Coast FIRE Calculator

What Is Coast FIRE?

Coast FIRE (also called “Coasting to FIRE”) is a milestone within the broader Financial Independence, Retire Early movement. It represents the point where your investment portfolio is large enough that compound interest alone will grow it to your full retirement needs by your target retirement age — without any additional contributions.

Unlike traditional FIRE, which requires accumulating 25× your annual expenses before quitting work entirely, Coast FIRE only requires reaching a smaller number earlier in life, then letting time and compounding do the rest.

Once you reach your Coast FIRE number, your relationship with work fundamentally changes: you still need income to cover current living expenses, but you no longer need to save any of it. Every dollar you earn is for today, not for retirement.

How the Coast FIRE Formula Works

The calculation works backward from your retirement goal:

Step 1: Calculate your FIRE number

Your FIRE number is the portfolio size needed to fund retirement. Using the 4% safe withdrawal rate:

FIRE Number = Annual Retirement Spending ÷ Safe Withdrawal Rate

For example, $40,000/year ÷ 0.04 = $1,000,000

Step 2: Discount back to today

Using compound interest in reverse, calculate how much you need today for it to grow to your FIRE number by retirement:

Coast FIRE Number = FIRE Number ÷ (1 + Real Return)^Years

Where Real Return = Expected Return – Inflation (e.g., 7% – 3% = 4%)

Step 3: Compare to your current investments

If your current invested assets exceed your Coast FIRE number, congratulations — you’ve reached Coast FIRE. If not, the gap tells you how much more you need to save before you can coast.

A Worked Example

Meet Alex, age 30. Alex expects to spend $40,000/year in retirement and plans to retire at 65. Assuming a 7% expected return, 3% inflation, and a 4% withdrawal rate:

• FIRE Number: $40,000 ÷ 0.04 = $1,000,000
• Years to retirement: 65 – 30 = 35 years
• Real return: 7% – 3% = 4%
• Coast FIRE Number: $1,000,000 ÷ (1.04)^35 = $253,415

If Alex has $253,415 invested today, Alex never needs to save another dollar for retirement. The portfolio will grow to $1,000,000 in today’s dollars by age 65 through compound growth alone.

Alex currently has $200,000 invested — about $53,000 short. At Alex’s current savings rate, Coast FIRE is approximately 18 months away.

Coast FIRE vs. Other FIRE Types

Coast FIRE is the most achievable milestone because it requires the smallest portfolio — but it still requires you to work (albeit without the pressure to save).

Key Assumptions and Limitations

Investment returns are not guaranteed. The calculator uses a fixed annual return, but real markets fluctuate significantly year to year. Extended bear markets early in your “coasting” phase could delay your retirement timeline.

Inflation may exceed expectations. The default 3% inflation assumption is based on historical US averages. Periods of higher inflation (like 2021–2023) erode purchasing power faster.

Your spending may change. Healthcare costs typically rise with age. Children, housing changes, and lifestyle shifts can materially affect retirement spending needs.

Social Security is not included. This calculator does not factor in Social Security benefits, which could reduce the amount your portfolio needs to provide. Many FIRE planners treat Social Security as a bonus rather than a dependency.

For a more conservative estimate, use a lower expected return (5–6%) and a lower safe withdrawal rate (3.25–3.5%).

Tips for Reaching Coast FIRE Faster

1. Front-load savings in your 20s and early 30s. Every dollar invested at 25 has 40 years to compound. Even small differences in early savings dramatically change your Coast FIRE number.

2. Minimize investment fees. A 1% annual fee on a $200,000 portfolio costs $2,000/year and compounds against you. Low-cost index funds (total market or S&P 500) with expense ratios under 0.10% are the standard recommendation.

3. Maximize tax-advantaged accounts. 401(k) employer match is free money. Roth IRA contributions grow tax-free. HSA accounts are triple-tax-advantaged. Use all of them before taxable accounts.

4. Track your number quarterly. Recalculate your Coast FIRE number each quarter with updated portfolio values. Watching the gap shrink is a powerful motivator.