How much you need

"You need 25 times your annual spending." The formula fits on one line. Almost everyone still gets it wrong. The mistake is never in the multiplication: it is in the two numbers being multiplied. Which spending, exactly? And which multiple, for your horizon, your portfolio, your taxes and your other income?

This page walks the whole calculation, from bank statement to capital target, and adds the corrections the barstool version leaves out. By the end you will be able to produce your number, with an honest range around it instead of false precision.

Step 1: what you actually spend§

The multiple amplifies everything. Miss $200 a month and your target comes out $60,000 to $80,000 short, which is one to two years of work. Your spending estimate therefore deserves more care than your choice of withdrawal rate.

Start from the statements, not from memory. Export 12 months, ideally 24, from every account and every card, and sort the lines. The gap between what people think they spend and what they spend is almost always 15 to 25%, and always in the same direction.

Put the lumpy items back in. A car every 8 years, a furnace every 15, a roof every 30, gifts, the one big trip, the vet: annualize all of it. A working rule for a homeowner is that long-run upkeep costs 1 to 2% of the property's value every year, even in a year that "cost nothing".

Budget the life you are aiming at, not the one you have. FIRE changes the shape of your spending. Less commuting, fewer meals bought out for lack of time, fewer of the small consolations a hard job pays for. More hobbies, more travel, more free hours to fill. And health coverage entirely on your own tab, which for an early retiree is usually the largest new line in the budget and the one most often underestimated (US health coverage and Social Security: the two ends of the gap).

Think in age steps. Real spending in retirement traces a smile: high and travel-heavy at the start, calmer in the middle, then rising again late in life with health care and long-term care (Real spending in retirement (the retirement smile, Die With Zero)). For the initial sizing, plan on the high early plateau and let the natural downslope work in your favor.

Step 2: the tax correction, the costliest thing people forget§

The founding studies leave taxes out, and so does the 25x multiple. Your withdrawals will be taxed in part. You are aiming at a net standard of living, but the withdrawal that funds it starts out gross.

How much is taken depends on which account the money sits in, on how much of each withdrawal is gain rather than your own capital, and on the rules where you live. That plumbing changes from country to country, and the US version of it is in US taxes in the withdrawal phase. The shape of the correction is the same everywhere. Only the gain inside a withdrawal is taxed. That embedded gain fraction climbs as the years pass. And a sheltered account defers or shrinks the bill without always erasing it. A retiree drawing a modest income can end up paying very little. One drawing a large income from an account holding decades of unrealized gains pays real money.

Turn it into a single number. Call it the friction: the share of a gross withdrawal that never reaches your bank account. Estimate it, write it down as an assumption, and gross your target spending up by it. A few percent for a modest, well-sheltered plan; more, sometimes a good deal more, for a large one drawn from a taxable account. Then replace the assumption with your own figure once you have run the numbers on your own accounts, and revisit it at the annual review, because it drifts upward as gains pile up.

Step 3: subtract your non-portfolio income§

The portfolio does not have to fund what other income will fund. Three categories, each handled differently.

Your state pension. Even someone who stops at 42 has usually banked years of contributions, and will draw a pension in their sixties: smaller than a full career would have earned, but real. Ignoring it is the most common cautious mistake, and it is paid for in years of work nobody needed. Do not subtract it from the budget, though, because it does not arrive for another twenty or twenty-five years. Model it as deferred income instead: a dated flow, in constant dollars, that lightens the withdrawals from the year it starts. Any serious simulator accepts such a flow. The effect on failure probability is often dramatic, because a pension relieves exactly the long scenarios, the ones where the portfolio runs dry (US health coverage and Social Security: the two ends of the gap).

Near-certain income: rent from a property you own (Real estate in a FIRE plan (your home, and rentals)), a pension already claimed, an annuity. Subtract these from the spending, after a prudence haircut for what can go wrong: vacancy, repairs, the tax they carry of their own.

Hoped-for but unguaranteed income: part-time work, a hobby that starts paying (Barista, Coast, side income: work you choose). Do not subtract these when you size the plan. Treat them as margin. The plan has to hold without them. All they do is soften the crossing, the years spent inside a bear market.

Step 4: choose the multiple§

The multiple is the inverse of the withdrawal rate (The 4% rule in ten minutes). Choosing one is choosing the other, so what you are really trading is extra years of work against the risk of having to adjust later. Here are the markers research offers (ERN's Safe Withdrawal Rate series: a reader's guide, Modern guardrails (Morningstar): the state of the art, Beyond the United States: Anarkulova, Cederburg and the world sample):

Two forces pull the choice in opposite directions, and naming them is what makes the trade honest. Pulling the rate down, the multiple up: a long horizon, expensive starting valuations (Valuations, the CAPE, and what they say about your withdrawal rate), a world wider than the American sample, fees and taxes. Pulling the rate up, the multiple down: genuine spending flexibility, future income (that pension again), the ability to go back to work, an adaptive withdrawal rule (Choosing your strategy: criteria, comparison, worked case). One more fact belongs on that list and usually gets left off. The simulator's binary "success" is too blunt a measure. Most of the historical "failures" at 3.5% are paths where the trouble was visible ten years ahead (When to worry, and when to let it ride).

Step 5: assemble it, then stress it§

The full calculation fits in five lines. Here it is on a realistic case.

The number you land on is not a sacred finish line. It is the center of a range. Stress it: plus or minus 10% on the spending, plus or minus half a point on the rate, the pension pushed back two years. If the conclusion (in practice, your departure date) survives those shakes, the plan is solid. If it flips, you know which input to work on. Replaying the plan under a handful of variants is the most profitable thing you can do with a simulator, and far more useful than staring at the first verdict it printed.

The shortcuts that fool people§

"I'll just use what I spend now." Almost always wrong, and wrong in both directions at once: too high (a mortgage that ends, children who leave, work expenses that disappear) and too low (health coverage, medical bills, the hobbies that fill freed-up hours, the home repairs you kept putting off). Do the real work of step 1.

"The simulator says 97%, so I'm fine." A simulator is worth its inputs and its model, and nothing more (Simulator traps: independence, US bias, survivorship). 97% on US historical windows, with spending underestimated by 15%, is false comfort. Better practice: several models read side by side (Historical windows, bootstrap, parametric: the three families of models), audited inputs, and margin.

"I'll count my home in my capital." The home you live in cuts your spending, since you pay no rent, but it produces no withdrawals. Counting it in the target capital counts that effect twice. It stays a reserve of last resort (selling, downsizing, a reverse mortgage, Real estate in a FIRE plan (your home, and rentals)): a margin, not an asset of the plan.

"I hit the number, then I stop thinking about it." The target moves with your life. Recompute it once a year, in ten minutes, at the annual review (The annual review: the retiree's checklist).

The essentials§

• The target = gross spending (tax on the withdrawals included) × 25 to 33.
• The shakiest term is the spending: 24 months of statements, lumpy items annualized, health coverage added, three budgets (floor, comfort, dream).
• A state pension goes into the simulator as deferred income. Leaving it out costs years of work nobody needed.
• The multiple is a posture call between 25 and 33. 28 to 29 (3.5%) is the modern working compromise for a diversified early exit.
• The result is a range to stress, not a number to carve. Next comes Using the FIRE simulator to put it to the test, then Withdrawal strategies: the map before the territory to choose how to spend it.

Going further§

• Early Retirement Now, SWR Series parts 2 (preserving capital against consuming it) and 28 (the toolbox): earlyretirementnow.com (ERN's Safe Withdrawal Rate series: a reader's guide).
• Your own pension statement, from whoever owes you the pension: the official estimate is the input for the deferred-income line of your plan. In the United States that is the Social Security Statement in a my Social Security account (ssa.gov/myaccount, US health coverage and Social Security: the two ends of the gap).
• Morningstar, The State of Retirement Income: the recommended rates, recomputed every year (Modern guardrails (Morningstar): the state of the art).
• The FIRE simulator (Using the FIRE simulator), and exactly what it computes (Under the hood: how the simulator computes this book).