How it works
Your FIRE number is yearly spending divided by the withdrawal rate. At 4%, that is spending × 25. The calculator then adds what you save each month (take-home pay minus spending) to what you already have, grows it at your return, and counts the months until it reaches the target.
Worked example
You are 30 with $60,000 invested, take home $90,000 and spend $50,000. You save $40,000 a year, a 44.4% savings rate. At 4% your FIRE number is $1,250,000. At 5% a year after inflation, you get there in 17 years and 7 months, at 47.
A more cautious 3.5% withdrawal rate raises the target to $1,428,571 and adds about 21 months. Cutting spending to $45,000 does more: it lowers the target to $1,125,000 and raises savings to $45,000 a year, bringing the date forward to 15 years and 1 month.
Why spending matters twice
Every dollar you stop spending is a dollar saved now and a dollar you never have to fund later. That is why the savings rate predicts your date better than income does. Starting from zero, someone saving half their pay reaches FIRE in about 16 years at 5% real returns, whatever they earn.
Picking a withdrawal rate
The 4% rule comes from historical studies of 30-year retirements. Retiring at 45 could mean 45 years or more of withdrawals, so many early retirees plan on 3.25–3.5%. The table shows how the target and date move at each rate.
What this leaves out
Returns are shown as a steady average, but real markets swing. Taxes on withdrawals, health insurance before Medicare, Social Security and changes in spending are not modelled. Treat the result as a direction, not a date to hand in notice. This is not financial advice.
If you are happy to keep working but want to stop saving, the Coast FIRE calculator shows how close you already are. Clearing high-interest debt usually beats investing; the debt payoff calculator shows how fast.