How to Calculate Your Coast FIRE Number

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Here is how to calculate your Coast FIRE number: take your FI number and discount it backwards by the years until you retire. That figure is the amount you need invested today so compounding alone carries you to full financial independence, with no further contributions. The arithmetic takes ninety seconds. For one woman sharing her story on TikTok, learning basic financial literacy took an immense amount of effort, because it felt completely gatekept when she was growing up. Nobody in her house talked about investing. The turning point was realising the information itself is free and public.

The short version: Calculate your Coast FIRE number with one line of arithmetic: FI number divided by (1 + real return) raised to your years until retirement. A woman targeting $1,000,000 in 20 years at a 7% real return needs $258,419 invested now, then nothing more.

What a Coast FIRE number actually is

Your Coast FIRE number is the balance at which your invested money can reach full financial independence on its own, without another dollar from you. Past that line, your portfolio is no longer waiting on your paycheck. It is waiting on time.

That is the whole difference between Coast FIRE and regular FIRE. Regular FIRE asks you to reach the full number, often 25 times your annual spending, before you stop. Coast FIRE asks a smaller question: what is the smallest pile that grows into that full number by itself? Once you hit it, your job only has to cover this year’s bills, not this year’s bills plus retirement savings.

So what changes the day you cross it? Nothing in your account, and quite a lot in your options. You can drop to four days a week, take the lower-paid work that fits around a child, or stop apologising for the year you spend rebuilding. The money keeps working whether or not you do.

The formula every Coast FIRE calculator runs

Every Coast FIRE calculator runs one closed-form equation: Coast FI number = FI number × (1 + real return) ^ −years to retirement. There is no hidden model underneath it. The tool is doing long division and telling you the answer.

Work the standard example. A $1,000,000 target, a 20-year runway, and a 7% real return give $1,000,000 × (1.07) ^ −20 = $258,419. Another way to see the same thing: at 7% real over 40 years, roughly 1.7 times your annual living expenses grows into 25 times your expenses, which is the classic 4% rule target. The algebra is trivial. The assumptions carry all the weight, and we come back to them below.

The mechanism is compounding read backwards. Forward compounding asks what $258,419 becomes in 20 years. The Coast FIRE version asks the reverse question, which is why the exponent is negative. If you would rather not do the arithmetic by hand, our Coast FIRE calculator runs this same equation for you. To watch the forward version move instead, the SEC’s free compound interest calculator plots it without asking for an email address.

How do you calculate your Coast FIRE number?

Take your annual retirement spending, multiply by 25 to get your FI number, count the years until you want to stop, then divide the FI number by (1 + your real return) raised to that number of years. The result is the balance you need invested today for compounding to finish the job alone.

Steps 1 and 2: your annual spending and your FI number

Start with what a year of your retired life costs, not what a magazine says it should. Add housing, food, insurance, and travel. Multiply that figure by 25. A woman planning on $40,000 a year lands on a $1,000,000 FI number.

What if you have no idea what retirement costs? Use this year’s spending minus the costs that end with work, which are commuting, childcare, and the retirement contributions themselves. That number is close enough to start, and you will refine it every year anyway.

Step 3: your runway in years

Count the years between today and the age you want the money to be ready. Not the age you plan to quit, the age the money has to be there. If Social Security is part of your plan, check your actual projected benefit on the Social Security Administration’s my Social Security portal rather than guessing, because that figure changes your required FI number more than any assumption on this page.

Step 4: pick your real return, then run it twice

Real return means growth after inflation. Most calculators default to 7%. Run yours at 7% and again at 5%, and treat the gap between the two answers as the honest range. The table below does that for a $1,000,000 target and a retirement age of 67.

Your age now	Years of runway	Coast number at 7%	Coast number at 5%
40	27	$160,900	$267,800
45	22	$225,700	$341,800
50	17	$316,600	$436,300
55	12	$444,000	$556,800
60	7	$622,700	$710,700

Read the two right-hand columns as a range, not as a choice. At 50 the answer is somewhere between $316,600 and $436,300, and the difference between those two figures is roughly three years of work.

Why the 7% assumption is the weak point

The 7% real return baked into most Coast FIRE examples describes what US stocks did historically, not what they promise next. Academic finance has a name for the gap between that historical figure and what theory predicts: the equity premium puzzle. As one commenter put it, “There’s an entire section of financial academia dealing with the greater than realistic returns that the US received called the equity premium puzzle.”

Why does a two-point difference matter so much? Because the exponent multiplies it. Over 27 years, 7% versus 5% moves the required balance from $160,900 to $267,800, which is a 66% increase in what you need today from a change most people would call small.

The more careful approach that thread pushes toward is Monte Carlo modelling in a spreadsheet, simulating returns as a distribution rather than a single number, with inflation of 3% to 4% given its own distribution. That is more machinery than most of us need. Running your number twice, at 7% and at 5%, captures most of the same honesty in ninety seconds.

What the arithmetic cannot tell you

The arithmetic cannot tell you whether you will hold on when the number drops, and holding on is the entire mechanism. Coasting only works if the money stays invested through every bad year between now and your target date.

The one-shot problem

One finance blogger worked the case of a woman who finishes a long training path, clears her student loans, and reaches $200,000 invested at 45. Twenty years later that is $641,427, well short of the $1,000,000 the same piece targets. Her structural objection is sharper than the arithmetic: “Coast FIRE is a bullet, you only get one chance to fire (pun intended) the gun.” Because no withdrawals happen for decades, an error in your spending estimate stays invisible until it is far too late to fix by working another year.

The behaviour problem

With zero market knowledge, someone answering a Quora thread about life’s biggest mistakes put 25,000 rupees into a single stock, drawn in after watching other people make fortunes trading. They held through most of the ride. Then it hit a correction, their emotions took over, and they sold at a loss. It is still climbing today. Watching that stock keep growing after that panic is the biggest investing mistake of their life. It is also the reason I treat “will I hold” as a real input, not a personality question.

So what protects the plan? A boring, broad, automatic portfolio you never have to make a decision about, and a written note to yourself explaining why you chose it. You are not planning for the market. You are planning for the version of you who reads bad headlines at 11pm.

Coasting at 50 is a different calculation

Starting late removes the mechanism Coast FIRE runs on, because compounding needs decades and a shorter runway hands most of the work back to your contributions. At 27 years out, compounding does 84% of the lifting. At 7 years out, it does 38%.

Does that mean starting at 55 is pointless? No. It means the number answers a different question. It changes what the number is for. Before 45, your Coast FIRE number is permission to earn less. After 55, it is a progress marker that tells you how much of the remaining gap your existing balance will close by itself, which is exactly the figure that tells you whether one more year of full contributions buys you two years of freedom later.

Three levers still move meaningfully after 50: catch-up contributions once you turn 50, delaying the target date by two or three years, and cutting the retirement spending estimate itself. Current contribution and catch-up limits are published on the IRS retirement contribution limits page, and they change most years, so check them rather than trusting a blog’s number. The third lever is usually the strongest, and it is the one nobody mentions, because trimming $4,000 a year off your spending estimate takes $100,000 off your FI number.

If the mindset piece is the part that keeps stalling you, the guide to what financial independence actually means for women covers the ground this article assumes you already agree with.

The thousand hours she spent modelling instead of living

Running the number once a year is enough, and one woman on r/financialindependence learned that the expensive way. She reached financial independence in her early 40s on a single income while raising young kids. For years before that she checked her accounts multiple times a day and reran her retirement projections more times than she could count.

Looking back, she probably spent a thousand hours on modelling and tracking when fifty hours would have covered everything she actually needed to know. Since settling into this life, she spends at least sixty percent less time on financial admin, and she has not missed a single thing that mattered.

How often should you recalculate, then? Once a year, plus any time your spending plan changes by more than 10%. A quarterly check feels responsible and mostly just teaches you to react to noise. If your money plan itself needs building first, the $5,000 travel fund plan is the same discipline at a scale you can finish this year.

What crossing the line actually feels like

The first milestone changes your self-image more than any number that comes after it. That shift is the part no calculator outputs, and it is the part that keeps people investing through the dull middle years, the way one r/leanfire commenter describes it:

The first time my investments crossed $100,000, it changed how I saw myself more than any number since. I had never inherited or been given a dime, so that milestone proved I did not have to stay poor. I built it with my own hands and discipline. My net worth is far bigger now, and so are the risks and the complexity, but nothing has matched that first six-figure line.

Keep the receipts. That same commenter still screenshots every new high and saves it to a folder, just for themselves. On the days the market takes back four months of progress, that folder is the evidence that the plan is working and the year is not.

Where your Coast FIRE number goes next

Once you have the number, the next decision is where the money actually sits. A Coast FIRE plan running in a savings account is not coasting, it is shrinking, because inflation removes the real return the whole formula depends on.

Which account comes first? The one with an employer match, then the tax-advantaged account you qualify for on your own. Three practical next steps, in order: open the tax-advantaged account you are eligible for, pick one broad low-cost index fund rather than five, and automate the contribution for the day after payday. When you are comparing where to hold it, the guide to the best robo-advisors for beginner investors compares the options on fees rather than on branding.

For the wider picture of how this fits with everything else on one income, the investing for beginners hub holds the rest of this section, and the small financial wins worth banking when you start over at 40 is the honest starting point if this article arrived at the wrong decade.

Another commenter in that same r/leanfire thread is a few weeks from $200,000 and feels giddy at every log-in. They actually missed their $100,000 milestone completely, because an old account was not pulling current numbers from a former employer’s 401(k), so they logged in one day and it was already above six figures on its own. Now every time the balance climbs another $25,000 to $50,000, they let themselves celebrate quietly, alone.

Nobody in their life would understand why a number on a screen makes them this happy. You will, though. You are not chasing a jackpot here. You are buying back weekdays, one deposit at a time, on one income, with nobody’s permission.

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