What Is Coast FIRE? A Plain-English Guide

What is Coast FIRE? Coast FIRE (often written Coast FI) is the point where the money you have already invested will grow into a full retirement nest egg on its own — without another dollar of contributions. Once you reach your Coast FIRE number, you can stop saving for retirement entirely and let compounding do the rest. You still need a job to pay today’s bills, but your future is already funded.

This guide covers the Coast FIRE number formula, a realistic 35-year-old example, how Coast FIRE compares to FIRE and Barista FIRE, and how to run your own numbers with the free Coast FIRE calculator on this page’s homepage.

The Coast FIRE Number, Explained

Retirement planning usually works forward: you estimate annual spending, multiply by a safe withdrawal rate, and get the “FIRE number” you need at retirement age. Coast FIRE works backward. It asks a simpler question: how much do I need invested today so that, without saving another dollar, it grows into my FIRE number by retirement?

That number is the Coast FIRE number. It is lower than the FIRE number because it gets 20–40 years of compounding growth before you ever touch it.

The Coast FIRE Formula

The formula has three parts:

• FIRE number = annual spending ÷ safe withdrawal rate
• Years until retirement = target retirement age − current age
• Coast FIRE number = FIRE number ÷ (1 + real return)^years

The real return matters. A 7% expected return with 3% inflation is not a 4% growth rate — it is roughly 3.88%, because the correct formula is (1 + 7%) ÷ (1 + 3%) − 1. The Coast FIRE calculator on this site uses that correct inflation-adjusted math.

A Worked Example: 35 Years Old, Spending $50,000 a Year

Assume you are 35, spend $50,000 a year, target retirement at 65, expect 7% nominal returns, and assume 3% inflation.

• FIRE number = $50,000 ÷ 4% = $1,250,000
• Years to retirement = 65 − 35 = 30 years
• Real return = (1.07 ÷ 1.03) − 1 ≈ 3.88%
• Coast FIRE number = $1,250,000 ÷ 1.0388³⁰ ≈ $398,578

So at 35, you need about $398,578 invested today to coast into retirement. That matches the number-by-age table on the calculator’s homepage: at 35 the table shows $398,578, at 45 it shows $583,417, and at 55 it shows $853,974.

See the full table and your own number with the free Coast FIRE calculator.

Coast FIRE vs. FIRE vs. Barista FIRE

Coast FIRE is the middle path: the smallest lump sum that guarantees your retirement, with the flexibility to keep earning.

Why Coast FI Matters

For savers in their 30s and 40s, Coast FI is powerful because it reframes the question. You do not need to reach $1.25 million to retire early. You only need to reach the Coast FIRE number — often hundreds of thousands less — and then let compounding work. Every extra dollar after that point shortens your retirement date or funds an earlier one.

Two caveats keep Coast FIRE honest: it assumes your spending and return assumptions hold, and it ignores the years between coasting and retirement, during which you still need income. That is why the calculator shows both your Coast FIRE number and the full year-by-year growth chart.

Coast FIRE FAQ

Do I still need to work at Coast FIRE?
Yes. Coast FIRE means you stop saving for retirement, not working. You still earn enough to cover current living expenses.

Is Coast FIRE safe?
It is as safe as your assumptions. The calculator uses a 4% safe withdrawal rate, a 7% expected return and 3% inflation by default — all adjustable before you calculate.

What return should I use?
7% nominal is a common long-run assumption for a broad stock portfolio. Lower it to 5–6% if you want a more conservative Coast FIRE number.

Can I use the calculator for free?
Yes. Everything runs locally in your browser, nothing is uploaded, and your numbers update as you type.

Run your Coast FIRE number now — free, no sign-up.