Can You Afford to Retire Early? Running the FIRE Numbers

The idea that will not leave you alone

There is a moment in every working person’s life — usually around 35, sometimes earlier — when the thought arrives: what if I did not have to do this forever? Not hating the job necessarily, just wondering whether there is a number, a specific amount of money, that would make working optional rather than mandatory.

That question is the core of the FIRE movement — Financial Independence, Retire Early. During my years in wealth management, I watched it evolve from a fringe idea on internet forums into a serious financial planning goal for a growing number of clients. The maths behind it is surprisingly simple. The discipline required to execute it is anything but.

The basic premise is this: if you can accumulate a portfolio large enough that its investment returns cover your annual living expenses, you no longer need employment income. You are financially independent. Whether you then choose to stop working, switch to something you love, or just enjoy the security of knowing you could walk away — that is up to you.

What is your FIRE number?

Your FIRE number is the total investment portfolio you need to sustain your lifestyle indefinitely. The most commonly used guideline is the 4% rule, which grew out of retirement-withdrawal research often traced back to the Trinity Study and later work by advisers such as William Bengen. The simple version is familiar: if you withdraw no more than 4% of your portfolio in the first year of retirement and adjust for inflation annually, your money has historically lasted at least 30 years.

Working backwards: if you need $40,000 per year to cover your expenses, your FIRE number is $40,000 divided by 0.04, which equals $1,000,000. If you need $60,000, it is $1,500,000. If you need $80,000, it is $2,000,000.

The formula is elegant but the inputs require honesty. You need to know what your annual expenses actually are — not your current income, but your actual spending in retirement. Many FIRE aspirants find they can live on significantly less than they earn, especially once commuting costs, work wardrobe expenses, and convenience spending disappear.

This is where I saw clients fool themselves most often. They would plug in today’s trimmed-down budget and forget to include health insurance, home maintenance, travel, irregular car replacement, family support, and taxes on withdrawals. If your true spending is $55,000 but you model $42,000 because that is the number you wish were true, the spreadsheet will look wonderfully efficient right up until real life arrives.

Let’s use the FIRE Calculator to find your personal target.

That number might look impossibly large or surprisingly achievable depending on your current expenses and savings. Either way, it is the destination. The right way to use it is as a planning range, not a magical line in the sand. Run it once with your leanest believable annual spending, once with a more comfortable number, and once with a deliberately cautious figure that includes taxes and healthcare.

If those versions produce, say, $1.1 million, $1.35 million, and $1.55 million, that spread is useful. It tells you how sensitive your plan is to spending assumptions. In my experience, that is far more valuable than telling yourself you have one precise FIRE number down to the dollar.

How your savings rate determines your timeline

In traditional retirement planning, you work for 40 years and save slowly. In FIRE, the savings rate is the primary lever. Someone saving 10% of their income might need to work 40+ years to reach financial independence. Someone saving 50% can potentially get there in about 17 years. At 70%, the timeline shrinks to roughly 8 to 10 years.

The relationship is not linear — it is exponential, because every dollar you save does double duty. It increases your investment balance and simultaneously proves you can live on less, which lowers your FIRE number. A person earning $100,000 who saves $50,000 and lives on $50,000 needs a much smaller portfolio than someone earning the same amount who saves $10,000 and lives on $90,000.

Let’s use the Retirement Calculator to model your timeline based on your current age, savings, and contribution rate.

Pay close attention to how the timeline changes when you adjust the savings rate. Even a five-percentage-point increase — from 20% to 25% of income — can shave years off your working life. This is the insight that motivates FIRE adherents to optimise their expenses: every dollar cut from your monthly spending accelerates the timeline from both directions.

It is also why the most useful question is not “What return do I need?” but “What savings rate can I sustain for ten years without becoming miserable?” A mathematically perfect plan that depends on you cutting every pleasure out of life usually fails long before the portfolio has a chance to compound.

What does compound growth actually change?

The reason FIRE works at all is compounding. Your investments generate returns, those returns generate their own returns, and over time the growth becomes self-reinforcing. The earlier you start and the more consistently you contribute, the more dramatic the compounding effect becomes.

Let’s use the Compound Interest Calculator to visualise how your money grows over different timeframes.

Try comparing a scenario where you invest $1,000 per month for 15 years versus 25 years at a 7% average return. The difference is not proportional — the last ten years produce dramatically more wealth than the first ten, because by then your existing portfolio is generating substantial returns on its own. This is why starting early matters so much, and why even small contributions in your twenties can have an outsized impact on your FIRE timeline.

But it is also why sequence risk matters so much near retirement. If a bad market hits in the first few years after you stop working, the elegant compounding curve can run in reverse while you are taking withdrawals. FIRE plans need enough margin that they still work through ugly early years, not just through the average return in a spreadsheet.

How much do you need to invest each month?

The final piece of the puzzle is connecting your savings rate to specific investment contributions and seeing whether the projected growth hits your FIRE number within your target timeframe.

Use the Investment Calculator to model your contributions and projected portfolio growth.

If the numbers show you reaching your FIRE number at age 50, ask yourself: is that realistic given your current spending, income trajectory, and risk tolerance? If you are relying on aggressive market returns to make the timeline work, build in a margin of safety. Markets do not deliver 10% every year — there will be downturns, and your plan needs to survive them.

I usually tell people to test three return assumptions: one optimistic, one boring, and one deliberately frustrating. If the plan only works at the optimistic rate, it is not really a plan yet. It is a hope wearing a spreadsheet.

Why the 4% rule is not the whole plan

This is the part many FIRE articles rush past. The 4% rule is a starting heuristic, not a personal guarantee. It was built around historical US market data and a conventional retirement length. Someone leaving full-time work at 38 is not planning for 30 years. They may be planning for 50 or more.

That does not mean FIRE is fantasy. It means the rule needs context. Many early retirees prefer to test their numbers at 3.5% or even 3.25%, especially if they expect a long retirement, want more conservatism, or know they will be uneasy cutting spending during a bear market. A $50,000 spending plan needs $1.25 million at 4%, but about $1.43 million at 3.5%. That difference is large enough to change the decision.

This article is informational, not personalised financial advice, and US rules vary by account type and tax situation. If you are making a real decision about leaving work, this is exactly the point where a qualified financial adviser or tax professional earns their fee. The spreadsheet should start the conversation, not finish it.

How do you fund the years before 59 1/2?

This is another classic FIRE blind spot. Plenty of people build a healthy retirement balance in tax-advantaged accounts, then realise much of it is awkward to access if they want to stop work in their forties or early fifties. In the US, pulling money out of pre-tax retirement accounts too early can trigger ordinary income tax and, in many cases, a 10% penalty unless an exception applies.

That is why many serious FIRE plans use a mix of account types: taxable brokerage money for flexibility, retirement accounts for long-term tax advantages, and sometimes Roth contributions or carefully planned conversion strategies to bridge the gap. Fidelity’s current guidance on early withdrawals and 72(t) distributions is a useful reminder that “I have enough saved” and “I can access it efficiently” are not the same question.

You do not need to master every withdrawal strategy before you begin. But you do need to know whether your plan assumes money you cannot easily touch for another decade. If it does, the timeline may be technically correct and practically useless.

What early retirees often miss about healthcare and Social Security

Two US-specific realities deserve a line item in any FIRE plan. First, Medicare eligibility usually begins at 65, which means anyone retiring earlier may need to fund private coverage, marketplace coverage, COBRA, or a spouse’s employer plan for years. That cost can be manageable, but it is rarely trivial.

Second, Social Security can start as early as 62, but claiming early reduces the monthly benefit for life. For some households, those benefits will later reduce pressure on the portfolio. For others, the gap between early retirement and age 62 or 65 is the expensive part that determines whether the plan is robust or fragile.

In practice, this means your spending number should not be flat. Your forties, fifties, early sixties, Medicare years, and later retirement may all look different. A one-line annual budget is better than nothing, but a phased plan is better than a one-line budget.

The honest conversation about FIRE

I would be doing you a disservice if I presented FIRE as universally achievable or without trade-offs. A few realities worth considering:

Income matters. FIRE is substantially easier on a high income. Saving 50% of a $150,000 salary leaves $75,000 to live on, which is comfortable. Saving 50% of a $40,000 salary leaves $20,000, which is extremely difficult in most areas. FIRE content online skews heavily toward high earners, and the timelines they share are not representative.

The 4% rule has limitations. It was designed for a 30-year retirement. If you retire at 35, you need your money to last 50 to 60 years, which is a different proposition. Many early retirees use a more conservative 3% or 3.5% withdrawal rate, which increases the required portfolio size.

Healthcare is expensive. If you retire before you are eligible for government healthcare programmes, private health insurance becomes a significant expense that must be factored into your annual spending number.

Life changes. Your expenses at 35 may not reflect your expenses at 55. Children, ageing parents, health issues, relocations — life has a way of revising your budget without asking permission.

None of this means FIRE is not worth pursuing. Financial independence — even if you never fully stop working — provides options, security, and peace of mind that are genuinely life-changing. But the plan needs to be built on realistic numbers, not optimistic projections and aspirational spreadsheets.

Run the calculators with honest inputs. Know your FIRE number, your timeline, and the savings rate required to get there. Then pressure-test the assumptions underneath them: taxes, healthcare, market volatility, account access, and how flexible you would be if returns disappoint.

The answer does not have to be all or nothing. Some people will aim for full early retirement. Some will land on Coast FIRE, semi-retirement, or simply a much stronger position by age 50 than they had imagined possible. From my years working with clients, that is usually the healthiest way to think about it: not as a purity test, but as a numbers-led way to buy more options.