
// Mortgage P&I
var monthlyInterest = (interestRate / 100) / 12;
var totalPayments = loanTermYears * 12;
var monthlyPI = 0;
if (monthlyInterest > 0 && totalPayments > 0) {
monthlyPI = loanAmount * (monthlyInterest * Math.pow(1 + monthlyInterest, totalPayments)) / (Math.pow(1 + monthlyInterest, totalPayments) – 1);
} else if (interestRate === 0) {
monthlyPI = loanAmount / totalPayments;
}
// Operating Expenses
var monthlyVacancyCost = rent * (vacancyRate / 100);
var monthlyMaintCost = rent * (maintRate / 100);
var monthlyTax = yearlyTax / 12;
var monthlyIns = yearlyIns / 12;
var totalOperatingExpenses = monthlyVacancyCost + monthlyMaintCost + monthlyTax + monthlyIns + monthlyHOA;
var variableOpLoss = monthlyVacancyCost + monthlyMaintCost;
var fixedOpLoss = monthlyTax + monthlyIns + monthlyHOA;
// NOI
var monthlyNOI = rent – totalOperatingExpenses;
var annualNOI = monthlyNOI * 12;
// Cash Flow
var monthlyCashFlow = monthlyNOI – monthlyPI;
var annualCashFlow = monthlyCashFlow * 12;
// Returns
var cocReturn = 0;
if (totalCashInvested > 0) {
cocReturn = (annualCashFlow / totalCashInvested) * 100;
}
var capRate = 0;
if (price > 0) {
capRate = (annualNOI / price) * 100;
}
// 3. Display Results
var formatMoney = function(num) {
return '$' + num.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
};
var formatPercent = function(num) {
return num.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}) + '%';
};
document.getElementById('dispRent').innerText = formatMoney(rent);
document.getElementById('dispOpLoss').innerText = '-' + formatMoney(variableOpLoss);
document.getElementById('dispFixedExp').innerText = '-' + formatMoney(fixedOpLoss);
document.getElementById('dispNOI').innerText = formatMoney(monthlyNOI);
document.getElementById('dispMortgage').innerText = '-' + formatMoney(monthlyPI);
var cfElem = document.getElementById('dispCashFlow');
cfElem.innerText = formatMoney(monthlyCashFlow);
if(monthlyCashFlow < 0) {
cfElem.classList.add('negative-flow');
cfElem.style.color = '#c0392b';
} else {
cfElem.classList.remove('negative-flow');
cfElem.style.color = '#27ae60';
}
document.getElementById('dispCoC').innerText = formatPercent(cocReturn);
document.getElementById('dispCapRate').innerText = formatPercent(capRate);
document.getElementById('results-area').style.display = 'block';
}
Mastering Rental Property Analysis
Investing in real estate is one of the most reliable ways to build long-term wealth, but success hinges on the numbers. A "gut feeling" isn't enough when hundreds of thousands of dollars are on the line. This Rental Property Cash Flow Calculator is designed to help investors objectively evaluate the profitability of a potential rental asset before signing the dotted line.
Why Cash Flow is King
Cash flow represents the net profit you pocket every month after all operating expenses and mortgage payments are made. Positive cash flow ensures the property pays for itself and provides you with passive income. If a property has negative cash flow, you are effectively paying monthly for the privilege of owning it, which is a risky strategy often reliant solely on appreciation.
Understanding Key Metrics
NOI (Net Operating Income): This is your total income minus operating expenses (taxes, insurance, vacancy, repairs) but excluding mortgage payments. It measures the raw profitability of the asset itself, regardless of financing.
Cash on Cash Return (CoC): This is arguably the most important metric for investors. It calculates the annual return on the actual cash you invested (down payment + closing costs). For example, if you invest $50,000 to buy a property and it generates $5,000 a year in cash flow, your CoC return is 10%.
Cap Rate: The Capitalization Rate measures the rate of return on the property assuming you paid all cash. It helps compare properties in different areas without the variable of financing structures.
Estimating Expenses Accurately
The biggest mistake new investors make is underestimating expenses. Always account for:
Vacancy: Your property won't be rented 365 days a year. A 5-8% vacancy rate is a standard conservative estimate.
CapEx & Maintenance: Roofs leak and water heaters break. Setting aside 5-10% of monthly rent creates a safety net for these inevitable costs.
Property Management: Even if you plan to self-manage, it is wise to calculate the numbers with a management fee (usually 8-10%) to ensure the deal still works if you eventually outsource the work.
How to Use This Calculator
To get the most accurate results, input the purchase price and loan details first. Then, research local rents to estimate income. Finally, be honest about expenses—input your property tax, insurance quotes, and set aside funds for repairs. The tool will instantly calculate your monthly cash flow and ROI metrics, allowing you to make data-driven investment decisions.