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 determine whether a potential property will generate positive income or become a financial drain.
Cash flow is the net amount of cash moving into or out of an investment after all operating expenses and debt service (mortgage payments) have been paid. A positive cash flow means the property is putting money in your pocket every month, while negative cash flow means you are paying out of pocket to hold the asset.
Key Metrics Calculated
Net Operating Income (NOI): This represents the profitability of the property before mortgage payments. It is calculated by subtracting operating expenses (taxes, insurance, maintenance, etc.) from the total revenue generated.
Cash on Cash Return (CoC): This is arguably the most important metric for investors using leverage. It measures the annual return on the actual cash invested (down payment + closing costs), giving you a percentage yield on your money.
Cap Rate (Capitalization Rate): This metric helps compare the profitability of different properties regardless of how they are financed. It is the ratio of NOI to the property's purchase price.
How to Improve Cash Flow
If your calculation shows a negative or low cash flow, consider these adjustments:
Increase Rent: Are you charging market rates? Small increases can significantly boost the bottom line.
Reduce Vacancy: High turnover kills returns. Improving tenant retention strategies can lower your vacancy rate.
Refinance: Securing a lower interest rate or extending the loan term can lower monthly mortgage payments.
Why Use a Rental Property Calculator?
Real estate markets are competitive. Successful investors do not rely on "gut feelings." By using a specialized calculator, you can account for often-overlooked expenses like vacancy provisions (money set aside for when the property is empty) and maintenance reserves. This ensures your projection reflects the reality of property ownership, protecting you from bad investments.
function calculateRentalCashFlow() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('purchasePrice').value);
var downPercent = parseFloat(document.getElementById('downPaymentPercent').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var termYears = parseFloat(document.getElementById('loanTerm').value);
var monthlyRent = parseFloat(document.getElementById('monthlyRent').value);
var vacancyRate = parseFloat(document.getElementById('vacancyRate').value);
var annualTax = parseFloat(document.getElementById('propertyTax').value);
var annualIns = parseFloat(document.getElementById('insurance').value);
var monthlyHOA = parseFloat(document.getElementById('hoaFees').value);
var maintPercent = parseFloat(document.getElementById('maintenance').value);
// Validate essential inputs
if (isNaN(price) || isNaN(downPercent) || isNaN(interestRate) || isNaN(termYears) || isNaN(monthlyRent)) {
alert("Please fill in all required fields (Price, Down Payment, Interest, Term, Rent) to calculate.");
return;
}
// Handle optional fields being empty (treat as 0)
if (isNaN(vacancyRate)) vacancyRate = 0;
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualIns)) annualIns = 0;
if (isNaN(monthlyHOA)) monthlyHOA = 0;
if (isNaN(maintPercent)) maintPercent = 0;
// 2. Perform Calculations
// Loan Calculation
var downPaymentAmount = price * (downPercent / 100);
var loanAmount = price – downPaymentAmount;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = termYears * 12;
// Mortgage Payment Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyMortgage = 0;
if (interestRate === 0) {
monthlyMortgage = loanAmount / numberOfPayments;
} else {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Expense Calculations
var monthlyTax = annualTax / 12;
var monthlyIns = annualIns / 12;
var vacancyCost = monthlyRent * (vacancyRate / 100);
var maintenanceCost = monthlyRent * (maintPercent / 100);
// Total Monthly Expenses (Operating + Mortgage)
var operatingExpenses = monthlyTax + monthlyIns + monthlyHOA + vacancyCost + maintenanceCost;
var totalMonthlyExpenses = operatingExpenses + monthlyMortgage;
// Income Metrics
var monthlyCashFlow = monthlyRent – totalMonthlyExpenses;
var annualCashFlow = monthlyCashFlow * 12;
// NOI (Net Operating Income) = Revenue – Operating Expenses (Excluding Mortgage)
var monthlyNOI = monthlyRent – operatingExpenses;
var annualNOI = monthlyNOI * 12;
// Returns
var capRate = (annualNOI / price) * 100;
// Cash on Cash Return = Annual Cash Flow / Total Cash Invested
// Assuming Closing Costs are roughly 2% of price for simplicity in this calculation,
// or we can strictly use Down Payment if not specified.
// Let's stick to Down Payment to match inputs precisely.
var totalCashInvested = downPaymentAmount;
// If downpayment is 0, avoid division by zero
var cocReturn = 0;
if (totalCashInvested > 0) {
cocReturn = (annualCashFlow / totalCashInvested) * 100;
}
// 3. Update DOM
document.getElementById('resMortgage').innerText = formatCurrency(monthlyMortgage);
document.getElementById('resExpenses').innerText = formatCurrency(totalMonthlyExpenses);
document.getElementById('resNOI').innerText = formatCurrency(monthlyNOI);
var cashFlowEl = document.getElementById('resCashFlow');
cashFlowEl.innerText = formatCurrency(monthlyCashFlow);
if (monthlyCashFlow >= 0) {
cashFlowEl.classList.remove('rp-negative');
cashFlowEl.classList.add('rp-highlight');
} else {
cashFlowEl.classList.remove('rp-highlight');
cashFlowEl.classList.add('rp-negative');
}
document.getElementById('resCoC').innerText = cocReturn.toFixed(2) + "%";
document.getElementById('resCapRate').innerText = capRate.toFixed(2) + "%";
// Show results
document.getElementById('rpResults').style.display = 'block';
}
function formatCurrency(num) {
return "$" + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}