function calculateRentalCashFlow() {
// 1. Get Input Values
var monthlyRent = parseFloat(document.getElementById('monthlyRent').value);
var otherIncome = parseFloat(document.getElementById('otherIncome').value);
var vacancyRate = parseFloat(document.getElementById('vacancyRate').value);
var mortgagePayment = parseFloat(document.getElementById('mortgagePayment').value);
var propertyTax = parseFloat(document.getElementById('propertyTax').value);
var insurance = parseFloat(document.getElementById('insurance').value);
var mgmtFeeRate = parseFloat(document.getElementById('mgmtFeeRate').value);
var repairRate = parseFloat(document.getElementById('repairRate').value);
var hoaFees = parseFloat(document.getElementById('hoaFees').value);
// 2. Validate Inputs (Simple check)
if (isNaN(monthlyRent) || isNaN(mortgagePayment) || isNaN(propertyTax) || isNaN(insurance)) {
alert("Please enter valid numbers for Rent, Mortgage, Tax, and Insurance.");
return;
}
// Handle default 0 for optional fields if they are NaN (empty)
if (isNaN(otherIncome)) otherIncome = 0;
if (isNaN(vacancyRate)) vacancyRate = 0;
if (isNaN(mgmtFeeRate)) mgmtFeeRate = 0;
if (isNaN(repairRate)) repairRate = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// 3. Perform Calculations
// Income Logic
var potentialGrossIncome = monthlyRent + otherIncome;
var vacancyLoss = monthlyRent * (vacancyRate / 100);
var effectiveGrossIncome = potentialGrossIncome – vacancyLoss;
// Expense Logic
var mgmtCost = monthlyRent * (mgmtFeeRate / 100);
var repairCost = monthlyRent * (repairRate / 100);
// Total Operating Expenses (Tax + Ins + Mgmt + Repairs + HOA/Misc)
var operatingExpenses = propertyTax + insurance + mgmtCost + repairCost + hoaFees;
// NOI
var netOperatingIncome = effectiveGrossIncome – operatingExpenses;
// Cash Flow
var cashFlow = netOperatingIncome – mortgagePayment;
// 4. Update UI
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
document.getElementById('displayGrossIncome').innerHTML = formatter.format(effectiveGrossIncome);
document.getElementById('displayVacancy').innerHTML = "-" + formatter.format(vacancyLoss);
document.getElementById('displayOpsExp').innerHTML = "-" + formatter.format(operatingExpenses);
document.getElementById('displayNOI').innerHTML = formatter.format(netOperatingIncome);
document.getElementById('displayCashFlow').innerHTML = formatter.format(cashFlow);
// Color coding for cash flow
var cfElement = document.getElementById('displayCashFlow');
if (cashFlow >= 0) {
cfElement.style.color = '#fff'; // Keep white on green background
document.querySelector('.rp-cashflow-highlight').style.background = '#27ae60';
} else {
cfElement.style.color = '#fff';
document.querySelector('.rp-cashflow-highlight').style.background = '#e74c3c';
}
// Show result box
document.getElementById('rpResult').style.display = 'block';
}
Mastering Rental Property Analysis with the Cash Flow Calculator
Investing in real estate is one of the most reliable ways to build wealth, but the difference between a profitable asset and a money pit often comes down to one metric: Cash Flow. Our Rental Property Cash Flow Calculator helps investors accurately project the monthly income of a potential rental unit by accounting for all hidden expenses, not just the mortgage.
What is Rental Cash Flow?
Cash flow is the net amount of money left in your pocket after all expenses are paid. It is calculated as:
Positive cash flow means the property pays for itself and generates profit. Negative cash flow implies you must contribute money monthly to keep the asset afloat.
Key Metrics Explained
When using this calculator, it is crucial to input realistic numbers. Here is what the specific fields mean:
Vacancy Rate: No property is occupied 100% of the time. A standard safe estimate is 5-8%, which accounts for turnover periods between tenants.
Management Fees: Even if you plan to self-manage, you should account for your time or future management needs. Property management companies typically charge 8-10% of the monthly rent.
Maintenance & Repairs: Things break. Setting aside 5-10% of the monthly rent ensures you have funds for plumbing issues, painting, or roof repairs when they arise.
Net Operating Income (NOI): This is the profitability of the property before the mortgage is paid. It is a critical number for determining the Cap Rate of an investment.
How to Interpret Your Results
After entering your mortgage details, taxes, and rental estimates, the calculator provides your Net Monthly Cash Flow. A general rule of thumb for many investors is the "$100/door" rule, aiming for at least $100 in pure profit per unit per month. However, in high-appreciation markets, investors might accept lower cash flow in exchange for long-term equity growth.
Use this tool to run "what-if" scenarios. What if you raise the rent by $50? What if taxes go up by 10%? Understanding these sensitivities is key to risk management in real estate investing.