Your FI Number in Plain English

How much you need so work becomes optional

Your FI number is the portfolio size at which your investments could cover your living expenses indefinitely, making paid work a choice rather than a requirement. The arithmetic behind it is genuinely simple, which is part of why the idea spread so quickly.

The calculation itself

Take your annual spending and multiply by twenty-five. Someone spending $40,000 a year has an FI number of $1,000,000; someone spending $60,000 has an FI number of $1.5 million.

The twenty-five comes from the inverse of a 4% withdrawal rate. It is shorthand for the idea that a portfolio can sustainably provide about 4% of its value each year.

Notice what drives the result. It is your spending, not your income. Two people earning identical salaries can have FI numbers hundreds of thousands of dollars apart.

Why spending is the whole story

Every $100 a month you permanently remove from your budget lowers your FI number by roughly $30,000, because you need twenty-five times the annual amount.

That cuts both ways. A lifestyle upgrade of $500 a month raises your target by about $150,000, which can add years to the timeline.

This is why FIRE discussions obsess over expenses. The spending figure appears on both sides of the equation — it sets the target and it determines how much you can save toward it.

Key takeaway Your FI number is annual spending times twenty-five. Because spending both sets the target and determines your savings rate, controlling it moves the timeline more than anything else you can do.

Key points

$40,000 of annual spending implies an FI number near $1,000,000.
Every $100 a month of permanent spending costs roughly $30,000 of target.
The multiplier comes from the 4% withdrawal rule of thumb.
Use a more cautious multiplier of 28 to 30 for very long retirements.
Health insurance is often the largest unknown for early retirees.

Adjust the multiplier for your situation

The twenty-five multiplier came from research on roughly thirty-year retirements. Someone retiring at 40 may be planning for fifty years or more.

For those longer horizons, many people use a multiplier of twenty-eight to thirty, which corresponds to a withdrawal rate closer to 3.3% or 3.5%.

That change is not trivial. Moving from twenty-five to thirty raises a $1,000,000 target to $1,200,000, which is worth knowing before you build a plan.

Use realistic spending, not aspirational spending

Base the number on what you actually spend over a full year, including the irregular items people forget: insurance, car repairs, medical costs, gifts, and travel.

Pull twelve months of real transactions rather than estimating. Most people underestimate their spending by a meaningful margin when working from memory.

If you plan to spend differently in early retirement — more travel, less commuting — build that version of the budget explicitly rather than assuming it nets out.

What the number does not include

Health insurance is the big one in the United States, since employer coverage disappears before Medicare eligibility. That cost can be substantial and belongs in the spending figure.

Taxes continue in retirement, particularly on withdrawals from traditional accounts. Your FI number should be based on spending including taxes, not spending after them.

Access matters too. Money in retirement accounts has age restrictions, so early retirees need either taxable holdings or one of the established strategies for early access.

None of these break the concept. They mean a genuine FI plan needs a bit more detail than the multiplication that makes the idea memorable.

                    Start here: add up your actual spending over the last twelve months from your bank and card statements. That single number is the foundation of everything else in FIRE planning.
                

Try these inputs

Standard: $40,000 annual spending times 25
Cautious: same spending times 30
With health insurance: add the real premium before multiplying

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