function calculateRentalCashFlow() {
// 1. Retrieve Inputs
var price = parseFloat(document.getElementById('rpc_price').value) || 0;
var down = parseFloat(document.getElementById('rpc_down').value) || 0;
var interestRate = parseFloat(document.getElementById('rpc_rate').value) || 0;
var loanTermYears = parseFloat(document.getElementById('rpc_term').value) || 0;
var rent = parseFloat(document.getElementById('rpc_rent').value) || 0;
var vacancyRate = parseFloat(document.getElementById('rpc_vacancy').value) || 0;
var tax = parseFloat(document.getElementById('rpc_tax').value) || 0;
var insurance = parseFloat(document.getElementById('rpc_insurance').value) || 0;
var hoa = parseFloat(document.getElementById('rpc_hoa').value) || 0;
var repairsRate = parseFloat(document.getElementById('rpc_repairs').value) || 0;
var capexRate = parseFloat(document.getElementById('rpc_capex').value) || 0;
var mgmtRate = parseFloat(document.getElementById('rpc_management').value) || 0;
// 2. Calculate Mortgage
var loanAmount = price – down;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyMortgage = 0;
if (loanAmount > 0 && interestRate > 0) {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
} else if (loanAmount > 0 && interestRate === 0) {
monthlyMortgage = loanAmount / numberOfPayments;
}
// 3. Calculate Operating Expenses & Income
var vacancyCost = rent * (vacancyRate / 100);
var effectiveGrossIncome = rent – vacancyCost;
var repairsCost = rent * (repairsRate / 100);
var capexCost = rent * (capexRate / 100);
var mgmtCost = rent * (mgmtRate / 100);
var totalOperatingExpenses = tax + insurance + hoa + repairsCost + capexCost + mgmtCost;
// 4. Calculate Key Metrics
var monthlyNOI = effectiveGrossIncome – totalOperatingExpenses;
var monthlyCashFlow = monthlyNOI – monthlyMortgage;
var annualCashFlow = monthlyCashFlow * 12;
var annualNOI = monthlyNOI * 12;
// Cash on Cash Return: Annual Cash Flow / Total Cash Invested (assuming closing costs roughly included in down for simplicity, strictly speaking it's down + closing)
var cashInvested = down;
var cocReturn = 0;
if (cashInvested > 0) {
cocReturn = (annualCashFlow / cashInvested) * 100;
}
// Cap Rate: Annual NOI / Purchase Price
var capRate = 0;
if (price > 0) {
capRate = (annualNOI / price) * 100;
}
// 5. Display Results
var formatter = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
var percentFormatter = new Intl.NumberFormat('en-US', { style: 'decimal', minimumFractionDigits: 2, maximumFractionDigits: 2 });
document.getElementById('res_gross').innerText = formatter.format(rent);
document.getElementById('res_mortgage').innerText = formatter.format(monthlyMortgage);
document.getElementById('res_expenses').innerText = formatter.format(totalOperatingExpenses);
document.getElementById('res_noi').innerText = formatter.format(monthlyNOI);
var cfEl = document.getElementById('res_cashflow');
cfEl.innerText = formatter.format(monthlyCashFlow);
cfEl.style.color = monthlyCashFlow >= 0 ? '#27ae60' : '#c0392b';
var annualCfEl = document.getElementById('res_annual_cf');
annualCfEl.innerText = formatter.format(annualCashFlow);
annualCfEl.style.color = annualCashFlow >= 0 ? '#27ae60' : '#c0392b';
document.getElementById('res_coc').innerText = percentFormatter.format(cocReturn) + "%";
document.getElementById('res_cap').innerText = percentFormatter.format(capRate) + "%";
document.getElementById('rpc_results').style.display = 'block';
}
Understanding Rental Property Cash Flow
Investing in real estate is one of the most reliable ways to build long-term wealth, but not every property is a good investment. The difference between a profitable asset and a financial burden often comes down to one metric: Cash Flow. This Rental Property Cash Flow Calculator helps investors analyze deals quickly to ensure profitability before signing on the dotted line.
What is Positive Cash Flow?
Positive cash flow occurs when a property's gross income (rent) exceeds the total of its mortgage payments and operating expenses. It is the profit you pocket every month. "Cash flow positive" is the holy grail for buy-and-hold investors because it means the asset pays for itself while potentially appreciating in value.
Conversely, negative cash flow means you must contribute your own money every month to keep the property running. While some investors accept this in high-appreciation markets, it carries significantly higher risk.
Key Metrics Calculated
Beyond simple monthly profit, this tool calculates three critical metrics for professional analysis:
Net Operating Income (NOI): This is your total revenue minus all operating expenses (taxes, insurance, maintenance), excluding the mortgage. It measures the profitability of the property itself, regardless of financing.
Cap Rate (Capitalization Rate): Calculated as NOI / Purchase Price, this percentage allows you to compare the profitability of different properties assuming an all-cash purchase. A higher Cap Rate generally indicates a better return, though often with higher risk.
Cash-on-Cash Return (CoC): Calculated as Annual Cash Flow / Total Cash Invested. This measures the efficiency of your specific investment capital. If you put $50,000 down and make $5,000 a year in profit, your CoC return is 10%.
Estimating Expenses Correctly
New investors often overestimate cash flow by forgetting hidden costs. To get an accurate result, ensure you account for:
Vacancy: Properties don't stay rented 365 days a year. A 5-8% vacancy allowance is standard industry practice.
CapEx (Capital Expenditures): Big-ticket items like roofs and HVAC systems eventually need replacement. Setting aside 5-10% of rent monthly prevents shock expenses later.
Property Management: Even if you self-manage now, factoring in an 8-10% management fee ensures the deal still works if you decide to hire a professional later.
How to Use This Calculator
1. Enter Purchase Details: Input the purchase price, your down payment, and loan details. The calculator automatically determines your monthly mortgage payment (Principal & Interest).
2. Input Income: Enter the expected monthly rent. Be realistic—check comparable rentals in the area (comps).
3. Detail Expenses: Fill out taxes, insurance, and HOA fees. Don't skip the percentage-based reserves for maintenance and vacancy.
4. Analyze: Click "Calculate Cash Flow" to see your monthly profit, CoC return, and Cap Rate. Use these numbers to decide if the property meets your investment criteria.