How to Calculate Your FIRE Number, Step by Step

Your FIRE number is the size of the investment portfolio that could pay for your lifestyle indefinitely, so that working becomes optional. FIRE stands for Financial Independence, Retire Early, and almost every plan in that movement starts with this one figure. Calculating your FIRE number takes two inputs: what you expect to spend each year, and the share of the portfolio you plan to withdraw each year. This guide shows the formula, works through examples with the same functions that power our Coast FIRE calculator and Barista FIRE calculator, and covers the adjustments that turn a rough target into a usable plan.

The FIRE number formula

The core formula fits on one line:

With a 4% withdrawal rate, dividing by 0.04 is the same as multiplying by 25, which is why you will often hear the "25x rule". With 3.5% the multiplier is about 28.6, and with 3% it is about 33.3.

Using example spending of $40,000 a year:

The withdrawal rate is not a small detail. Moving from 4% to 3% raises the target from $1,000,000 to $1,333,333, a third more money to save. Our guide to the 4% rule and safe withdrawal rates explains where these percentages come from and why early retirees often choose something below 4%.

Step 1: Work out your annual spending

The spending figure matters more than any other input, because the FIRE number scales directly with it. Every extra $1,000 of annual spending adds $25,000 to the target at a 4% withdrawal rate. A few practical rules help:

• Start from real data. Pull twelve months of bank and card statements and total what you actually spent. Budgets tend to be optimistic; statements are not.
• Adjust for life changes. Some costs fall in early retirement (commuting, work clothes, retirement contributions themselves). Others rise, especially health insurance. In the US, people who retire before Medicare eligibility at 65 typically buy coverage through the ACA marketplace or another source, and that premium belongs in the budget.
• Include irregular costs. Car replacements, home repairs, travel and gifts happen every few years. Spread them into an annual average.
• Think about taxes. Withdrawals from traditional 401(k)s and IRAs are taxed as ordinary income, and gains in taxable accounts may be taxed too. Either add an estimate of taxes to your spending, or use a lower withdrawal rate to leave room for them.

Step 2: Choose a withdrawal rate

The withdrawal rate encodes how long the money needs to last and how much risk you accept. A 30-year traditional retirement has been the classic setting for the 4% rule. A 45- or 50-year early retirement is a harder test, and many FIRE planners use 3.25% to 3.5% for it. Others keep 4% but commit to cutting spending in bad markets. There is no single correct answer, so it is worth computing the number at two or three rates and seeing how sensitive your plan is.

Step 3: Decide what currency your number is in

A FIRE number calculated from today's spending is in today's dollars. If you are 25 years away, prices will be higher by then. At an example 3% inflation rate, $40,000 of spending today costs about $83,751 in 25 years, according to our inflation calculator. That does not mean you need to multiply the FIRE number by the same factor and then project nominal returns. The simpler and more consistent approach is to keep everything in today's dollars and use a real (after-inflation) return when projecting your portfolio. Our guide to real vs nominal returns explains why mixing the two is the most common mistake in retirement projections.

Step 4: Estimate when you will get there

Once you have a target, the next question is how long it takes. That depends on your current portfolio, how much you add each month and the real return you assume. With example inputs of $50,000 invested at age 30, $2,000 saved per month and a 5% real return, the projection crosses:

• the 4% target of $1,000,000 at about age 51;
• the 3.5% target of $1,142,857 at about age 53;
• the 3% target of $1,333,333 at about age 55.

Raising monthly savings from $2,000 to $3,000 brings the 4% date forward to about age 47. Savings rate usually matters more than return assumptions during the first decade, because the portfolio is small and new money is a large share of each year's growth.

Step 5: Account for income you will still have

Many people do not stop earning completely. Part-time work, freelance projects, rental income or a pension all reduce what the portfolio has to provide. The adjusted formula is:

This is the idea behind Barista FIRE. If the example household earns $20,000 a year from part-time work, the portfolio needs to cover only $20,000, and the target falls from $1,000,000 to $500,000, a reduction of 50%. Our comparison of Coast FIRE vs Barista FIRE shows how each variation changes the plan.

Social Security deserves its own mention. Benefits typically begin between 62 and 70, which is often decades after an early retirement date. One way to handle this is to compute two numbers: a larger bridge amount that covers full spending until benefits start, and a smaller long-term amount once benefits reduce what the portfolio must supply. Check your own estimated benefit on your Social Security statement rather than assuming a figure.

Step 6: Check whether you have already "coasted"

You may not need to keep saving to reach your number. If your existing portfolio would grow to the FIRE number by a normal retirement age without any new contributions, you have reached Coast FIRE. In the example, the 4% target of $1,000,000 at age 60 has a Coast FIRE number of $231,377 at age 30, assuming a 5% real return. The example portfolio of $50,000 is 22% of the way there. With contributions continuing, it would cross the coast threshold at about age 40.

Variations on the FIRE number

The basic formula supports several named variations. They differ only in the spending figure:

• Lean FIRE targets a frugal budget. Our calculator uses 75% of base spending as a reference point for lean, which gives $750,000 in this example.
• Fat FIRE targets a more comfortable budget, shown at 150% of base spending: $1,500,000.
• Barista FIRE subtracts reliable part-time income before dividing.
• Coast FIRE asks whether today's portfolio will grow into the target on its own.

These are labels, not rules. You can read more about the trade-offs in our guide to Lean FIRE vs Fat FIRE.

How sensitive is the number?

Because the formula divides spending by a percentage, small changes in spending move the target a lot. At a 4% withdrawal rate, every $1,000 of annual spending adds $25,000 to the FIRE number. At 3.5% it adds $28,571, and at 3% it adds $33,333. Put another way, a $100-a-month subscription you keep in retirement requires roughly $30,000 of extra savings at the 4% rule.

This sensitivity cuts both ways. Trimming recurring costs before you retire lowers the target and raises your savings rate at the same time, which is why many FIRE plans focus first on spending rather than on investment returns. It also means your FIRE number deserves a careful spending estimate, not a rounded guess.

Line items people often miss

A FIRE budget looks different from a working budget. Some costs disappear, such as commuting and payroll retirement contributions, but others appear or grow:

• Health insurance before Medicare. Medicare eligibility generally begins at 65. Anyone retiring earlier needs coverage for the gap, through a marketplace plan, a spouse's employer plan, or another source. Premiums and out-of-pocket costs can be one of the largest line items in an early retirement budget, so price them for your age, state and household before fixing your number.
• Income taxes on withdrawals. Withdrawals from traditional 401(k) and IRA accounts are generally taxed as ordinary income. Withdrawals from taxable brokerage accounts may produce capital gains. Roth withdrawals that meet the rules are tax-free. Your FIRE number should cover spending plus whatever tax your withdrawals generate.
• Irregular big expenses. Cars, roofs, appliances and travel do not appear every month. Average them into the annual figure.
• Supporting family. College costs, help for aging parents or adult children can be significant and are easy to leave out.
• Account access rules. Retirement accounts generally have penalties for withdrawals before age 59½, with some exceptions. Early retirees usually plan which accounts fund which years. This affects timing more than the total number, but it can require holding more in taxable accounts than a simple plan assumes.

Social Security and the FIRE number

Social Security retirement benefits can start as early as age 62, with larger monthly benefits for those who wait longer. Many early retirees treat benefits as a later income source that lowers the portfolio withdrawals needed after a certain age. That can reduce the FIRE number, but only for the years after benefits begin. A common approach is to plan two phases: a bridge phase from early retirement until benefits start, funded entirely by the portfolio, and a later phase where benefits cover part of spending. Your own estimated benefit is available from the Social Security Administration through a my Social Security account. Because benefit rules can change, conservative planners include only part of their estimate, or none, in their FIRE number.

Recalculate as you go

A FIRE number is not set once. Spending habits change, family situations change, and your view on a safe withdrawal rate may change as you read more and get closer to the date. A practical habit is to recompute the number once a year, using the past twelve months of actual spending, and compare it with your portfolio. Tracking the ratio of portfolio to FIRE number over time shows progress more clearly than the raw balance, because it accounts for changes on both sides. When the ratio reaches 1.0 at your chosen withdrawal rate, you have reached your number on paper.

Common mistakes when calculating a FIRE number

1. Using gross income instead of spending. Your target depends on what you spend, not what you earn. High earners who spend modestly can reach FIRE quickly; high spenders cannot, whatever their salary.
2. Forgetting healthcare. Pre-Medicare health insurance can be one of the largest line items for US early retirees.
3. Mixing nominal returns with today's dollars. If your spending is in today's dollars, project the portfolio with a real return.
4. Treating the number as exact. A plan built on $1,000,000 versus $1,142,857 differs by years of work. Use a range, and revisit it as your spending and markets change.
5. Counting home equity. A paid-off home lowers your spending, which lowers the target, but the house itself does not produce withdrawals unless you sell or borrow against it.

Putting it together

Calculating a FIRE number is straightforward arithmetic: spending divided by a withdrawal rate. The judgment lies in the inputs. Base the spending figure on real statements, add healthcare and taxes, choose a withdrawal rate that matches the length of your retirement, keep everything in today's dollars, and subtract any income you are confident you will keep. Then run the numbers at a few withdrawal rates to see the range. The Coast FIRE calculator shows your target, your coast number and a year-by-year schedule, and the Barista FIRE calculator shows how part-time income shrinks the target.