function calculateFIRE() {
// 1. Get input values
var incomeInput = document.getElementById("annualIncome").value;
var expensesInput = document.getElementById("annualExpenses").value;
var netWorthInput = document.getElementById("currentNetWorth").value;
var returnInput = document.getElementById("investmentReturn").value;
var swrInput = document.getElementById("swr").value;
var resultsDiv = document.getElementById("resultsArea");
var errorDiv = document.getElementById("errorDisplay");
// 2. Clear previous errors/results
errorDiv.style.display = "none";
resultsDiv.style.display = "none";
// 3. Parse floats
var income = parseFloat(incomeInput);
var expenses = parseFloat(expensesInput);
var netWorth = parseFloat(netWorthInput) || 0; // Default to 0 if empty
var returnRate = parseFloat(returnInput) || 7;
var swr = parseFloat(swrInput) || 4;
// 4. Validation
if (isNaN(income) || isNaN(expenses) || income < 0 || expenses income) {
errorDiv.innerHTML = "Your expenses exceed your income. You must spend less than you earn to achieve FIRE.";
errorDiv.style.display = "block";
return;
}
if (swr 0) {
savingsRate = (annualSavings / income) * 100;
}
var fireTarget = expenses / (swr / 100);
// 6. Calculate Years to FI
// If already reached
var years = 0;
var currentPot = netWorth;
var r = returnRate / 100;
// Safety Break for infinite loops
var maxYears = 100;
var reached = false;
if (currentPot >= fireTarget) {
years = 0;
reached = true;
} else {
// Loop year by year
while (years = fireTarget) {
reached = true;
break;
}
}
}
// 7. Format Output
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
document.getElementById("displaySavingsRate").innerHTML = savingsRate.toFixed(1) + "%";
document.getElementById("displayAnnualSavings").innerHTML = formatter.format(annualSavings);
document.getElementById("displayTargetNumber").innerHTML = formatter.format(fireTarget);
var yearText = "";
if (years === 0 && reached) {
yearText = "You are FI now!";
} else if (!reached) {
yearText = "> 100 Years";
} else {
yearText = years + " Years";
}
document.getElementById("displayYears").innerHTML = yearText;
// 8. Show Results
resultsDiv.style.display = "block";
}
Understanding the Physics of FIRE
The FIRE (Financial Independence, Retire Early) movement is built on a simple mathematical truth: your time to retirement is not based on how much money you make, but on your Savings Rate. This FIRE calculator helps you visualize exactly how long you need to work based on the percentage of your take-home pay that you invest.
What is Savings Rate?
Your savings rate is the most critical metric in early retirement planning. It is calculated as:
((Income – Expenses) / Income) * 100
While a standard financial advisor might recommend saving 10-15% of your income for a retirement at age 65, FIRE adherents aim for savings rates of 50%, 60%, or even 70%. As shown in the calculator above, increasing your savings rate has a double effect:
It increases the amount of capital you are adding to your investments annually.
It decreases the amount of money you learn to live on, lowering your total "FI Number" (target).
Key Inputs Explained
Annual Take-Home Income: Use your net pay (after taxes). This is the actual money hitting your bank account available for spending or saving.
Annual Spending: Be honest with this number. It should include housing, food, insurance, travel, and discretionary spending.
Current Invested Assets: The current value of your 401k, IRAs, and taxable brokerage accounts. Do not include the equity in your primary home unless you plan to sell it to fund your retirement.
Expected Annual Return: The historical inflation-adjusted return of the stock market (S&P 500) is roughly 7%. However, conservative planners may use 5% or 6% to add a margin of safety.
Safe Withdrawal Rate (SWR): The percentage of your portfolio you can withdraw annually without running out of money. The "4% Rule" is the standard benchmark derived from the Trinity Study.
How to Interpret Your Results
Once you run the calculation, pay attention to your FIRE Target Number. This is derived by dividing your annual expenses by your Safe Withdrawal Rate (or multiplying expenses by 25 if using the 4% rule). This is the "Finish Line."
If the Time to Financial Independence seems too long, you have two levers to pull: increase your income (side hustles, career advancement) or decrease your expenses (geo-arbitrage, frugal living). Mathematically, cutting expenses is often more powerful because it lowers your target number immediately.
The Power of Compound Interest
This calculator assumes your investments compound annually. In the early years of your journey, your contributions (savings) do the heavy lifting. As your portfolio grows, the investment returns eventually surpass your annual contributions, accelerating your path to freedom. This is often called the "tipping point" in the FIRE community.