Most FIRE calculators give you a deterministic 4% projection with no taxes. Retirement Lab simulates 10,000 Monte Carlo market paths and applies bracket-based tax modeling on the withdrawal phase, so the answer is a probability of success — not a yes/no on a single number. Free, no account needed.
Calculate my FIRE numberThe classic FIRE formula starts from a single equation: target portfolio equals annual spending divided by your withdrawal rate.
fire_number = annual_spending / withdrawal_rate
At a 4% safe withdrawal rate, $50,000 of annual spending implies a $1.25M FIRE number ($50,000 / 0.04). Lower the withdrawal rate and the target rises; raise it and the safety margin falls.
Real markets do not return 4% every year — sequence-of-returns risk is exactly the thing the 4% rule averages away. Retirement Lab runs 10,000 Monte Carlo paths drawn from a return distribution calibrated to long-run global equities, including fat-tail risk and an adjustable skewness factor. The output is a probability distribution of where your portfolio actually lands — not a yes/no on a single number.
The 4% rule everyone quotes is pre-tax. The simulation applies bracket-based income tax, capital gains, and (where applicable) wealth tax for the residency country you choose, so the projection reflects the after-tax draw your portfolio can actually support — not a flat 15% retirement tax assumption. See how the return and tax assumptions work for the full walkthrough.
Your FIRE number is the size of an invested portfolio that, withdrawn at a sustainable rate, can fund your annual spending indefinitely. It is the threshold at which work becomes optional — investment returns can cover living expenses without further contributions. The number depends on how much you spend each year and the withdrawal rate you assume the portfolio can sustain.
The textbook formula is annual_spending / withdrawal_rate. At a 4% safe withdrawal rate, $50,000 of annual spending implies a $1.25M FIRE number. Lower the withdrawal rate to 3.5% and the FIRE number rises to about $1.43M; raise it to 5% and it drops to $1.0M. Retirement Lab refines that single-rate estimate by simulating 10,000 market sequences for your inputs — including bracket-based taxes on the withdrawal phase — so the result is a probability distribution rather than a single number.
A 4%-rule projection assumes the future plays out at the average. Real markets cluster — bad years tend to come in runs, and a poor sequence of early returns can ruin a plan that looked safe on a spreadsheet. Monte Carlo runs 10,000 different sequences for the same inputs and reports how often each outcome happens, so you can see the unlucky paths instead of averaging them away.
Yes. The simulation applies bracket-based income tax, capital gains, and (where applicable) wealth tax for the residency country you choose, on the withdrawal phase. That matters because a portfolio sized for a pre-tax FIRE target can fall short once you account for the bracket you actually retire into. Most online FIRE calculators apply a flat retirement tax rate, or skip taxes entirely.