function calculateRentalResults() {
// 1. Retrieve and Parse Inputs
var price = parseFloat(document.getElementById('rp_price').value);
var downPercent = parseFloat(document.getElementById('rp_down_payment').value);
var interestRate = parseFloat(document.getElementById('rp_interest').value);
var termYears = parseFloat(document.getElementById('rp_term').value);
var closingCosts = parseFloat(document.getElementById('rp_closing_costs').value);
var monthlyRent = parseFloat(document.getElementById('rp_rent').value);
var annualTax = parseFloat(document.getElementById('rp_property_tax').value);
var annualInsurance = parseFloat(document.getElementById('rp_insurance').value);
var monthlyHOA = parseFloat(document.getElementById('rp_hoa').value);
var vacancyRate = parseFloat(document.getElementById('rp_vacancy').value);
var capexRate = parseFloat(document.getElementById('rp_capex').value);
// 2. Validation
var errorDiv = document.getElementById('rp-error-msg');
var resultsDiv = document.getElementById('rp-results');
if (isNaN(price) || isNaN(downPercent) || isNaN(interestRate) || isNaN(termYears) ||
isNaN(monthlyRent) || isNaN(annualTax) || isNaN(annualInsurance)) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
errorDiv.style.display = 'none';
// 3. Loan Calculation
var downPaymentAmount = price * (downPercent / 100);
var loanAmount = price – downPaymentAmount;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = termYears * 12;
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var mortgagePayment = 0;
if (interestRate > 0) {
mortgagePayment = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
} else {
mortgagePayment = loanAmount / numberOfPayments;
}
// 4. Operating Expenses Calculation
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var monthlyVacancyCost = monthlyRent * (vacancyRate / 100);
var monthlyCapexCost = monthlyRent * (capexRate / 100);
var totalOperatingExpenses = monthlyTax + monthlyInsurance + monthlyHOA + monthlyVacancyCost + monthlyCapexCost;
var totalExpensesIncludingMortgage = totalOperatingExpenses + mortgagePayment;
// 5. Income Metrics
var netOperatingIncome = monthlyRent – totalOperatingExpenses; // NOI does NOT include mortgage
var cashFlow = monthlyRent – totalExpensesIncludingMortgage;
// 6. Return Metrics
var annualNOI = netOperatingIncome * 12;
var annualCashFlow = cashFlow * 12;
var totalCashInvested = downPaymentAmount + closingCosts;
// Cap Rate = (Annual NOI / Purchase Price) * 100
var capRate = (annualNOI / price) * 100;
// Cash on Cash Return = (Annual Cash Flow / Total Cash Invested) * 100
var cashOnCash = (annualCashFlow / totalCashInvested) * 100;
// 7. Display Results
resultsDiv.style.display = 'block';
// Formatting function
function formatCurrency(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
document.getElementById('rp_final_cashflow').innerHTML = formatCurrency(cashFlow);
document.getElementById('rp_coc').innerHTML = cashOnCash.toFixed(2) + '%';
document.getElementById('rp_cap_rate').innerHTML = capRate.toFixed(2) + '%';
document.getElementById('rp_noi').innerHTML = formatCurrency(netOperatingIncome);
document.getElementById('rp_total_expenses').innerHTML = formatCurrency(totalExpensesIncludingMortgage);
}
Mastering Your Rental Property Analysis
Investing in real estate is a numbers game. Whether you are analyzing a single-family home, a duplex, or a multi-unit apartment complex, the difference between a profitable investment and a financial burden lies in the accuracy of your cash flow analysis. This Rental Property Cash Flow Calculator helps investors objectively evaluate the profitability of a potential purchase.
Understanding the Key Metrics
1. Monthly Cash Flow
This is the most immediate metric for buy-and-hold investors. It represents the money left in your pocket after all expenses (operating costs + debt service) are paid. Positive cash flow ensures the property pays for itself, while negative cash flow requires monthly contributions from your personal funds.
2. Cash on Cash Return (CoC)
Unlike simple ROI, Cash on Cash Return measures the efficiency of the actual capital you invested. It is calculated by dividing your Annual Pre-Tax Cash Flow by your Total Cash Invested (Down Payment + Closing Costs + Rehab Costs). A CoC of 8-12% is often considered a strong target for residential real estate.
3. Cap Rate (Capitalization Rate)
Cap Rate measures the property's natural rate of return assuming you bought it in cash. It is calculated as Net Operating Income (NOI) / Purchase Price. This metric allows you to compare properties with different financing structures side-by-side.
How to Use This Calculator
Purchase Price & Loan: Enter the negotiated price and your loan terms. The calculator separates principal and interest to determine debt service.
Vacancy Rate: Always account for vacancy. A standard rate is 5% to 8% (roughly 2-4 weeks vacant per year).
Repairs & CapEx: Many beginners forget this. Set aside 5-10% of rent for repairs (broken toilets, painting) and Capital Expenditures (new roof, HVAC replacement).
Note: This calculator assumes a fixed-rate mortgage. If you are using an ARM or interest-only loan, the monthly debt service calculations may differ.