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Insight · CoastFIRE
Coast FIRE Calculator: Have You Already Saved Enough?
What if you've already done the hard part? If you've been saving consistently for years, there's a real chance that compound growth alone could carry your portfolio the rest of the way to retirement. This guide walks through exactly how the Coast FIRE concept works, the math behind it, and how to think about whether you might already be there.
What If You Could Stop Contributing to Retirement Today and Still Be Fine?
Imagine checking your retirement account balance this month and realizing that, if the math holds, you might never need to contribute another dollar to it. Your existing savings, left alone to grow, could theoretically reach your target by the time you want to retire. That idea sits at the heart of Coast FIRE, and for consistent savers in their 30s, 40s, and 50s, it is worth understanding in detail.
Coast FIRE is a subset of the broader FIRE (Financial Independence, Retire Early) movement. Unlike traditional FIRE, which focuses on accumulating enough to retire immediately, Coast FIRE is about identifying the earlier milestone where your portfolio is large enough that compound growth can theoretically do the rest of the work by your target retirement age. Once you hit that number, some people choose to shift to lower-pressure work, reduce hours, or simply redirect their savings toward other goals. Understanding the concept does not require you to make any immediate changes. It simply gives you a clearer picture of where you stand.
This guide breaks down the Coast FIRE calculation step by step, walks through multiple illustrative scenarios with different assumptions, and highlights the important caveats that every honest treatment of this topic needs to include.
Step 1: Understand the Four Variables Behind Your Coast FIRE Number
The Coast FIRE calculation is built on four inputs. Changing any one of them shifts your number significantly, which is why it is important to understand what each variable does before running any figures.
Current portfolio balance: This is your starting point. It typically includes all tax-advantaged retirement savings such as 401(k), IRA, and Roth IRA balances, though some people also include taxable investment accounts depending on their plan.
Assumed annual growth rate: This is the average annual return you project your investments to earn between now and retirement. A commonly discussed range in long-term financial planning is 5% to 7% after adjusting for inflation, though this is illustrative and not a guarantee. Markets do not deliver smooth, predictable returns year to year.
Target retirement age: The age at which you plan to stop relying on employment income. This determines how many years of compound growth your portfolio has to work with.
Target retirement number (your FIRE number): The total portfolio value you would need at retirement to fund your spending. Many people calculate their FIRE number using the 4% rule as a starting framework, meaning a portfolio of 25 times your expected annual expenses.
The relationship between these variables is straightforward in concept: your Coast FIRE number is the present value of your target retirement number, discounted back by your assumed growth rate over the years remaining until retirement. In plain terms, it answers the question: how much do I need today so that, growing at an assumed rate, it reaches my target by retirement age?
Step 2: See the Math in Action With Illustrative Scenarios
The formula behind Coast FIRE is present value math. Your Coast FIRE number equals your target retirement number divided by (1 + annual growth rate) raised to the power of years until retirement.
Coast FIRE Number = Target Retirement Number / (1 + r)^n
Where r is the assumed annual growth rate and n is the number of years until your target retirement age. These scenarios below are purely hypothetical and illustrative. They are not projections or predictions of any individual's situation.
Scenario A: Age 35, retiring at 65, 30 years of growth Hypothetical target retirement number: $1,500,000. Assumed growth rate: 6% annually. Coast FIRE number: $1,500,000 / (1.06)^30 = approximately $261,000. If a hypothetical 35-year-old already has around $261,000 saved, their portfolio would be projected to reach $1.5 million by age 65 at this assumed rate, with no further contributions.
Scenario B: Age 45, retiring at 65, 20 years of growth Hypothetical target retirement number: $1,500,000. Assumed growth rate: 6% annually. Coast FIRE number: $1,500,000 / (1.06)^20 = approximately $468,000. At 45, the same target requires nearly twice as much already saved, because there are only 20 years of compounding available instead of 30.
Scenario C: Age 45, more conservative growth assumption Hypothetical target retirement number: $1,500,000. Assumed growth rate: 4% annually. Coast FIRE number: $1,500,000 / (1.04)^20 = approximately $684,000. Lowering the assumed growth rate from 6% to 4% raises the required Coast FIRE number by more than $200,000. This illustrates how sensitive the calculation is to the growth assumption you choose.
The pattern here is a useful mental model. Time is the most powerful lever in this calculation. The earlier your Coast FIRE age, the smaller the portfolio you need to have reached that milestone. This is why starting to save earlier can have such a dramatic effect on long-term outcomes.
Step 3: Account for the Variables That Can Change Your Number
Running a Coast FIRE calculation is a useful exercise. But several real-world factors can shift the picture considerably, and honest planning means taking each of them seriously.
Growth rate assumptions are not guarantees. This cannot be overstated. A fixed 6% or 7% annual return is a modelling convention, not a reliable forecast. Actual investment returns are variable and can be negative in some years. Sequence-of-returns risk, meaning the order in which gains and losses occur, can affect outcomes significantly. Running your numbers at multiple assumed rates, including conservative ones such as 4% or 5%, gives a more complete picture of the range of possibilities.
Inflation matters. If your target retirement number is expressed in today's dollars but you are 25 years from retirement, the real purchasing power you will need in the future is higher than that figure suggests. Many planners prefer to work with inflation-adjusted (real) return assumptions rather than nominal ones. For context on why this matters over a long horizon, the question of whether a $1 million portfolio will be sufficient in 2040 is worth thinking through carefully.
Account types and tax treatment differ. A 401(k) or traditional IRA balance will be subject to ordinary income tax on withdrawals in retirement. A Roth IRA balance, by contrast, may be withdrawn tax-free in retirement, subject to IRS rules. Your Coast FIRE number in pre-tax dollars is not the same as the same number in after-tax dollars. This is a genuine planning complexity that a financial adviser can help untangle for your specific situation.
Required Minimum Distributions (RMDs) begin at age 73 under current IRS rules for most tax-deferred accounts. If your retirement plan extends well beyond that age, RMD rules become part of the withdrawal picture.
Social Security benefits are often excluded from FIRE calculations because they are not portfolio-derived. However, Social Security income, which is based on your earnings history and the age at which you claim, can meaningfully reduce the portfolio withdrawal burden in retirement. Some planners subtract an estimated present value of Social Security from their target number; others prefer to treat it as a buffer rather than a dependency.
Step 4: Understand What Reaching Coast FIRE Actually Means (and Doesn't)
Reaching your Coast FIRE number is a milestone, not a finish line. It is worth being precise about what changes and what does not.
What it potentially means: If your projections hold and your assumed growth rate is realized over time, you may not need to make additional contributions to your retirement accounts to reach your target. Some people at this stage redirect savings toward other goals, reduce working hours, transition to less demanding work, or explore semi-retirement arrangements. The flexibility this creates is the real appeal of the concept.
For those interested in exploring the broader landscape of FIRE strategies, a comparison of LeanFIRE, FatFIRE, and CoastFIRE approaches can help clarify which framework fits different financial situations and lifestyle goals.
What it does not mean: Coast FIRE is not a guarantee that you will have enough at retirement. It is a projection based on assumptions that may not be realized. Markets can underperform. Expenses can rise. Life circumstances change. Reaching your Coast FIRE number under one set of assumptions does not eliminate the need to monitor your plan over time.
It also does not mean you stop working entirely, unless you have sufficient income from other sources to cover current living expenses. Coast FIRE addresses the retirement savings question specifically. Day-to-day living costs still need to be covered, and many people who reach their Coast FIRE number continue working, just without the same pressure to maximize retirement contributions.
Common Misconceptions About Coast FIRE
A few persistent misunderstandings tend to surface when people first encounter the Coast FIRE concept. Clearing them up makes the whole framework more useful.
Misconception 1: Coast FIRE means you can stop working. Not necessarily. Reaching your Coast FIRE number means your retirement savings may be on track without further contributions. It says nothing about how you fund your life between now and retirement. Many people who reach Coast FIRE continue earning income, just with more flexibility in how they do it.
Misconception 2: A higher assumed growth rate is always more realistic. It is tempting to use a high growth rate to arrive at a more flattering Coast FIRE number. But using an optimistic assumption that does not reflect actual long-term market conditions creates a false sense of security. Conservative assumptions leave a margin for real-world variability.
Misconception 3: Once you hit your Coast FIRE number, the calculation is done. Your target retirement number, your expected expenses, your tax situation, and your timeline can all shift over time. Coast FIRE is a useful snapshot, not a permanent verdict. Revisiting the numbers periodically, especially after major life changes, is a sensible practice.
Misconception 4: Coast FIRE only applies to FIRE enthusiasts. The underlying math applies to anyone with a retirement savings goal and a long time horizon. You do not need to be pursuing early retirement or identifying with the FIRE movement to find value in understanding whether your existing savings might be sufficient to grow to your target without further contributions.
Frequently Asked Questions
What is a Coast FIRE number and how is it different from a regular FIRE number?
A regular FIRE number (sometimes called your full retirement number) is the total portfolio value you need at retirement to fund your spending, often calculated as 25 times your expected annual expenses. Your Coast FIRE number is a different, earlier milestone: it is the portfolio balance you need today so that, growing at an assumed rate over the years until retirement, it reaches your full FIRE number without any additional contributions. The Coast FIRE number is almost always smaller than your full FIRE number because it accounts for the growth that will occur between now and retirement.
What growth rate should I use in a Coast FIRE calculation?
There is no single correct answer, and this is one of the most important variables to think carefully about. Many long-term financial planning models use nominal annual return assumptions in the range of 6% to 8% for equity-heavy portfolios, or lower inflation-adjusted (real) return assumptions of roughly 4% to 6%. The right approach is to run the calculation at several different rates, including conservative ones, to understand the range of possible outcomes. Growth rate assumptions are not guarantees of future performance, and using a single optimistic number without testing lower scenarios creates risk. A qualified financial adviser can help you select assumptions appropriate to your situation and investment mix.
Does reaching my Coast FIRE number mean I should stop contributing to my 401(k) or IRA?
Not necessarily, and this is a decision with real financial and tax implications that goes beyond what a general calculation can answer. Contributing to a 401(k) up to any employer match, for example, is a widely discussed consideration because the match represents additional compensation. Stopping contributions also means forgoing the tax advantages of those accounts for any future savings. Additionally, if your circumstances change, such as higher expected expenses in retirement, a different retirement age, or market underperformance, having additional contributions as a buffer could be valuable. Whether it makes sense to reduce or pause contributions after reaching a Coast FIRE milestone is a personal planning decision best made with the guidance of a qualified financial adviser.
Disclaimer: This article is intended for general educational purposes only. It does not constitute personalised financial, tax, or investment advice. The hypothetical scenarios and calculations presented here are illustrative only and do not represent projections or guarantees of future investment performance. Individual circumstances vary widely. Readers are strongly encouraged to consult a qualified financial adviser, tax professional, or other licensed professional before making any decisions about retirement contributions, investment strategy, or financial planning.
Curious Where You Stand on the Coast FIRE Spectrum?
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