function calculateRental() {
// 1. Get Inputs by ID
var price = parseFloat(document.getElementById('purchasePrice').value);
var downPercent = parseFloat(document.getElementById('downPayment').value);
var rate = parseFloat(document.getElementById('interestRate').value);
var termYears = parseFloat(document.getElementById('loanTerm').value);
var closing = parseFloat(document.getElementById('closingCosts').value);
var rent = parseFloat(document.getElementById('monthlyRent').value);
var annualTax = parseFloat(document.getElementById('propertyTax').value);
var annualIns = parseFloat(document.getElementById('homeInsurance').value);
var hoa = parseFloat(document.getElementById('hoaFees').value);
var vacancyPct = parseFloat(document.getElementById('vacancyRate').value);
var maintPct = parseFloat(document.getElementById('maintenanceRate').value);
// 2. Validation
if (isNaN(price) || isNaN(downPercent) || isNaN(rate) || isNaN(termYears) || isNaN(rent)) {
document.getElementById('error-message').style.display = 'block';
document.getElementById('calc-results').style.display = 'none';
return;
} else {
document.getElementById('error-message').style.display = 'none';
}
// Handle optional empty fields as 0
if(isNaN(closing)) closing = 0;
if(isNaN(annualTax)) annualTax = 0;
if(isNaN(annualIns)) annualIns = 0;
if(isNaN(hoa)) hoa = 0;
if(isNaN(vacancyPct)) vacancyPct = 0;
if(isNaN(maintPct)) maintPct = 0;
// 3. Calculation Logic
// Loan Calculations
var downPaymentAmount = price * (downPercent / 100);
var loanAmount = price – downPaymentAmount;
var monthlyRate = (rate / 100) / 12;
var totalMonths = termYears * 12;
// Mortgage P&I
var monthlyPI = 0;
if (rate > 0) {
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, totalMonths)) / (Math.pow(1 + monthlyRate, totalMonths) – 1);
} else {
monthlyPI = loanAmount / totalMonths;
}
// Expense Calculations
var monthlyTax = annualTax / 12;
var monthlyIns = annualIns / 12;
var vacancyCost = rent * (vacancyPct / 100);
var maintCost = rent * (maintPct / 100);
var totalMonthlyExpenses = monthlyPI + monthlyTax + monthlyIns + hoa + vacancyCost + maintCost;
var operatingExpensesOnly = monthlyTax + monthlyIns + hoa + vacancyCost + maintCost;
// Income Calculations
var cashFlow = rent – totalMonthlyExpenses;
var annualCashFlow = cashFlow * 12;
var netOperatingIncome = (rent * 12) – (operatingExpensesOnly * 12);
// ROI Calculations
var totalCashInvested = downPaymentAmount + closing;
var cashOnCash = 0;
if (totalCashInvested > 0) {
cashOnCash = (annualCashFlow / totalCashInvested) * 100;
}
var capRate = 0;
if (price > 0) {
capRate = (netOperatingIncome / price) * 100;
}
// 4. Update UI
var resultsDiv = document.getElementById('calc-results');
resultsDiv.style.display = 'block';
// Helper for currency
var fmt = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
document.getElementById('res-mortgage').innerText = fmt.format(monthlyPI);
document.getElementById('res-expenses').innerText = fmt.format(totalMonthlyExpenses);
document.getElementById('res-noi').innerText = fmt.format(netOperatingIncome / 12);
document.getElementById('res-cashflow').innerText = fmt.format(cashFlow);
document.getElementById('res-coc').innerText = cashOnCash.toFixed(2) + '%';
document.getElementById('res-cap').innerText = capRate.toFixed(2) + '%';
// Styling for negative/positive cash flow
var cfRow = document.getElementById('cashflow-row');
if (cashFlow < 0) {
cfRow.classList.add('negative');
cfRow.style.color = '#e74c3c';
} else {
cfRow.classList.remove('negative');
cfRow.style.color = '#2ecc71';
}
}
Mastering Real Estate Investment with a Rental Cash Flow Calculator
Investing in rental properties is one of the most reliable ways to build long-term wealth, but not every property is a good deal. The difference between a profitable asset and a money pit often comes down to one metric: Cash Flow. Our Rental Property Cash Flow Calculator is designed to help investors rigorously analyze potential deals by accounting for all income and expenses, ensuring you make data-driven decisions.
What is Rental Property Cash Flow?
Cash flow is the net amount of money left over after all property-related expenses are paid. It is calculated by taking your total rental income and subtracting vacancy costs, mortgage payments (principal and interest), taxes, insurance, homeowner association (HOA) fees, and maintenance reserves.
Positive Cash Flow: You are making a profit every month. This is the goal for most buy-and-hold investors.
Negative Cash Flow: The property costs more to hold than it generates in income. While some investors accept this for appreciation plays, it carries significantly higher risk.
Key Metrics Explained
This calculator provides several critical outputs to evaluate your investment:
Net Operating Income (NOI): This represents the profitability of the property before mortgage financing is considered. It is calculated as Gross Income – Operating Expenses. Note that mortgage payments are not included in operating expenses.
Cash on Cash ROI: This is arguably the most important metric for investors. It measures the annual return on the actual cash you invested (Down Payment + Closing Costs). A 10% Cash on Cash return means you earn $10 for every $100 invested per year.
Cap Rate (Capitalization Rate): This metric helps compare the profitability of different properties regardless of how they are financed. It is calculated as NOI / Purchase Price.
How to Estimate Expenses Accurately
One of the biggest mistakes new investors make is underestimating expenses. When using the calculator above, be sure to include:
Vacancy Rate: No property is occupied 100% of the time. A standard conservative estimate is 5% to 8% (roughly 2-3 weeks of vacancy per year).
Maintenance & Repairs: Things break. Setting aside 5% to 10% of monthly rent ensures you have funds when the water heater fails or the roof needs patching.
Property Management: Even if you plan to manage it yourself, it is wise to calculate the deal with a management fee (typically 8-10%) to see if the deal still works if you decide to hire a professional later.
Use this tool to run "what-if" scenarios. What happens to your cash flow if the interest rate rises by 1%? What if the vacancy rate doubles? By stress-testing your numbers, you can invest with confidence.