FIRE and the 4% Rule: How Much Do You Need to Stop Working?

FIRE — Financial Independence, Retire Early — is the idea of saving and investing enough that your portfolio can cover your living costs, so work becomes optional. You don't have to retire at 40 to benefit from it. Knowing your number, and how far away it is, turns a vague hope into a plan.

This guide explains how to calculate your FIRE number, where the 4% rule comes from, its limits, and the practical levers that get you there sooner.

What is a FIRE number?

Your FIRE number is the amount you'd need invested so that withdrawing a safe percentage each year could cover your spending. The calculation is simple:

FIRE number = yearly spending ÷ safe withdrawal rate

At a 4% withdrawal rate, that's 25 times your yearly spending. Spend $40,000 a year, and your number is $1,000,000.

Notice how much your spending drives the target. Every $1,000 cut from yearly spending reduces the number by $25,000 at 4%.

Where the 4% rule comes from

The 4% rule comes from US research in the 1990s, most famously by financial planner William Bengen and the later "Trinity study". They looked at historical US stock and bond returns and asked: what's the highest starting withdrawal, adjusted for inflation each year, that would have lasted 30 years in almost every historical period?

The answer was around 4%. In most periods tested, a portfolio of mostly stocks with some bonds survived 30 years of withdrawals at that rate.

The limits of the 4% rule

It's a useful rule of thumb, not a guarantee:

• It was built for 30-year retirements. Retiring at 40 could mean 50 years of withdrawals. Longer periods leave more room for bad luck.
• It's based on US history, which was strong. Other markets have done worse.
• Sequence matters. A big market fall in the first few years of retirement does more damage than the same fall later.
• It ignores flexibility. Real people cut spending in bad years and spend more in good ones — which improves the odds a lot.

For these reasons, many early retirees plan with 3–3.5%, or keep the flexibility to cut spending or earn some income when markets are down.

How long will it take?

Here's an illustration: spending $40,000 a year (in today's money), starting with $50,000 invested and saving $1,500 a month, at an assumed 7% return before fees with 2.5% inflation:

A more cautious withdrawal rate adds a few years — the price of extra safety.

The levers that matter most

1. Your savings rate. This is the biggest lever. In the example above, saving an extra $500 a month ($2,000 instead of $1,500) brings the 4% target forward from about 25.8 to about 21.9 years.

2. Your spending. Spending less works twice: it lowers your FIRE number and frees more money to invest. Cutting spending from $40,000 to $35,000 drops the target to $875,000 and the time to about 23.7 years — before even counting the extra savings.

3. Your costs. Fees come straight out of your real return. A 1% yearly fee can add years to your timeline. See the real cost of ETF fees.

4. Time. Starting earlier gives compounding longer to work. Even small amounts invested in your twenties carry a lot of weight.

5. Income after "retiring". Earning even a little — part-time work, consulting, a project you enjoy — sharply reduces what you need to withdraw, and lowers your effective number.

Plan in today's money

Your spending will rise with inflation, so a FIRE number set in today's money needs to grow with prices. The FIRE Calculator handles this by using a real return — the investment return minus inflation — so your target and progress are always in today's money. Read more in inflation and real returns.

A worked FIRE plan

Let's build a plan for someone who wants to spend $48,000 a year, has $120,000 invested and can save $2,500 a month. Because they're aiming to stop work early, they choose a cautious 3.5% withdrawal rate.

1. FIRE number: $48,000 ÷ 3.5% = about $1,371,429.
2. Assumptions: 7% return, a 0.14% fund fee and 2.5% inflation — a real return of a little over 4%.
3. Time to reach it: about 22 years.

From there, the plan can be stress-tested: what if returns are lower? What if spending falls to $42,000? What if part-time income covers $10,000 a year? Each change moves the date, and seeing those trade-offs is the point of the exercise.

Sequence of returns risk — and how to manage it

The order of returns matters once you start withdrawing. Two retirees can earn the same average return over 30 years, but the one who meets a large fall in the first few years can run out of money far sooner, because they're selling more units at low prices just as the portfolio shrinks.

Common ways people manage this:

• A cash buffer. Holding one to two years of spending in cash means you don't have to sell investments during a downturn.
• Flexible spending. Cutting withdrawals by, say, 10% in bad years dramatically improves the odds that a portfolio lasts.
• Guardrails. Some retirees set rules in advance: spend a little more after strong years, a little less after weak ones.
• A lower starting rate. Starting at 3–3.5% leaves more margin if early returns are poor.
• Some income. Even modest part-time earnings in the first years of early retirement greatly reduce the strain on the portfolio.

Flavours of FIRE

• Lean FIRE — a modest spending target, reached sooner.
• Fat FIRE — a higher spending target with more comfort and margin.
• Barista or semi-FIRE — enough invested to cover most costs, with part-time income covering the rest.
• Coast FIRE — enough invested early that, without further saving, it should grow to your full number by traditional retirement age.

What a FIRE plan should also consider

• Taxes on withdrawals, which depend on where you live and the accounts you use.
• Healthcare costs, especially before any state or employer coverage applies.
• Pensions and state benefits, which can reduce what your portfolio must cover later.
• A cash buffer, so you aren't forced to sell investments after a market fall.
• Your asset mix — see building a simple long-term portfolio.

The bottom line

Your FIRE number is your yearly spending divided by a safe withdrawal rate — 25 times spending at 4%, more if you want extra safety for a long early retirement. Your savings rate and spending are the strongest levers, low costs keep more of your return working, and planning in today's money keeps the target honest. Even if you never fully retire early, knowing your number gives you options.