function calculateCashFlow() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('purchasePrice').value);
var downPercent = parseFloat(document.getElementById('downPaymentPercent').value);
var rate = parseFloat(document.getElementById('interestRate').value);
var termYears = parseFloat(document.getElementById('loanTerm').value);
var closingCosts = parseFloat(document.getElementById('closingCosts').value);
var rent = parseFloat(document.getElementById('monthlyRent').value);
var taxYear = parseFloat(document.getElementById('propertyTaxYear').value);
var insuranceYear = parseFloat(document.getElementById('insuranceYear').value);
var hoaMonth = parseFloat(document.getElementById('monthlyHOA').value);
var vacancyPercent = parseFloat(document.getElementById('vacancyRate').value);
// 2. Validate Inputs
if (isNaN(price) || isNaN(rent) || isNaN(rate) || isNaN(termYears)) {
alert("Please fill in all required fields (Price, Rent, Interest Rate, Term) with valid numbers.");
return;
}
// Handle optional empty fields as 0
if (isNaN(closingCosts)) closingCosts = 0;
if (isNaN(taxYear)) taxYear = 0;
if (isNaN(insuranceYear)) insuranceYear = 0;
if (isNaN(hoaMonth)) hoaMonth = 0;
if (isNaN(vacancyPercent)) vacancyPercent = 0;
if (isNaN(downPercent)) downPercent = 0;
// 3. Perform Calculations
// Loan Calculations
var downPaymentAmount = price * (downPercent / 100);
var loanAmount = price – downPaymentAmount;
var monthlyRate = rate / 100 / 12;
var numberOfPayments = termYears * 12;
// Mortgage P&I Formula: M = P[r(1+r)^n]/[(1+r)^n – 1]
var monthlyMortgage = 0;
if (rate > 0) {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
} else {
monthlyMortgage = loanAmount / numberOfPayments;
}
// Expense Calculations
var monthlyTax = taxYear / 12;
var monthlyInsurance = insuranceYear / 12;
var monthlyVacancyCost = rent * (vacancyPercent / 100);
var totalMonthlyExpenses = monthlyMortgage + monthlyTax + monthlyInsurance + hoaMonth + monthlyVacancyCost;
var operatingExpensesOnly = monthlyTax + monthlyInsurance + hoaMonth + monthlyVacancyCost; // For Cap Rate (excludes debt service)
// Cash Flow
var monthlyCashFlow = rent – totalMonthlyExpenses;
var annualCashFlow = monthlyCashFlow * 12;
// Investment Returns
var totalInitialInvestment = downPaymentAmount + closingCosts;
var cashOnCashReturn = 0;
if (totalInitialInvestment > 0) {
cashOnCashReturn = (annualCashFlow / totalInitialInvestment) * 100;
}
// Cap Rate = Net Operating Income (NOI) / Current Market Value
// NOI = Annual Rent – Annual Operating Expenses (Excluding Mortgage)
var annualNOI = (rent * 12) – (operatingExpensesOnly * 12);
var capRate = (annualNOI / price) * 100;
// 4. Update UI
document.getElementById('resInitialInvest').innerText = "$" + totalInitialInvestment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resMortgage').innerText = "$" + monthlyMortgage.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Show Operating Expenses + Mortgage in total output logic, or just Operating expenses?
// Label says "Total Monthly Expenses (incl. Vacancy)". Usually this implies Operating + Debt Service for the renter's pocket.
document.getElementById('resExpenses').innerText = "$" + totalMonthlyExpenses.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
var cfElement = document.getElementById('resCashFlow');
cfElement.innerText = "$" + monthlyCashFlow.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Style positive/negative
if (monthlyCashFlow >= 0) {
cfElement.className = "result-value positive-flow";
} else {
cfElement.className = "result-value negative-flow";
}
document.getElementById('resAnnualFlow').innerText = "$" + annualCashFlow.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resCoC').innerText = cashOnCashReturn.toFixed(2) + "%";
document.getElementById('resCapRate').innerText = capRate.toFixed(2) + "%";
document.getElementById('result-area').style.display = 'block';
}
Understanding Rental Property Cash Flow
Investing in real estate is one of the most reliable ways to build wealth, but the success of any rental property hinges on its numbers. This Rental Property Cash Flow Calculator is designed to help investors analyze the profitability of a potential deal by breaking down income, expenses, and financing costs.
What is Cash Flow?
Cash flow is the profit remaining after you have paid all operating expenses and debt service (mortgage payments) associated with the property.
Positive Cash Flow: You earn more in rent than it costs to own the property. This is the goal for most buy-and-hold investors.
Negative Cash Flow: The property costs you money every month. While some investors accept this banking on appreciation, it is generally considered higher risk.
Key Metrics Calculated
Beyond simple monthly profit, this tool provides two critical metrics for comparing investments:
1. Cash on Cash Return (CoC)
This metric measures the annual return on the actual cash you invested (Down Payment + Closing Costs), rather than the total value of the property. It answers the question: "For every dollar I put in, how many pennies do I get back this year?" A CoC return of 8-12% is often considered a solid benchmark for rental properties.
2. Cap Rate (Capitalization Rate)
Cap Rate measures the property's natural rate of return assuming you bought it with all cash (no mortgage). It is calculated by dividing the Net Operating Income (NOI) by the property price. This helps you compare the profitability of different properties regardless of how they are financed.
Factors That Impact ROI
When using this calculator, pay close attention to the Vacancy Rate and Maintenance Reserves (included here under HOA/Maintenance or Expenses). Novice investors often overlook these costs. A standard vacancy rate is 5-8% (about 2-3 weeks of emptiness per year), and setting aside 10-15% of rent for repairs is a prudent safety net.