FIRE Simulator
Today's money · Monte Carlo
Money lasts in
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Wealth over time
Each thin line is one simulated future, coloured from poorest (purple) to richest (yellow); red lines run out of money. The black line is the median, the shaded bands hold the middle 50% and 80% of futures.
When can you stop, at which spending?
Safe spending by FIRE year
Stress test: a crash right after you stop
A historical-style crash forced into the first FIRE years, random markets after it. A bad start is what breaks a withdrawal plan: you sell low to pay the bills, and that money never recovers.
Wealth at the FIRE start
Year the money runs out
How the simulation works, and what it can't see
Everything is in today's money
Returns are drawn after inflation, so a spending of 40,000 a year means today's purchasing power in every year. Nominal inputs (returns, interest rates) are converted using the inflation you set.
One year at a time
- The portfolio, and the retirement accounts invested like it, is a mix of stocks and bonds rebalanced every year. Stocks and bonds each get one random return per year, drawn independently from a log-normal distribution with the median and spread you set (or a fat-tailed one).
- Cash and fixed-interest accounts grow at their interest minus inflation. Until the FIRE year you keep saving.
- When its owner reaches the unlock age, a retirement account is withdrawn, taxed, and added to the portfolio.
- From the FIRE year, income lines (pensions, part-time work) and extra costs count, and one-off events happen in their year.
- Spending comes from cash first, then the portfolio. A future fails in the first year the reachable money can't pay the bills, even if a retirement account would unlock later.
Spending rules
- Fixed: the same real amount every year (the classic safe withdrawal rate).
- Guardrails (Guyton & Klinger): cut spending when the withdrawal rate drifts above where it started by more than the band, raise it when it drifts below.
- Vanguard dynamic: aim at a fixed share of wealth, but change spending by at most +up / −down a year.
- Spend-down (die with zero): each year, aim at what could be spent every year until the end if the money earned the assumed return, with the locked accounts and future income counted as money today; until an account unlocks, only the reachable money pays. Spending moves towards it by at most +raise / −cut a year.
- The flexible rules stay between the floor (costs you can't cut) and the ceiling. They fail only when even the floor can't be paid.
What it can't see
- Real markets have long bad stretches and recoveries; independent random years are a simplification. Use the stress test for bad starts.
- The return assumption moves the answer more than anything else. Try a cautious case.
- Taxes are simplified: a flat rate on each retirement account when it unlocks, and optionally an effective rate on portfolio withdrawals. Put everything else (income tax, wealth tax, health insurance) into the spending.
- Not modelled: pension reforms, costs rising faster than prices, how long each person lives (the plan simply runs until the age you set), currency risk.
This is a tool for thinking about a plan, not financial advice.