Net Worth and FIRE: How to Know You've Hit Your Number

The question that anyone serious about financial independence eventually asks is the same one: "How do I know when I'm done?"

The answer is a number. Your FIRE number is the level of investable wealth at which the income it generates covers your spending indefinitely. Hit the number, and you can stop working. Fall short, and you can't. The arithmetic is simple even if the execution isn't.

Less obvious is what kind of number we're talking about. Your FIRE number isn't a savings rate, isn't an income target, isn't a salary you need to reach. It's a net worth target. More specifically, it's a target for the investable portion of your net worth, which makes it both narrower than total net worth and more rigorous than the casual usage suggests.

This is the practical look at how those two concepts connect. How your FIRE number derives from your net worth, what the 4% rule is for and where it breaks, and the structural details of your net worth (composition, location, liquidity) that determine whether the headline figure actually delivers what it promises. For a broader treatment of why net worth matters in the first place, see our hub guide on personal net worth.

Your FIRE number is a net worth target

A FIRE number is the investable portfolio value at which your withdrawals can sustain your lifestyle for the rest of your life with high enough probability of success.

The basic formula:

FIRE number = Annual spending × Withdrawal multiplier

For the conventional 4% rule (we'll come back to this), the multiplier is 25. £40,000 of annual spending × 25 = £1,000,000 FIRE number. £80,000 of spending × 25 = £2,000,000.

What that number actually is, in balance-sheet terms, is the level your investable net worth needs to reach. Not your total net worth (because not all assets generate income, and not all are accessible). Not your income (which is irrelevant once you've stopped working). Not your savings rate (which determines how quickly you reach the number, not what the number is).

The implication is uncomfortable: most casual FIRE talk treats the number as abstract, but the day-to-day discipline of pursuing it is just net worth tracking, applied with intent. Every monthly increment to your investable net worth is incremental progress toward the target. Every dip is a setback. The number doesn't get to you because you "feel ready"; it gets to you when the portfolio crosses the line.

The 4% rule, and where it comes from

The 4% rule is the most-cited piece of advice in the FIRE community, and it's worth understanding its actual origin before you use it.

In 1994, financial planner William Bengen studied US historical market data going back to 1926. The question he was trying to answer: what's the maximum annual withdrawal rate, expressed as a percentage of starting portfolio, that survives every 30-year period without depleting the portfolio?

The answer, in his data, was about 4%. A portfolio of 50% stocks and 50% bonds, withdrawing 4% in year one and adjusting for inflation each subsequent year, would have survived every rolling 30-year period in the dataset, including starting points like 1929, 1966 and 1973 that include severe market drawdowns.

That's the rule. Withdraw 4% annually, adjusted for inflation, and the portfolio should last 30 years. The 25x multiplier (which gets you to the FIRE number) is the inverse: 1 / 0.04 = 25.

What the rule is not, and what people often miss:

• It's not a guarantee but a "no historical failure" benchmark in a specific US dataset.
• It's not designed for retirements longer than 30 years (which most FIRE retirees plan for).
• The dataset is US-only.
• Fees, taxes and sequence-of-returns risk aren't meaningfully adjusted for.
• It's a benchmark for sustainable withdrawal under specific historical conditions, not a withdrawal strategy.

The rule is a useful starting point. It's a poor stopping point.

Calculating your FIRE number from spending

The arithmetic is simple. The hard part is knowing what your annual spending actually is.

The standard process:

Step 1: Calculate your real annual spending. Not your gross income. Not what your bank statement shows minus pension contributions. Your actual money-out: rent or mortgage, utilities, food, transport, insurance, holidays, subscriptions, taxes if relevant, and anything else that leaves your accounts in a normal year.

Step 2: Adjust for retirement-specific changes. Some costs go down in retirement (commuting, work clothes, sometimes housing if downsizing). Some go up (healthcare in countries without universal coverage, more time and money for travel). The adjustment is personal but should be conservative.

Step 3: Multiply. At a 4% withdrawal rate, multiply by 25. At a more conservative 3.5%, multiply by 28.5. At a more aggressive 5%, multiply by 20. The multiplier is your assumption about sustainable withdrawal rate; the lower it is, the larger the buffer.

Step 4: Mark the date you got the number. Your FIRE number, in today's pounds or dollars, has a date attached. Inflation will move it. A £1,000,000 target this year doesn't stay £1,000,000 in nominal terms five years from now.

We cover the detailed calculation with realistic worked examples for the UK, EU and US in How much do you need to retire early? The variation by country is significant because of tax and pension treatment.

Why the 4% rule needs adjustment outside the US

The biggest unstated assumption in the 4% rule is that you have US-style market returns. Bengen used the S&P 500 and similar. A diversified non-US portfolio doesn't necessarily get the same returns or the same volatility profile.

Recent research (notably Wade Pfau's work on international withdrawal rates) suggests that for UK and many EU investors, the "safe" withdrawal rate is closer to 3 to 3.5%. The multiplier should be 28-33, not 25. The implication is a meaningfully higher FIRE number for non-US investors targeting the same spending level.

Other adjustments worth understanding:

Sequence risk is higher with bond-heavy portfolios. If you're risk-averse and lean toward bonds, the safe withdrawal rate drops. Bengen's 50/50 wasn't conservative for the modern environment.

Fees matter. A 1% annual fee on a portfolio is approximately a 25% reduction in safe withdrawal rate over a long retirement.

Tax drag matters. UK ISA withdrawals are tax-free; US 401(k) withdrawals are income-taxed. The "real" spending the 4% rule supports depends on which wrapper the withdrawal comes from.

Longer retirement horizons reduce the safe rate. The 4% rule was for 30 years. At 50 years (an early retiree's reality), 3.25-3.5% is closer to the no-failure rate.

We've covered the international issue specifically in Why the 4% rule breaks outside the US. The summary: if you're British, European or anywhere not in the US, the 4% rule is the optimistic version of the truth. 3.5% is closer to honest.

Coast FIRE: a net worth milestone, not an end state

Coast FIRE is a useful concept once you understand it correctly. It's a milestone on the way to FIRE, not a final state.

The definition: Coast FIRE is the level of investable net worth from which compound growth alone, without any further contributions, will reach your full FIRE number by your conventional retirement age.

If your FIRE number is £1,000,000 at age 65, and you assume 5% real return on invested assets, then Coast FIRE at age 35 is approximately £230,000. A £230,000 portfolio growing at 5% real for 30 years compounds to roughly £1,000,000. At Coast FIRE you no longer need to save for retirement; you just need to keep what you have invested.

The implication is that Coast FIRE arrives much earlier than full FIRE. It's the moment you can stop saving, continue working at whatever level supports your current spending, and let compound growth do the rest.

For people who reach Coast FIRE, the work calculus changes. You don't need to maximise income any more. You can take a lower-paying job you enjoy, reduce hours, retrain, travel. The FIRE number is still arriving on schedule even if you stop contributing.

The thing to be clear about: Coast FIRE is still a net worth target. The Coast FIRE figure is a smaller, intermediate version of the same calculation. It depends on the same assumptions about returns, inflation, and your eventual retirement spending level.

Full coverage of Coast, Barista and Lean variations sits in Coast FIRE, Barista FIRE, Lean FIRE: pick your target.

Lean FIRE, Fat FIRE, Barista FIRE: variations on the same target

The same arithmetic, different lifestyle assumptions.

Lean FIRE. A FIRE number based on minimal spending. £20,000 to £25,000 of annual spending in the UK; perhaps $25,000 to $35,000 in the US. The FIRE number is smaller (£500,000 to £625,000 at 4%) and reaches faster, at the cost of a stricter lifestyle. Geographic arbitrage often features: lean FIRE in low-cost countries is a common pattern.

Fat FIRE. The opposite. Annual spending of £80,000 to £150,000 or more. FIRE number above £2,000,000. Slower to reach, but allows more discretionary spending in retirement.

Barista FIRE. A hybrid where you reach a partial FIRE number (enough to cover most expenses) and supplement with part-time work that produces just enough income to cover the gap. The "barista" framing comes from the US health insurance system, where part-time work at a company like Starbucks provides healthcare access. In the UK or EU it's less applicable as a label but the concept (partial FIRE plus optional flexible income) is the same.

In each case, the FIRE number is a target for net worth. The differences are in the spending assumption, which scales the target up or down.

It's not just the total. It's the composition.

A £1,000,000 net worth isn't the same as a £1,000,000 net worth.

Three examples:

Asset-heavy, liquid-light. £900,000 in a primary residence and £100,000 in investments. The total is £1,000,000. The investable portion is £100,000. The FIRE math breaks: 4% of £100,000 is £4,000 a year, nowhere near enough to live on. The house can be sold, but selling is slow, expensive and leaves you needing to rent. The net worth target is met on the headline, not in practice.

Balanced. £400,000 in property, £600,000 in investments. Same total. The investable portion is £600,000. 4% of £600,000 is £24,000 a year. Still short of a £40,000 spending target, but the gap is smaller and the assets can do real work.

Investable-heavy. £200,000 in property, £800,000 in investments. Same total. 4% of £800,000 is £32,000. Closer to the target.

The composition matters because not all assets produce income, and not all are accessible. A FIRE plan that conflates "net worth" with "investable wealth" overstates readiness.

Within investable wealth, further breakdown matters:

• Liquid investments (taxable brokerage, GIA): accessible any time, taxable on gain or income.
• Tax-advantaged investments (ISA, 401(k), SIPP, Roth IRA): accessible at the right age, more tax-efficient.
• Restricted investments (pensions you can't touch yet, locked-in retirement accounts): not accessible until pension age.

A FIRE plan retiring at 50 needs investable wealth that's accessible before the standard pension age in your country. That's where the bridge problem comes in.

Sequence of returns risk

Sequence of returns risk is the technical name for one of the bigger threats to a FIRE plan. It's the risk that your portfolio experiences poor returns in the early years of retirement, when withdrawals are taking money out of a portfolio that hasn't had time to recover.

Two retirees with identical 30-year average returns can have very different outcomes if the returns arrive in different order. If the first five years are below average, the early withdrawals deplete the portfolio more than expected, and the recovery has to work harder to catch up. Sometimes it can't.

The 4% rule incorporates this risk implicitly (Bengen's data included sequences with bad starts). What it doesn't do is tell you what to do about it.

Common mitigations:

Cash buffer. Hold 1 to 2 years of spending in cash or near-cash, so you don't have to sell into a falling market. Reduces sequence risk at the cost of returns.

Bond glidepath. Increase bond allocation in the first few years of retirement, then gradually reduce. Reduces volatility when you're most vulnerable.

Flexible withdrawal. Cut spending in years following major drawdowns. The Guyton-Klinger rules formalise this. Counterintuitively, willingness to take a 10% spending cut after a bad year can raise the safe withdrawal rate.

Earnings flexibility. Plan to do some paid work during early retirement years. Even modest income reduces withdrawals during the high-risk window.

The point is that the FIRE number assumes a particular kind of retirement. If your portfolio is heavily concentrated in volatile assets or you have no flexibility on spending, the same number is less robust than it looks. We've written about this in detail in Sequence of returns risk, explained plainly.

The bridge problem

For anyone retiring before standard pension age (55 to 67, depending on country), there's a structural problem: most retirement savings are locked in accounts you can't access yet.

A UK retiree at 50 can't touch their workplace pension or SIPP until 55 (rising to 57 in 2028). A US retiree at 50 with most savings in a 401(k) faces 10% early withdrawal penalties plus full income tax. A French retiree at 50 can't touch their PEA without losing tax advantages. Australian retirees can't access super until 60 in most cases.

The bridge is the gap between the desired retirement age and pension access age. Funding it requires accessible wealth outside locked accounts: taxable brokerage, ISAs (UK), cash savings, or sometimes property income.

But the FIRE number, properly calculated, includes the bridge. If you want to retire at 50 and your pension access is at 57, you need 7 years of spending in accessible form before the pension wealth even comes into play.

This is where pure 4% rule math falls apart for early retirees. A £1,000,000 FIRE number that's 70% in a SIPP and 30% in an ISA can't be drawn at 4% across the whole portfolio from age 50. You can draw on the ISA only, which means you're really withdrawing closer to 13% from a £300,000 sub-portfolio. That's not sustainable until the SIPP unlocks.

The detailed treatment is in The bridge problem: how to fund early retirement before pension age.

Inflation: the moving target

Your FIRE number is in today's pounds or dollars. Inflation moves it.

At 2% inflation, a £1,000,000 target becomes £1,219,000 in 10 years and £1,486,000 in 20 years in nominal terms (same purchasing power). At 4% inflation, the same target becomes £1,480,000 in 10 years and £2,191,000 in 20 years.

This is why the 4% rule's "adjust withdrawals for inflation each year" is more important than it looks. The portfolio has to grow at least at inflation just to maintain purchasing power.

Higher-inflation environments increase the FIRE number significantly. The post-2021 inflation shock pushed many UK and European retirees' real purchasing power down by 10 to 15% even though their nominal portfolio values held up.

If you're early in the accumulation phase, this is fine: your savings are also accumulating at higher nominal rates. If you're close to or in early retirement, it's a substantive risk that can blow a plan.

We covered the practical question in What if inflation stays at 4% for a decade?

Monte Carlo: probability, not certainty

The 4% rule gives a binary answer: this portfolio survives the historical sequences or it doesn't. Monte Carlo simulation gives a probabilistic one.

A Monte Carlo simulation generates thousands of randomised market scenarios using observed return distributions, then checks what percentage of scenarios produce a successful retirement (portfolio doesn't deplete). A "95% probability of success" plan succeeds in 95% of simulated futures.

This is a better way to think about FIRE readiness than the binary 4% rule answer. It captures the reality that markets aren't deterministic and that even a "safe" plan has a non-zero chance of failure.

Useful probability bands for FIRE planning:

• Below 80%: the plan is materially risky. Reconsider spending or work longer.
• 80-90%: plausible but with meaningful failure risk. Either accept some flexibility in retirement spending or build a larger buffer.
• 90-95%: generally accepted as "safe enough" for early retirement.
• Above 95%: very conservative. Most plans get there by saving more than strictly needed.

The probability is sensitive to assumptions. Return assumptions, inflation assumptions, sequence assumptions, lifespan assumptions. Garbage in, garbage out.

We treat the stress-testing approach in Stress-testing your FIRE number.

How Endute treats this

Endute's FIRE planning is built around the assumption that your FIRE number is a net worth target, derived from your actual spending data, and refined against the structure of your real portfolio.

Practically, that means a few things.

Multi-phase life plans. You can configure your projected life as discrete phases (Working, Coast, Retired, Drawdown) with different income sources and spending levels in each. The net worth projection covers the whole sequence.

Per-phase budgets and income. Each phase has its own spending and income assumptions, including lump sums (sale of a business, inheritance) and changes in earning patterns.

Deterministic projection. A month-by-month forecast of your net worth across the plan, using your real starting balances and assumed returns.

Monte Carlo simulation. Thousands of randomised market scenarios, producing a probability of success figure rather than a binary yes or no.

Historical backtesting. Run your plan through every historical period using Shiller data, to see how the same starting position would have fared in 1929, 1966, 2000 and every other vintage.

Sensitivity analysis. A tornado chart showing which assumptions matter most to the outcome. Often it's not the ones people expect.

Drift monitoring. Once you're in retirement, the dashboard tracks projected vs actual portfolio value and savings rate, with nudge alerts if you're falling behind plan.

Plan versioning. Save a snapshot of the plan today, change assumptions, save another, compare side by side. The historical record of how the plan changed is preserved.

The point of all of this is that the FIRE number stops being abstract. It's anchored to your real spending data, your real portfolio composition, your real geography. The probability of success is computed from your actual situation, not a US-default assumption.

For a broader overview of the approach, Retirement Planning That Actually Uses Your Numbers is the place to start.

How to know you've hit your number

A practical checklist.

Your investable net worth covers the FIRE multiplier. Total investable wealth divided by your safe withdrawal rate (3.5% if you're conservative, 4% if you're US-default) is at least equal to your annual spending. The headline check.

The composition is right. Enough accessible wealth to cover the bridge to pension age. Enough liquid wealth to cover sequence risk in the first few years. Not too concentrated in volatile assets without flexibility to ride out drawdowns.

The plan survives Monte Carlo at acceptable probability. A 90%+ probability of success across thousands of randomised scenarios, given your specific portfolio and spending. Not just a 4% rule check.

Your spending data is accurate. The FIRE number is only as good as the spending estimate. If you've been tracking spending properly for 12+ months, the number is grounded. If you're guessing, it isn't.

You've stress-tested the assumptions. What happens if inflation is 4% for a decade? What happens if there's a 30% market drop in your first year? What if you live to 100? A plan that survives these tests is more credible than one that hasn't been tested.

When all of these are true, you've hit your number.

Most people, before they reach this point, have a moment where they think they might be ready but the figures aren't quite there. The mistake at that point is either retiring anyway (and discovering the bridge or sequence problem later) or deferring indefinitely (and dying before reaching the target). The right move is to test the plan rigorously enough to know whether the gap is real or imagined.

The point of treating the FIRE number as a net worth target is that it makes the question answerable. Either the portfolio is there and structured correctly, or it isn't. The answer is in the data.

Once you have the answer, the rest is timing.