Fat FIRE Calculator

How the calculator works

Fat FIRE means retiring early without trimming your lifestyle, which usually means a big spending figure and a big target. The calculator starts at $120,000 a year, but the formula doesn’t care what you call your budget.

Fat FIRE number = yearly spending ÷ withdrawal rate
real return     = (1 + expected return) ÷ (1 + inflation) − 1

Every month, starting from what you have invested today:
balance  = balance × (1 + real return)^(1/12) + monthly contribution
FIRE age = your age in the first month the balance reaches the fat FIRE number

It grows your balance by a month of the real return, adds your contribution, and repeats until you hit the target. That first month gives you your FIRE age, your years to go and the chart. Everything is in today’s dollars.

There’s no official definition of fat FIRE. Most people use it for spending of $100,000 a year or more, and some reserve it for well above that.

A worked example

Rachel is 40, has $900,000 invested and saves $6,000 a month. She and her partner spend about $120,000 a year and don’t want to spend less once they stop working.

At a 4% withdrawal rate their fat FIRE number is $3,000,000. With 7% returns and 3% inflation, they get there when Rachel is 55. That’s a real early retirement, but it’s also 15 more years of saving $6,000 a month, and it depends heavily on the spending figure:

Every extra $10,000 a year of spending adds $250,000 to the target at 4%. At this level, lifestyle decisions are retirement-date decisions.

How much do taxes change the number?

More than most people expect, because the spending you enter has to come out of the portfolio after tax.

Where the money sits matters. Withdrawals from traditional 401(k)s and IRAs are taxed as ordinary income. In a taxable brokerage account, you’re taxed on the gain when you sell rather than on the whole withdrawal, and long-term gains get the 0%, 15% or 20% capital gains rates depending on your income. Qualified Roth withdrawals aren’t taxed at all. Large taxable portfolios can also run into the 3.8% net investment income tax, which can apply once modified adjusted gross income passes $200,000 for single filers or $250,000 for married couples filing jointly.

The calculator doesn’t model any of this, so build it into your spending. As an illustration, suppose 15% of every dollar Rachel withdraws goes to tax. To keep $120,000, she’d need to take out $141,176 a year. Enter that instead, and the target becomes $3,529,412 and the date moves to 58. Your own rate could be well above or below 15%; it depends on your account mix and where you live. A tax professional can give you a better figure than any calculator, and a mix of account types gives you more control over it each year.

Why does fat FIRE take so long?

Because the target grows with every dollar of spending, and a high income doesn’t automatically mean a high savings rate. People on a fat FIRE path tend to earn a lot and spend a lot, which is the whole point. The savings rate is what sets the timeline. A household that earns $300,000 and spends $120,000 isn’t necessarily closer to early retirement than one that earns $120,000 and spends $50,000.

It also means a stretch of weak markets costs you more in absolute dollars. When your target is several million, a flat decade is a lot of money that didn’t grow.

Is Coast FIRE a useful stepping stone?

For most people aiming this high, yes. Your Coast FIRE number is the balance that grows into your target by a later retirement age with no more contributions.

Say Rachel would be happy to fully retire at 60 if fat FIRE took too long. Her Coast FIRE number for that is $1,400,200, and at $6,000 a month she crosses it at 48. From that point, $120,000 a year at 60 is covered even if she never saves another dollar. Everything she invests after 48 is buying an earlier date, not security, and that tends to change how people feel about a big job. The Coast FIRE calculator will show you where you stand.

What the calculator leaves out

It assumes steady returns, which is optimistic over any 15-year stretch. It ignores taxes, Social Security and the 10% additional tax on most retirement-account withdrawals before 59½. And it assumes your spending stays flat in today’s dollars, when plenty of people expect to spend more in their active early years and less later on. If your plan works only with every one of these going right, add some margin.