function calculateRental() {
// 1. Get Values
var price = parseFloat(document.getElementById('purchase_price').value);
var downPercent = parseFloat(document.getElementById('down_payment').value);
var rate = parseFloat(document.getElementById('interest_rate').value);
var years = parseFloat(document.getElementById('loan_term').value);
var closingCosts = parseFloat(document.getElementById('closing_costs').value);
var rehabCosts = parseFloat(document.getElementById('rehab_costs').value);
var rent = parseFloat(document.getElementById('monthly_rent').value);
var annualTax = parseFloat(document.getElementById('property_tax').value);
var annualIns = parseFloat(document.getElementById('insurance').value);
var hoa = parseFloat(document.getElementById('hoa_fee').value);
var vacancyPct = parseFloat(document.getElementById('vacancy_rate').value);
var maintPct = parseFloat(document.getElementById('maintenance_rate').value);
var capexPct = parseFloat(document.getElementById('capex_rate').value);
var mgmtPct = parseFloat(document.getElementById('management_rate').value);
// 2. Validation
if (isNaN(price) || isNaN(downPercent) || isNaN(rate) || isNaN(years) || isNaN(rent) || isNaN(annualTax) || isNaN(annualIns)) {
document.getElementById('calc-error').style.display = 'block';
document.getElementById('results-area').style.display = 'none';
return;
}
document.getElementById('calc-error').style.display = 'none';
// 3. Loan Calculations
var downAmount = price * (downPercent / 100);
var loanAmount = price – downAmount;
var monthlyRate = rate / 100 / 12;
var numPayments = years * 12;
var monthlyMortgage = 0;
if (rate === 0) {
monthlyMortgage = loanAmount / numPayments;
} else {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) / (Math.pow(1 + monthlyRate, numPayments) – 1);
}
// 4. Monthly Expenses Calculations
var monthlyTax = annualTax / 12;
var monthlyIns = annualIns / 12;
var vacancyCost = rent * (vacancyPct / 100);
var maintCost = rent * (maintPct / 100);
var capexCost = rent * (capexPct / 100);
var mgmtCost = rent * (mgmtPct / 100);
var operatingExpenses = monthlyTax + monthlyIns + hoa + vacancyCost + maintCost + capexCost + mgmtCost;
var totalMonthlyCosts = monthlyMortgage + operatingExpenses;
// 5. Profit Metrics
var monthlyCashFlow = rent – totalMonthlyCosts;
var annualCashFlow = monthlyCashFlow * 12;
// Net Operating Income (NOI) = Income – Operating Expenses (Excludes Mortgage)
var monthlyNOI = rent – operatingExpenses;
var annualNOI = monthlyNOI * 12;
// Total Cash Invested
var totalInvested = downAmount + closingCosts + rehabCosts;
// Cash on Cash ROI
var cocROI = 0;
if (totalInvested > 0) {
cocROI = (annualCashFlow / totalInvested) * 100;
}
// Cap Rate
var capRate = 0;
if (price > 0) {
capRate = (annualNOI / price) * 100;
}
// 6. Display Results
var fmtMoney = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
var fmtPct = new Intl.NumberFormat('en-US', { style: 'percent', minimumFractionDigits: 2 });
document.getElementById('out_mortgage').innerHTML = fmtMoney.format(monthlyMortgage);
document.getElementById('out_opex').innerHTML = fmtMoney.format(operatingExpenses);
document.getElementById('out_total_costs').innerHTML = fmtMoney.format(totalMonthlyCosts);
document.getElementById('out_noi').innerHTML = fmtMoney.format(monthlyNOI);
var cfElement = document.getElementById('out_cashflow');
cfElement.innerHTML = fmtMoney.format(monthlyCashFlow);
if(monthlyCashFlow >= 0) {
cfElement.className = "result-value highlight-positive";
} else {
cfElement.className = "result-value highlight-negative";
}
var roiElement = document.getElementById('out_coc_roi');
roiElement.innerHTML = cocROI.toFixed(2) + "%";
if(cocROI >= 0) {
roiElement.className = "result-value highlight-positive";
} else {
roiElement.className = "result-value highlight-negative";
}
document.getElementById('out_cap_rate').innerHTML = capRate.toFixed(2) + "%";
document.getElementById('results-area').style.display = 'block';
}
Mastering Your Rental Property Cash Flow Analysis
Investing in real estate is one of the most reliable ways to build wealth, but the difference between a profitable asset and a financial burden often comes down to the numbers. A Rental Property Cash Flow Calculator is an essential tool for investors to evaluate potential deals, forecast returns, and ensure liquidity.
What is Positive Cash Flow?
Cash flow is the net amount of money left over each month after all operating expenses and debt service (mortgage payments) have been paid. Positive cash flow means the property is generating income for you every month. Conversely, negative cash flow implies you are paying out of pocket to hold the property. While appreciation is a long-term benefit, positive cash flow is critical for sustaining your investment business day-to-day.
Understanding the Key Metrics
This calculator provides three critical metrics to help you judge the quality of a real estate investment:
Net Operating Income (NOI): This represents the profitability of the property before leverage (loans) is considered. It is calculated by subtracting operating expenses (vacancy, taxes, insurance, maintenance) from the gross rental income. NOI is crucial for calculating the Cap Rate.
Cap Rate (Capitalization Rate): Calculated as (NOI / Purchase Price) × 100, the Cap Rate helps you compare the return on investment of different properties regardless of how they are financed. A higher cap rate generally indicates a better return, though it may come with higher risk.
Cash on Cash ROI: This is arguably the most important metric for investors using leverage. It measures the annual cash return relative to the actual cash you invested (down payment + closing costs + rehab). It answers the question: "For every dollar I put into this deal, how much am I getting back this year?"
Hidden Expenses that Kill Cash Flow
Novice investors often overestimate cash flow by ignoring non-monthly expenses. Our calculator includes fields for these crucial "hidden" costs:
Vacancy Rate: Properties won't be rented 100% of the time. Setting aside 5-10% of rent helps cover periods between tenants.
CapEx (Capital Expenditures): Roofs, HVAC systems, and water heaters eventually need replacement. Allocating a percentage of rent (typically 5-10%) to a reserve fund prevents large repair bills from destroying your returns.
Property Management: Even if you manage the property yourself, it is wise to account for this cost (typically 8-10%) to ensure the deal still makes sense if you decide to hire a professional later.
How to Improve Your Numbers
If the calculator shows negative cash flow or low ROI, consider negotiating a lower purchase price to reduce the loan amount, shopping for better insurance rates, or looking for value-add opportunities to justify higher rent. Remember, successful real estate investing is not just about buying property; it's about buying the right property at the right numbers.