How to Calculate Your FIRE Number (and Why the 4 Percent Rule Is a Study, Not a Promise)

The problem: the math is one division, the judgment is everything else

The FIRE number is the portfolio size at which you can stop working for money. The acronym is Financial Independence, Retire Early. The math is one division: annual expenses divided by the safe withdrawal rate. At the conventional 4% safe withdrawal rate, this simplifies to annual expenses times 25. Spend 40,000 dollars a year, multiply by 25, your FIRE number is 1,000,000 dollars.

The 4% safe withdrawal rate comes from the Trinity Study, a 1998 paper by three professors at Trinity University that tested withdrawal rates against US market returns from 1926 to 1995. The study found that a portfolio of 50% stocks and 50% bonds, with an initial withdrawal of 4% of the starting portfolio and subsequent withdrawals adjusted for inflation each year, survived 30 years in 95% of the historical scenarios. A 75% stock portfolio survived 100% of scenarios at 4%. At 5%, survival dropped below 90%. At 6%, it dropped below 80%.

The 4% rule is a study, not a promise. It is based on US market returns, which are one country's history. It assumes a 30-year retirement, which is short for someone retiring at 40. It assumes a stock-bond allocation, which is not the only option. It has a 5% failure rate, which means one in twenty 30-year retirements starting with a 4% withdrawal rate would have run out of money. For a retiree who needs the money to last 50 or 60 years — the FIRE case — the failure rate at 4% is higher than the Trinity Study's 30-year number, which is why many in the FIRE movement use 3.5% or 3% as a safer withdrawal rate.

The math of the FIRE number is one division. The judgment is everything else — which withdrawal rate to use, whether to model a longer retirement, whether to include Social Security, whether to assume real or nominal returns, whether to model taxes. A FIRE calculator does the math. You have to bring the judgment.

The FIRE Number Calculator does the math and offers the four FIRE variants (Lean, Fat, Coast, Barista) plus a sensitivity table that shows how years-to-FIRE changes with your savings rate. It is the right tool for the math. It is not a retirement plan.

Fastest path

Open the FIRE Number Calculator. Type your annual expenses (say, 55,000). Set your safe withdrawal rate (default 4%). The headline FIRE number appears: 1,375,000. Toggle to auto savings and enter your annual income (90,000). Set your expected real return (default 5%, post-inflation). Set your current savings (50,000) and current age (30). The tool computes years to FIRE. Scroll down for the variants and the savings-rate sensitivity table.

The 25x rule, and where it comes from

The 25x rule is the simplification of annual expenses divided by 0.04. At a 4% withdrawal rate, you need 25 times your annual expenses invested to withdraw 4% of the portfolio each year and have it match your expenses.

The math: 40,000 / 0.04 = 1,000,000. Or: 40,000 x 25 = 1,000,000. Both produce the same number. The 25x form is easier to remember and to do in your head.

The rule depends on the withdrawal rate. At 3%, the multiplier is 33x (1 / 0.03 = 33.3). At 3.5%, it is 28.6x. At 5%, it is 20x. The lower the withdrawal rate, the higher the multiplier, the larger the FIRE number. A conservative retiree using 3% needs 33x expenses. An aggressive retiree using 5% needs 20x. The same 40,000 of expenses gives FIRE numbers from 800,000 (at 5%) to 1,333,000 (at 3%).

The 4% rule is the default in most FIRE calculators because it is the Trinity Study's finding. It is not the only option. If your retirement is longer than 30 years, or you want a lower failure rate, or you expect lower market returns than the historical US average, you should use 3.5% or 3%. The trade-off is a higher FIRE number and more years to get there.

The four FIRE variants, and what each one actually means

The FIRE movement has split into variants, each with a different lifestyle assumption. The four common ones are Lean, Fat, Coast, and Barista.

Lean FIRE is the minimalist version. The assumption is that you can live on 70% of what most people would consider a standard expense level, and your FIRE number is 70% of your expenses times 25. The Lean FIRE number is what you need to retire with a frugal lifestyle — no car, no travel, no restaurants. The trade-off is a smaller FIRE number and a faster path, at the cost of a tighter lifestyle in retirement.

Fat FIRE is the upgraded version. The assumption is that you want to spend 200% of a standard expense level — a nicer house, travel, restaurants, a car. Your FIRE number is 200% of your expenses times 25. The Fat FIRE number is what you need to retire with a comfortable lifestyle. The trade-off is a larger FIRE number and a slower path.

Coast FIRE is the milestone, not the destination. The assumption is that you have enough invested today that, with no further contributions, compound growth alone gets you to your full FIRE number by your traditional retirement age (usually 65). The Coast FIRE number is the present value of your FIRE number, discounted back to today at your expected real return rate. The formula is FIRE / (1+r)^(years to traditional retirement). If your FIRE number is 1,000,000, your real return is 5%, and you have 35 years to traditional retirement, your Coast FIRE number is 1,000,000 / (1.05)^35 = 181,290. If you have 181,290 invested today, you can stop contributing to retirement and let it grow. You still need to cover your living expenses with work, but your retirement is funded.

Barista FIRE is the part-time version. The assumption is that you reach half your FIRE number and then cover the other half of your expenses with part-time work. The Barista FIRE number is half the full FIRE number. The trade-off is that you retire earlier (you only need half the portfolio) but you still work part-time.

The catch with Barista FIRE is that most calculators, including the FIRE Number Calculator, do not actually model the part-time income. They compute half the FIRE number and label it "Barista FIRE + part-time work." The part-time income is a label, not a computation. There is no input for how much you earn part-time, no calculation of how many years of part-time work you need, no present value of the part-time earnings. The Barista FIRE number is a threshold, and the rest of the modeling is up to you.

Why the return rate should be real, not nominal

The return rate is the most consequential input after the withdrawal rate. It determines how fast your portfolio grows during accumulation and how long it lasts during withdrawal. The mistake to avoid is using the nominal return rate when you should use the real return rate.

The nominal return rate is the headline rate. The S&P 500 has returned about 10% per year nominally since 1926. A diversified 60/40 portfolio has returned about 8%. These are the numbers you see in investment reports.

The real return rate is the nominal rate minus inflation. With 3% inflation, a 10% nominal return is a 7% real return. An 8% nominal return is a 5% real return. The real return is what your money actually buys after inflation.

For FIRE, you must use the real return, because your retirement spans decades. If you use 10% nominal and ignore inflation, you will overestimate your portfolio's growth by a factor of two or more. A 1,000,000 portfolio at 10% nominal for 30 years looks like 17.4 million. At 7% real, it is 7.6 million. At 5% real, it is 4.3 million. The difference is the difference between retiring at 40 and retiring at 55.

The FIRE Number Calculator asks for the real return directly, with a default of 5% and a helper text that says "Post-inflation." This is the right design. If your estimate of real return is too high, your FIRE number is too low and you retire too early. If your estimate is too low, your FIRE number is too high and you work too long. The default of 5% is conservative for a diversified stock-heavy portfolio, reasonable for a 60/40 portfolio, and aggressive for a bond-heavy portfolio.

Why Coast FIRE is a present-value calculation

The Coast FIRE number is the portfolio you need today so that, with no further contributions, it compounds to your full FIRE number by your traditional retirement age. The math is a present-value discount: FIRE / (1+r)^years.

If your FIRE number is 1,000,000, your real return is 5%, and you have 35 years to traditional retirement, your Coast FIRE number is 1,000,000 / (1.05)^35 = 181,290. If you have 181,290 invested today, you can stop contributing and let it grow. By the time you are 65, the portfolio will be 1,000,000 in today's dollars.

The Coast FIRE number is the threshold at which your retirement is funded by past savings alone. It does not mean you can retire. It means you no longer need to save for retirement. You still need to cover your living expenses, which means you still need to work — but you can take a lower-paying job, switch careers, or work part-time, because you do not need to save.

The common mistake with Coast FIRE is to compare it to your current savings and conclude you have coasted. The calculator shows the Coast FIRE threshold. It does not check whether your current savings exceed the threshold. You have to do that comparison yourself.

Why the years-to-FIRE calculation assumes constant contributions

The years-to-FIRE calculation is a closed-form annuity formula. It assumes you contribute a constant amount every year, the portfolio grows at a constant rate, and you stop when the portfolio reaches the FIRE number. The formula is the future value of an annuity, solved for the number of periods:

n = log((target * r + PMT) / (P * r + PMT)) / log(1 + r)

where P is the current savings, PMT is the annual contribution, r is the real return rate, and target is the FIRE number.

The assumption is constant contributions. No salary growth. No inflation escalation (because the rate is real). No career-stage income changes. No periods of unemployment. No one-time windfalls. No market crashes. The formula gives you the years to FIRE under the assumption of steady contributions and steady returns, which is the average case, not the actual case.

The sensitivity table in the FIRE Number Calculator shows how the years-to-FIRE changes with your savings rate. The clever part is that as you save more, your expenses drop (because expenses = income - savings in the table), which drops your FIRE target, which compounds the effect. A 50% savings rate does not halve the years to FIRE — it cuts it by more than half, because the FIRE target shrinks at the same time the contributions grow.

Gotchas

• The 4% rule is a study, not a promise. The Trinity Study found that 4% survived 30 years in 95% of historical US scenarios. For a 40-year retirement, the failure rate is higher. For a 50-year retirement, it is higher still. Use 3.5% or 3% if you want a lower failure rate, and accept the higher FIRE number that follows.
• Use the real return rate, not the nominal. The default of 5% is post-inflation. If you type 10% (the nominal S&P 500 return), you are double-counting inflation, and your FIRE number will be too low by a factor of two. The calculator asks for real return directly — do not convert it yourself.
• Lean and Fat FIRE ignore your SWR. The calculator computes Lean as expenses x 0.7 x 25 and Fat as expenses x 2 x 25, with the 25 hard-coded. If you set your SWR to 3.5%, the headline FIRE number rises (expenses / 0.035), but Lean and Fat do not. They are stuck at the 4% multiplier. Coast and Barista do respect the SWR, because they derive from the main FIRE number.
• Barista FIRE is not actually modeled. The calculator shows half the FIRE number and labels it "Barista FIRE + part-time work," but there is no part-time income input, no computation of how many years of part-time work you need, no present value of part-time earnings. The Barista number is a threshold, and the part-time aspect is a label.
• The years-to-FIRE assumes constant contributions. No salary growth, no inflation escalation (the rate is real, so this is fine), no periods of unemployment, no one-time windfalls, no market crashes. The formula gives the average case. Your actual path will be lumpier.
• No sequence-of-returns risk. The calculator uses a constant average return. It does not model the risk that a market crash in the first years of retirement depletes the portfolio faster than a crash in later years. This is the biggest risk in early retirement, and it is not in the calculator.
• No taxes. The calculator does not model taxes on withdrawals, taxes on investment gains, or the difference between traditional and Roth accounts. A 1,000,000 portfolio in a traditional 401(k) is not the same as 1,000,000 in a taxable brokerage, because the 401(k) withdrawals are taxed as income.
• No Social Security or pension. The calculator does not include Social Security, pensions, or other income streams in retirement. For most retirees, Social Security covers a meaningful chunk of expenses, which reduces the FIRE number needed. The calculator's FIRE number is the portfolio-only number.
• No asset allocation or glide path. The calculator uses a single return rate for the whole accumulation phase and retirement. It does not model the shift from stocks to bonds as you approach retirement, which is what most target-date funds do.
• No withdrawal-phase simulation. The calculator computes the FIRE number and stops. It does not simulate whether the portfolio survives 30, 40, or 50 years of withdrawals. The 4% rule is the Trinity Study's finding for a 30-year retirement; for a longer retirement, the failure rate is higher, and the calculator does not warn about this.
• The 25x Rule Target card is hard-coded. The overview card always shows expenses x 25, regardless of the SWR you set. If you set SWR to 3%, the headline FIRE number is expenses x 33, but the 25x Rule Target card still shows expenses x 25. Use the headline number, not the card.
• No Coast FIRE status check. The calculator reports the Coast FIRE threshold but does not compare it to your current savings to tell you whether you have coasted. You have to do that comparison yourself.
• Hard-coded USD. The calculator formats numbers as USD regardless of the locale. If you are not in the US, the numbers are in USD by convention, not your local currency.
• No persistence. The calculator state is in React, not in localStorage or the URL. Refreshing the page resets to defaults. If you want to save a scenario, take a screenshot or write down the numbers.

Summary

• The FIRE number is annual expenses divided by the safe withdrawal rate. At 4%, this is annual expenses times 25. The 4% rule comes from the Trinity Study, which found that 4% survived 30 years in 95% of historical US scenarios. For a longer retirement, use 3.5% or 3%, and accept the higher FIRE number. The 4% rule is a study, not a promise.
• The four FIRE variants are Lean (70% of expenses x 25, for a frugal lifestyle), Fat (200% of expenses x 25, for an upgraded lifestyle), Coast (the portfolio you need today so compound growth alone gets you to FIRE by traditional retirement age — formula is FIRE / (1+r)^years), and Barista (half the FIRE number, plus part-time work — but most calculators do not model the part-time income).
• Use the real return rate, not the nominal. The default of 5% is post-inflation. If you use the 10% nominal S&P 500 return, you will overestimate your portfolio's growth by a factor of two. The FIRE Number Calculator asks for real return directly, with a default of 5%.
• The years-to-FIRE calculation assumes constant contributions, no salary growth, no market crashes, no sequence-of-returns risk, no taxes, no Social Security, no asset allocation glide path, and no withdrawal-phase simulation. It gives the average case. Use the FIRE Number Calculator for the math, the Compound Interest Calculator to project a single portfolio's growth, the Emergency Fund Calculator for the pre-retirement cash buffer, and the Loan Calculator for any debt you are paying off on the way to FIRE.