Amount currently saved/invested towards retirement.
Adjusted for inflation (real return). Typically 5-7%.
Percentage of portfolio withdrawn annually (Rule of 4%).
Savings Rate:0%
Annual Savings:$0
FI Target Number:$0
Years to Financial Independence:0 Years
function calculateFI() {
// 1. Get input values
var income = parseFloat(document.getElementById('fi_income').value);
var expenses = parseFloat(document.getElementById('fi_expenses').value);
var currentPortfolio = parseFloat(document.getElementById('fi_portfolio').value) || 0;
var annualReturnPercent = parseFloat(document.getElementById('fi_return').value);
var swrPercent = parseFloat(document.getElementById('fi_swr').value);
// 2. Validation
if (isNaN(income) || isNaN(expenses) || isNaN(annualReturnPercent) || isNaN(swrPercent)) {
alert("Please enter valid numbers for Income, Expenses, Return Rate, and SWR.");
return;
}
if (expenses >= income) {
document.getElementById('fi-results').style.display = 'block';
document.getElementById('res_rate').innerText = "0% (or negative)";
document.getElementById('res_savings').innerText = "$0";
document.getElementById('res_target').innerText = "N/A";
document.getElementById('res_years').innerText = "Never (Expenses exceed Income)";
return;
}
if (expenses = fiNumber) {
document.getElementById('fi-results').style.display = 'block';
document.getElementById('res_rate').innerText = savingsRate.toFixed(2) + "%";
document.getElementById('res_savings').innerText = "$" + annualSavings.toLocaleString();
document.getElementById('res_target').innerText = "$" + fiNumber.toLocaleString();
document.getElementById('res_years').innerText = "0 Years (You are FI!)";
return;
}
// 4. Time to FI Calculation (Logarithmic Formula for Compound Interest with Contributions)
// Formula: n = ln((Target * r + c) / (Current * r + c)) / ln(1 + r)
// Where: Target = FI Number, r = annual return decimal, c = annual contribution
var r = annualReturnPercent / 100;
var yearsToFI = 0;
if (r === 0) {
// Simple division if return is 0%
var remaining = fiNumber – currentPortfolio;
yearsToFI = remaining / annualSavings;
} else {
var numerator = (fiNumber * r) + annualSavings;
var denominator = (currentPortfolio * r) + annualSavings;
// Avoid log of negative or zero errors
if (denominator <= 0) {
yearsToFI = Infinity;
} else {
yearsToFI = Math.log(numerator / denominator) / Math.log(1 + r);
}
}
// 5. Output Formatting
document.getElementById('fi-results').style.display = 'block';
document.getElementById('res_rate').innerText = savingsRate.toFixed(1) + "%";
document.getElementById('res_savings').innerText = "$" + annualSavings.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('res_target').innerText = "$" + fiNumber.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Format years
var yearsText = "";
if (!isFinite(yearsToFI) || yearsToFI 0 ? ", " + months + " Months" : "");
}
document.getElementById('res_years').innerText = yearsText;
}
Understanding Your FI Savings Rate
The Financial Independence (FI) Savings Rate Calculator helps you determine the most critical metric in the FIRE (Financial Independence, Retire Early) movement: your savings rate. Unlike traditional retirement calculators that focus on total accumulated wealth, this tool focuses on the gap between your income and expenses.
How the FI Math Works
The time it takes to reach financial independence is primarily determined by your savings rate, assuming you invest the difference between your income and expenses. The logic relies on two main components:
The Savings Rate: Calculated as (Income - Expenses) / Income. A higher rate means you are saving more AND living on less, which doubly accelerates your path to freedom.
The FI Number: This is the target portfolio size needed to sustain your lifestyle indefinitely. It is calculated as Annual Expenses / Safe Withdrawal Rate.
Safe Withdrawal Rate (SWR)
The default setting in this calculator is 4%, based on the famous "Trinity Study." This rule suggests that if you have a portfolio invested in a mix of stocks and bonds, you can withdraw 4% of the initial value (adjusted for inflation) every year with a very high probability of never running out of money.
For example, if you spend $40,000 per year, your FI Number using the 4% rule is $1,000,000 ($40,000 / 0.04).
Interpreting Your Results
Savings Rate: If your savings rate is 10%, it takes 9 years of work to save for 1 year of living expenses. If your savings rate is 50%, 1 year of work buys 1 year of freedom. At 75%, 1 year of work buys 3 years of freedom.
Years to FI: This calculation accounts for compound interest (Investment Return). While saving money is the fuel, investment returns act as an accelerant. The calculator uses a logarithmic formula to determine exactly how long until your invested assets grow large enough to support your annual expenses based on your SWR.