function calculateRentalMetrics() {
// 1. Get Input Values using var
var price = parseFloat(document.getElementById("purchasePrice").value);
var rent = parseFloat(document.getElementById("monthlyRent").value);
var downPerc = parseFloat(document.getElementById("downPaymentPerc").value);
var interest = parseFloat(document.getElementById("interestRate").value);
var term = parseFloat(document.getElementById("loanTerm").value);
var taxIns = parseFloat(document.getElementById("propertyTaxInsurance").value);
var maint = parseFloat(document.getElementById("maintenance").value);
var vacancy = parseFloat(document.getElementById("vacancyRate").value);
// Validation: Check if inputs are numbers
if (isNaN(price) || isNaN(rent) || isNaN(downPerc) || isNaN(interest) || isNaN(term) || isNaN(taxIns) || isNaN(maint) || isNaN(vacancy)) {
alert("Please fill in all fields with valid numbers.");
return;
}
// 2. Core Calculations
// Initial Cash Invested
var downPaymentAmount = price * (downPerc / 100);
var loanAmount = price – downPaymentAmount;
// Mortgage Calculation (Principal + Interest)
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyRate = (interest / 100) / 12;
var totalPayments = term * 12;
var monthlyMortgage = 0;
if (interest === 0) {
monthlyMortgage = loanAmount / totalPayments;
} else {
monthlyMortgage = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, totalPayments)) / (Math.pow(1 + monthlyRate, totalPayments) – 1);
}
// Income Calculations
var monthlyVacancyLoss = rent * (vacancy / 100);
var effectiveGrossIncome = rent – monthlyVacancyLoss;
// Expense Calculations (Operating Expenses excluding mortgage)
var totalOperatingExpenses = taxIns + maint;
// Net Operating Income (NOI)
// NOI = Income – Operating Expenses
var monthlyNOI = effectiveGrossIncome – totalOperatingExpenses;
var annualNOI = monthlyNOI * 12;
// Cash Flow
// Cash Flow = NOI – Mortgage
var monthlyCashFlow = monthlyNOI – monthlyMortgage;
var annualCashFlow = monthlyCashFlow * 12;
// ROI Metrics
// Cap Rate = Annual NOI / Purchase Price
var capRate = (annualNOI / price) * 100;
// Cash on Cash Return = Annual Cash Flow / Total Cash Invested
var cashOnCash = (annualCashFlow / downPaymentAmount) * 100;
// 3. Update DOM with Results
document.getElementById("resDownPayment").innerText = "$" + downPaymentAmount.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resMortgage").innerText = "$" + monthlyMortgage.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resNOI").innerText = "$" + monthlyNOI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Format Cash Flow color
var cfElement = document.getElementById("resCashFlow");
cfElement.innerText = "$" + monthlyCashFlow.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
if (monthlyCashFlow >= 0) {
cfElement.className = "rental-result-value rental-highlight";
} else {
cfElement.className = "rental-result-value rental-negative";
}
document.getElementById("resCapRate").innerText = capRate.toFixed(2) + "%";
// Format CoC color
var cocElement = document.getElementById("resCoC");
cocElement.innerText = cashOnCash.toFixed(2) + "%";
if (cashOnCash >= 0) {
cocElement.className = "rental-result-value rental-highlight";
} else {
cocElement.className = "rental-result-value rental-negative";
}
// Show results area
document.getElementById("resultsArea").style.display = "block";
}
Why Use a Rental Property Cash Flow Calculator?
Investing in real estate is one of the most reliable ways to build wealth, but it requires precise math. A Rental Property Cash Flow Calculator helps investors determine if a potential property is an asset or a liability. By analyzing purchase price, financing costs, and operating expenses, you can predict the profitability of a real estate deal before you sign any contracts.
Key Metrics Explained
1. Net Operating Income (NOI)
NOI is the total income a property generates minus all necessary operating expenses. This includes rent minus vacancy losses, property management fees, taxes, insurance, and repairs. Crucially, NOI excludes mortgage payments and interest tax. It represents the property's ability to generate revenue purely from operations.
2. Cap Rate (Capitalization Rate)
The Cap Rate measures the natural rate of return on the property independent of debt. It is calculated by dividing the Annual NOI by the Property's Purchase Price. A higher Cap Rate generally indicates a higher potential return, though it may come with higher risk. In many markets, a Cap Rate between 5% and 10% is considered good.
3. Cash on Cash Return (CoC)
This is arguably the most important metric for investors using leverage (mortgages). It calculates the cash income earned on the cash invested. If you put $50,000 down and the property generates $5,000 in net cash flow per year, your Cash on Cash return is 10%. This allows you to compare real estate returns directly against other investments like stocks or bonds.
How to Improve Cash Flow
Increase Rent: Keeping rents at market value is the fastest way to boost NOI.
Reduce Vacancy: Long-term tenants reduce turnover costs and vacancy losses.
Refinance: Securing a lower interest rate can significantly reduce monthly mortgage payments, instantly boosting cash flow.
Use the calculator above to run multiple scenarios. Adjust the "Vacancy Rate" or "Maintenance" costs to see how sensitive your investment is to unexpected expenses. Successful investors always account for the worst-case scenario to ensure they remain solvent.