function calculate401kReturn() {
// 1. Get input values by ID
var startBal = parseFloat(document.getElementById("k401StartBalance").value);
var contrib = parseFloat(document.getElementById("k401Contributions").value);
var endBal = parseFloat(document.getElementById("k401EndBalance").value);
var years = parseFloat(document.getElementById("k401Years").value);
// 2. Validation: Check for NaNs or invalid logic
if (isNaN(startBal) || isNaN(contrib) || isNaN(endBal) || isNaN(years)) {
alert("Please enter valid numerical values for all fields.");
return;
}
if (years 0) {
simpleReturn = (netGain / totalInvested) * 100;
}
// Annualized Return (CAGR)
// Formula: (Ending Balance / Total Invested) ^ (1/Years) – 1
// Note: This is an approximation treating contributions as if they were present at the start (or simply part of the basis).
// For precise cash-flow weighting, XIRR is needed, but this is the standard approximation for web calculators.
var cagr = 0;
if (totalInvested > 0 && endBal > 0) {
var growthFactor = endBal / totalInvested;
cagr = (Math.pow(growthFactor, (1 / years)) – 1) * 100;
} else if (totalInvested > 0 && endBal === 0) {
cagr = -100; // Total loss
}
// 4. Update the DOM with results
// Formatting numbers to currency and percentages
document.getElementById("netGainResult").innerText = "$" + netGain.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Add color for profit vs loss
var gainElement = document.getElementById("netGainResult");
if (netGain >= 0) {
gainElement.style.color = "#27ae60"; // Green
} else {
gainElement.style.color = "#c0392b"; // Red
}
document.getElementById("simpleReturnResult").innerText = simpleReturn.toFixed(2) + "%";
document.getElementById("cagrResult").innerText = cagr.toFixed(2) + "%";
// Show results
document.getElementById("resultsArea").style.display = "block";
}
How is Rate of Return Calculated in a 401(k)?
Calculating the true performance of your 401(k) can be confusing because the balance changes not just from investment gains, but also from your bi-weekly contributions and potential employer matching. Simply looking at your account balance growth does not tell you how well your investments are actually performing.
This guide explains the specific formulas used to determine your retirement portfolio's efficiency and helps you distinguish between simple growth and annualized returns.
The Components of 401(k) Growth
Before calculating returns, you must isolate investment performance from savings accumulation. Your account balance grows from three sources:
Principal (Start Balance): The money already in the account at the beginning of the period.
Contributions: New money added (from your paycheck and employer match) during the period.
Investment Returns: Capital gains, dividends, and interest generated by the market.
Method 1: Simple Return on Investment (ROI)
The Simple Return metric answers the question: "What percentage profit did I make on the total money involved?" It is useful for a quick snapshot of total profitability over a specific timeframe.
Net Gain = Ending Balance – (Beginning Balance + Contributions)
Simple Return = (Net Gain / (Beginning Balance + Contributions)) × 100
Example: You started with $100,000. You contributed $10,000. Your ending balance is $121,000.
Total Invested: $110,000
Net Gain: $121,000 – $110,000 = $11,000
Simple Return: ($11,000 / $110,000) = 10%
Method 2: Compound Annual Growth Rate (CAGR)
The Simple Return doesn't account for how long it took to earn that money. Earning 10% over 1 year is fantastic; earning 10% over 10 years is poor. The Annualized Return (CAGR) smooths out the return to show what you effectively earned per year.
This calculation assumes that your total basis (start balance + contributions) grew at a steady rate to reach your ending balance. While professional money managers use "Time-Weighted Return" or "Internal Rate of Return" (IRR) for pinpoint accuracy regarding cash flow timing, the CAGR provided by this calculator offers a solid approximation for personal finance tracking.
Why Does My 401(k) Website Show a Different Number?
If you compare the result of this calculator with your brokerage statement (e.g., Fidelity, Vanguard), you might see a discrepancy. This is usually due to:
Time-Weighted Returns: Brokerages often remove the impact of cash inflows/outflows to show pure fund manager performance.
Transaction Timing: This calculator assumes contributions are part of the total investment basis. If you contributed $10,000 on December 31st, it didn't have time to grow, which might skew a simple calculation compared to a date-specific IRR calculation.
What is a "Good" 401(k) Rate of Return?
Historically, the stock market (S&P 500) has returned an average of about 10% annually before inflation. However, 401(k) accounts are often diversified with bonds and international stocks, which lowers volatility but may also lower returns. A typical balanced portfolio often targets an annualized return of 6% to 8%.