How to Calculate Your FIRE Number, Step by Step

Almost every FIRE explainer gives you the same one-line recipe: multiply your annual spending by 25 and you have your number. It is a good recipe in the sense that it is memorable and roughly right for one specific person — someone retiring at 65 with a 30-year horizon and a balanced portfolio. If that is not you, the multiple quietly answers a question you did not ask.

This guide builds the number the other way round: from your own retirement length, your own return assumptions, and the things you will actually have to pay for. It takes about ten minutes and five inputs you already know.

Step 1: Work out how long your retirement actually is

This is the single number that the 25× multiple throws away, and it is the one that moves your target most.

Retirement length is life expectancy minus the age you stop earning. Someone stopping at 50 and planning to 90 is funding 40 years. Someone stopping at 62 and planning to 88 is funding 26. Those two people can have identical spending and identical portfolios and still need targets that differ by hundreds of thousands of dollars.

Pick a life expectancy you would be comfortable being wrong about in the wrong direction. Period life tables in most developed countries put average remaining life at 65 somewhere in the low-to-mid 80s, and roughly a quarter of 65-year-olds will see 90. Planning to average is planning for a coin flip. Planning to 90 or 95 costs you a few extra working months and removes the failure mode you cannot fix from inside it.

Step 2: Establish what you actually spend

Not your income minus your savings. Not last month. Twelve months of real outflow, including the annual and irregular items that never show up in a typical month: insurance premiums, car maintenance, the dentist, the flights home at Christmas, the appliance that dies.

Two adjustments are worth making before you go on:

• Strip out costs that end with work. Commuting, a second car, the wardrobe, the lunches. For many people this is 5–15% of current spending.
• Add back costs that start with retirement. Health insurance you no longer get through an employer is the big one in the US, and it is often the largest single line in an early retiree’s budget between the day they stop working and the day public coverage begins. More free time also tends to cost money rather than save it.

The number you want is your steady-state annual spending in today’s money. Keep it in today’s money for the whole calculation and handle inflation once, in the return assumption, rather than twice.

Step 3: Pick two return numbers, not one

Most calculators ask for one growth rate and apply it to your whole life. That is wrong in an interesting way: a portfolio you are still contributing to and one you are drawing down from should not be invested the same way, so they should not be modelled the same way either.

• Pre-retirement return. You have decades of runway and can absorb drawdowns, so this is your growth allocation. 7% nominal is a common long-run assumption for a stock-heavy portfolio.
• Post-retirement return. Once withdrawals start, most people de-risk — more bonds, more cash, a shorter duration on the money they will need first. 5% nominal is a reasonable planning figure for that mix.

Then subtract inflation. What compounds against your spending is the real return:

real return = (1 + nominal) / (1 + inflation) − 1

At 5% nominal and 3% inflation that is 1.94%, not 2%. The difference looks trivial and compounds to real money over 40 years, which is why the calculator does the division rather than the subtraction.

Step 4: Price the spending itself

Here is the actual question your FIRE number answers:

How much money, sitting in a portfolio earning my post-retirement real return, funds my inflation-adjusted spending every month from the day I stop working to the day I die?

That is the present value of an annuity, and it has a closed form. With monthly withdrawals taken at the start of each month:

corpus = (annual spending / 12) × (1 − (1 + m)^−n) / m × (1 + m)

where m = monthly real return, n = months in retirement

Work it for a concrete person: $50,000 a year, retiring at 50, planning to 90, 5% post-retirement return, 3% inflation.

The 25× rule sits in the middle of that range and is only correct at one point on it. The 40-year retiree who stops at $1.25M is $144,000 short of what their own horizon asks for — about three years of spending, discovered in their eighties.

Note the last column too. Longer horizons force a lower sustainable withdrawal rate. The 4% rule was tested on 30 years; nothing about it promises to hold for 50.

Step 5: Apply the withdrawal rate as a cap, not a definition

There is a problem with pure horizon math: it lands the portfolio on exactly zero at life expectancy. Zero margin. A bad first decade and the plan is not slightly off, it is over.

So use your safe withdrawal rate the other way round — not as the definition of the target, but as a floor under how conservative the first year has to be. Compute the first-year draw the horizon corpus implies (annual spending ÷ corpus). If it comes out above your SWR, raise the corpus until it does not.

For the 30-year retiree above, horizon math implies a 4.39% first-year draw. Against a 4% SWR the cap binds, and the target rises from $1,138,856 to $1,250,000. For the 40-year retiree, the implied draw is 3.59% — already inside 4% — so horizon math governs and the target stays at $1,394,107.

Whichever of the two is larger is your spending corpus. It is worth knowing which one decided it, because that tells you which assumption to argue with.

Step 6: Add the things that are not monthly spending

Two more items belong in the target, and both are routinely left out.

An emergency buffer. In accumulation, an emergency fund stops you selling investments at the worst moment. In retirement it does the same job and matters more, because you no longer have income to rebuild it. Six months of expenses is a common floor; twelve is defensible if your spending is lumpy or your portfolio is volatile. On $50,000 a year, six months is $25,000.

One-time goals falling after you retire. A house deposit, a wedding, a child’s education, a sabbatical year, replacing the car at 70. If the money comes out of the same portfolio, it belongs in the same target. Price each one in today’s money and discount it back at your post-retirement real return; a $40,000 expense fifteen years into retirement is not $40,000 of corpus today, it is roughly $30,000.

Your FIRE number is the sum:

FIRE number = spending corpus + emergency buffer + present value of post-retirement goals

For our example: $1,394,107 + $25,000 + goals. Call it $1.42 million with a modest goal or two, against the $1.25 million the multiple would have suggested.

Step 7: Check whether your current plan gets there

The target is half the work. The other half is the trajectory: does what you are doing now actually land on it?

Take your current invested balance, add your monthly contribution, compound at your pre-retirement return, and step the contribution up each year by the rate your income realistically grows. A 5% annual step-up is not optimistic — it is what a normal career with normal raises looks like, and leaving it out understates most people’s outcome badly.

Same starting point, same returns, same effort in year one — a 44% difference in the outcome, entirely from raises you were going to get anyway.

One trap to avoid when you compare that balance to your target: compare like with like. The $1,394,107 target is in today’s money. A balance twenty years out is in future money. At 3% inflation, that target inflates to $2,517,913 by the time our 50-year-old reaches it. Either inflate the target or deflate the balance, but never compare one of each — it is the most common arithmetic error in DIY FIRE spreadsheets, and it flatters your plan by a factor of about 1.8 over twenty years.

What to do with the number once you have it

Treat it as a hypothesis with error bars, not a finish line painted on the road. Three habits make it useful:

1. Re-run it annually, and after anything structural: a move, a child, a career change, a market year that ends far from your assumption.
2. Test the assumptions that scare you, not the ones that flatter you. Drop the post-retirement return by a point. Add five years to life expectancy. Push inflation to 4%. If the plan survives all three at once, the headline number is close to irrelevant.
3. Watch which constraint binds. If the SWR cap is setting your target, your plan is governed by early-retirement risk, and flexible spending helps most. If horizon math is setting it, your plan is governed by longevity, and working slightly longer or lowering spending helps most. Those call for different responses.

The calculator on the home page does every step above live, shows the breakdown line by line, and tells you which constraint is binding. Start with the defaults, then change one input at a time and watch what moves — that sensitivity is more informative than any single headline figure.