Coast FIRE Calculator

How the Coast FIRE Calculator Works

The tool follows a two-step process:

1. Estimate your full financial independence (FI) target using a safe withdrawal rate.
2. Discount that future target back to today using compound growth to find your Coast FIRE number.

Step 1: Estimate Your FI Number

A common rule of thumb in the FIRE community is the 4% safe withdrawal rate. It suggests that, based on historical data, many retirees could withdraw about 4% of their starting portfolio value each year (adjusted for inflation) and have a high chance of the money lasting 30+ years.

If you assume a 4% withdrawal rate, your financial independence number (FI number) is:

FI number = 25 × annual expenses in retirement

For example, if you expect to spend $40,000 per year in retirement (in today’s dollars), your FI number is:

$40,000 × 25 = $1,000,000

Step 2: Discount Your FI Number Back to Today

Next, we calculate how much you would need invested today so that, if it grows at your assumed annual return and you add nothing else, it reaches your FI number by your target retirement age.

The core idea is the present value of a future amount:

Coast FIRE number = FI number ÷ (1 + r)ⁿ

• r = expected annual investment return (as a decimal, e.g., 0.07 for 7%)
• n = number of years between your current age and target retirement age

The same formula in MathML form is:

Where C is your Coast FIRE number (how much you need today) and F is your FI number (how much you want by retirement).

Worked Example

Imagine you:

• Are currently 35 years old.
• Want to retire at age 65.
• Expect to spend $40,000 per year in retirement (today’s dollars).
• Assume a 7% average annual investment return.

First, calculate your FI number using the 4% rule:

FI number = $40,000 × 25 = $1,000,000

You have 30 years until retirement:

n = 65 − 35 = 30 years

Now discount that $1,000,000 back 30 years at 7%:

Coast FIRE number = $1,000,000 ÷ (1.07^30)

1.07³⁰ is about 7.612, so:

Coast FIRE number ≈ $1,000,000 ÷ 7.612 ≈ $131,000

If you already have around $131,000 invested and you truly earn 7% per year on average, you could, in theory, stop contributing new money. By age 65, your investments would grow to around $1,000,000, giving you the ability to withdraw around $40,000 per year at a 4% withdrawal rate.

How to Interpret Your Results

When you use the calculator, you’ll typically see:

• Your FI number — the estimated portfolio size you need at retirement.
• Your Coast FIRE number — how much you need invested today for your current portfolio to coast to that FI number.
• Gap to Coast FIRE — how far above or below the Coast FIRE number your current investments are.

Here is how to think about the result:

• If your current investments are below your Coast FIRE number: you haven’t reached Coast FIRE yet. You’ll need to keep saving and investing until you close the gap.
• If your current investments are roughly equal to your Coast FIRE number: you are near Coast FIRE. Small changes in return assumptions or expenses could move you above or below the threshold, so treat this as an approximation.
• If your current investments are above your Coast FIRE number: you are past Coast FIRE based on the assumptions you entered. In principle, you can reach your FI target by your retirement age even if you stop adding new money, assuming returns and expenses behave as modeled.

Remember that this is a long-term planning tool. Being a bit above or below the Coast FIRE number doesn’t require an immediate, drastic change. Instead, use it as a reference point when deciding how hard you need to push on income, savings, and lifestyle.

Example Coast FIRE Scenarios

The table below shows approximate Coast FIRE numbers for different time horizons and FI goals, assuming a 7% annual return.

Use this as a rough reference only. Small changes in your assumptions about returns, retirement age, or spending can shift these values significantly.

Who Coast FIRE Is Best Suited For

Coast FIRE can be especially attractive if:

• You are starting your career and can save aggressively in your 20s and 30s.
• You like your work, but want the flexibility to scale back later without sacrificing retirement security.
• You value lifestyle freedom and time more than maximizing your net worth.
• You work in a high-stress or high-burnout field and want a planned off-ramp.

It can be less suitable if you:

• Plan to make significant withdrawals from investments before your main retirement age.
• Have highly uncertain income or may need to support others financially.
• Are uncomfortable with stock-market volatility and prefer very conservative investments.

Coast FIRE vs. Traditional FIRE

Both Coast FIRE and traditional FIRE focus on building enough investments to support your future lifestyle, but they emphasize different trade-offs between time, savings rate, and flexibility.

Some people treat Coast FIRE as a milestone on the way to full FIRE. Others reach Coast FIRE, downshift their lifestyle, and never aim for very early full retirement.

Key Assumptions and Limitations

This calculator is a simplified model. It does not capture every real-world factor. Important assumptions include:

• Constant average return: The tool assumes a steady average annual return (for example, 7%). Actual investment returns are volatile and can be much higher or lower in any given year.
• No additional contributions after Coast FIRE: The definition of Coast FIRE assumes that, once you reach your Coast number, you stop adding new money to your retirement portfolio. In reality, you may continue contributing, which would put you ahead of these projections.
• No withdrawals before retirement: The calculation assumes you do not take money out of the portfolio until your target retirement age. Early withdrawals would reduce your ending balance.
• 4% withdrawal rule as a guideline: Using 25× expenses is based on historical research, not a guarantee. A different safe withdrawal rate (for example, 3.5% or 3%) would change your FI and Coast numbers.
• Expenses in today’s dollars: The calculator typically works with retirement expenses entered in today’s dollars. It then assumes that your FI number accounts implicitly for inflation over time. Actual inflation may be higher or lower.
• Taxes and fees not modeled in detail: The outputs generally ignore the effects of taxes, investment fees, and changes in tax law, which can materially affect your real-world results.
• Stable life circumstances: The model assumes your retirement age and desired spending remain fixed. Life events — health issues, family changes, job loss, or major economic shocks — can alter your plans.

Because of these limitations, use the results as rough planning inputs, not precise forecasts. It is often helpful to rerun the calculator under different return rates, retirement ages, and expense levels to see a range of possible outcomes.

Using This Calculator in Your Planning

To get the most value from this tool:

• Experiment with different retirement ages to see how extra working years reduce your Coast FIRE number.
• Adjust your expected annual expenses and note how spending less in retirement reduces how much you need today.
• Try several return rate assumptions (for example, 5%, 6%, 7%) to understand how sensitive the plan is to your market expectations.
• Revisit your numbers every year or two as your income, savings, and life goals change.

Coast FIRE is less about hitting a single “magic number” and more about giving yourself flexibility. Knowing how close you are can make it easier to decide whether to keep pushing hard, or to start coasting and reclaiming more of your time.

Important Disclaimer

This Coast FIRE calculator is for educational and illustrative purposes only. It does not provide personalized financial, investment, tax, or legal advice, and it does not account for your full financial situation.

All projections are based on user inputs and simplified assumptions about investment returns, inflation, taxes, and spending. Actual results will differ, potentially by a wide margin. Past market performance does not guarantee future results, and there is always a risk of losing money when you invest.

Before making major financial decisions or changes to your work, savings, or retirement plans, consider consulting a qualified financial professional who can review your specific goals, risk tolerance, and circumstances.