function calculateRental() {
// 1. Get Inputs
var price = parseFloat(document.getElementById('rc-price').value);
var closing = parseFloat(document.getElementById('rc-closing').value) || 0;
var down = parseFloat(document.getElementById('rc-down').value);
var rehab = parseFloat(document.getElementById('rc-rehab').value) || 0;
var rate = parseFloat(document.getElementById('rc-rate').value);
var term = parseFloat(document.getElementById('rc-term').value);
var rent = parseFloat(document.getElementById('rc-rent').value);
var taxYear = parseFloat(document.getElementById('rc-tax').value) || 0;
var insYear = parseFloat(document.getElementById('rc-insurance').value) || 0;
var hoaMonth = parseFloat(document.getElementById('rc-hoa').value) || 0;
var vacancyPct = parseFloat(document.getElementById('rc-vacancy').value) || 0;
var capexPct = parseFloat(document.getElementById('rc-capex').value) || 0;
// Validation
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(term) || isNaN(rent)) {
alert("Please fill in all required fields (Price, Down Payment, Rate, Term, Rent).");
return;
}
// 2. Loan Calculations
var loanAmount = price – down;
var monthlyRate = (rate / 100) / 12;
var numberOfPayments = term * 12;
var monthlyPI = 0;
if (rate === 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// 3. Expense Calculations
var taxMonth = taxYear / 12;
var insMonth = insYear / 12;
var vacancyMonth = rent * (vacancyPct / 100);
var capexMonth = rent * (capexPct / 100);
var totalMonthlyExpenses = monthlyPI + taxMonth + insMonth + hoaMonth + vacancyMonth + capexMonth;
var operatingExpenses = taxMonth + insMonth + hoaMonth + vacancyMonth + capexMonth; // Excludes Mortgage for NOI
// 4. Profit Metrics
var monthlyCashFlow = rent – totalMonthlyExpenses;
var annualCashFlow = monthlyCashFlow * 12;
var noiMonth = rent – operatingExpenses;
var noiYear = noiMonth * 12;
var totalCashInvested = down + closing + rehab;
// ROI & Cap Rate
var cocReturn = (annualCashFlow / totalCashInvested) * 100;
var capRate = (noiYear / price) * 100;
// 5. Update UI
document.getElementById('res-mortgage').innerText = formatCurrency(monthlyPI);
document.getElementById('res-expenses').innerText = formatCurrency(totalMonthlyExpenses);
document.getElementById('res-noi').innerText = formatCurrency(noiMonth);
var cashFlowEl = document.getElementById('res-cashflow');
cashFlowEl.innerText = formatCurrency(monthlyCashFlow);
if(monthlyCashFlow < 0) {
cashFlowEl.className = "rc-result-value rc-highlight rc-bad";
} else {
cashFlowEl.className = "rc-result-value rc-highlight";
}
document.getElementById('res-coc').innerText = cocReturn.toFixed(2) + "%";
document.getElementById('res-cap').innerText = capRate.toFixed(2) + "%";
document.getElementById('res-cash-needed').innerText = formatCurrency(totalCashInvested);
// Show results
document.getElementById('rc-results').style.display = "block";
}
function formatCurrency(num) {
return "$" + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
Mastering the Numbers: How to Use a Rental Property Calculator
Investing in real estate is one of the most reliable ways to build wealth, but it relies heavily on the accuracy of your math. A Rental Property Cash Flow Calculator is an essential tool for investors to determine whether a potential property will be a profitable asset or a financial drain. This guide explains how to analyze a deal and what the metrics mean.
Understanding the Inputs
To get an accurate result, you need realistic data. Here is what each field in the calculator represents:
Purchase Price & Down Payment: The agreed price of the home and the cash you are putting down upfront. Typically, investment properties require 20-25% down.
Rehab/Repair Costs: If you are buying a "fixer-upper," estimate the immediate costs needed to make the property rentable. This is added to your total cash investment.
Operating Expenses: These are often underestimated. Beyond mortgage payments, you must account for property taxes, insurance, and HOA fees.
Vacancy & Maintenance (CapEx): Smart investors set aside a percentage of rent (usually 5-10% each) for months when the unit is empty and for future big-ticket repairs like a new roof or HVAC system.
Pro Tip: Never calculate deals based on "best case" scenarios. Always factor in vacancy and maintenance reserves to ensure you have positive cash flow even when things go wrong.
Key Metrics Explained: Cash Flow, CoC, and Cap Rate
Once you hit "Calculate," you'll see three critical numbers. Here is how to interpret them:
1. Monthly Cash Flow
This is the net amount of money left in your pocket after all expenses (including the mortgage) are paid.
Formula: Total Rental Income – Total Expenses Goal: Most investors look for at least $100-$300 per door in positive monthly cash flow.
2. Cash on Cash Return (CoC ROI)
This percentage tells you how hard your actual cash investment is working. It compares your annual profit to the cash you actually put into the deal (Down Payment + Closing Costs + Rehab).
Formula: (Annual Cash Flow / Total Cash Invested) * 100 Goal: A "good" CoC return varies by market, but 8-12% is a common benchmark for residential rentals.
3. Cap Rate (Capitalization Rate)
The Cap Rate measures the property's natural rate of return assuming you bought it in all cash (no loan). It allows you to compare properties apples-to-apples regardless of financing.
Formula: (Net Operating Income / Purchase Price) * 100 Goal: Higher is generally better, but higher cap rates often come with higher risk (worse neighborhoods). A 5-8% Cap Rate is typical for stable areas.
Why Negative Cash Flow is a Warning Sign
If the calculator shows a negative cash flow (highlighted in red), the property costs more to hold than it generates in income. Unless you are banking entirely on massive appreciation (which is speculative and risky), you should negotiate a lower price, increase the down payment, or walk away from the deal.