Coast FIRE Explained: What It Means and How to Work Out Your Number

Coast FIRE Explained: What It Means and How to Work Out Your Number

Most retirement advice assumes one thing: you keep saving a slice of every paycheck until the day you stop working. Coast FIRE turns that idea around. It asks a different question. Have you already saved enough that, if you never added another dollar, your investments could still grow into a full retirement fund by the time you want to retire?

If the answer is yes, you have reached what the personal finance world calls "Coast FIRE." You still work, because you still need to pay today's bills. But you no longer have to put money aside for retirement. This guide explains what the term means, how the basic math works, how it compares with other versions of FIRE, and the assumptions that can make a Coast FIRE number look better on paper than it turns out in real life.

This is general educational information, not personal financial or investment advice. Your own numbers depend on your income, taxes, accounts, health and goals, and a licensed financial professional can help you apply them to your situation.

What "Coast FIRE" means

FIRE stands for "Financial Independence, Retire Early." In its classic form, the goal is to build an investment portfolio large enough to cover your living costs, so that work becomes optional, often long before a traditional retirement age.

Coast FIRE (also written CoastFI or Coast FI) is a milestone along that road rather than the finish line. ProjectionLab, a retirement-planning software company, defines it as the point where "you've saved enough that your investments will grow to support retirement on their own, without any additional contributions." Once you get there, your paycheck only has to cover your current spending.

Why people find the idea appealing

The traditional FIRE path asks for very high savings rates, often for many years. That can mean living on a tight budget while friends spend freely, and it can feel unrealistic for people with average incomes, children, or high housing costs.

Coast FIRE splits the job into two phases:

  1. A saving phase. You save and invest aggressively for a limited window, usually early in your career, when your investments have the most years ahead of them to compound.
  2. A coasting phase. Once your balance reaches your Coast FIRE number, you only need to earn enough to cover your current expenses. Your retirement money is left alone to grow.

What people do with that freedom varies. Some switch to a lower-paying job they enjoy more, some work part-time, some take a career break, and some simply keep saving but with less pressure. ProjectionLab lists taking a lower-paying job, cutting back to part-time, or switching careers as examples.

How to work out a Coast FIRE number

The basic calculation has two steps.

Step 1: Estimate your retirement target

Many FIRE planners start from the "4% rule," which suggests that a portfolio roughly 25 times your yearly spending may be able to support withdrawals for a long retirement. So the rough target is:

Retirement target = expected yearly spending in retirement x 25

The 4% rule is a rule of thumb drawn from historical market data, not a promise. It does not know your tax situation, how long you will live, or what markets will do. Treat it as a starting point for planning.

Step 2: Work backwards to today

Next, you ask how much you would need invested today for it to grow into that target on its own. ProjectionLab gives the formula as:

Coast FIRE number = retirement target / (1 + r)^n

  • r is the yearly return you expect after inflation (a "real" return).
  • n is the number of years until your target retirement age.

A simple hypothetical example

The figures below are made up for illustration only. They are not a forecast or a recommendation.

Imagine someone who expects to spend 40,000 dollars a year in retirement. Using the 25x shortcut, their target is 1,000,000 dollars.

Now suppose they are 35, want to retire at 65 (30 years away), and assume a 5% real return each year. The math is 1,000,000 divided by 1.05 to the power of 30. Because 1.05^30 is about 4.32, the Coast FIRE number comes out at roughly 231,000 dollars.

If the same person instead assumed a 4% real return, 1.04^30 is about 3.24, and the number rises to about 308,000 dollars. A one-point change in the assumption moved the answer by tens of thousands of dollars. That is the single most important lesson of Coast FIRE math: small changes in assumptions make big differences over long periods.

You can check this kind of compounding yourself with the free compound interest calculator run by Investor.gov, the investor-education site of the U.S. Securities and Exchange Commission.

Coast FIRE vs. full FIRE vs. Barista FIRE

These terms are easy to mix up. Here is how they differ in plain language:

  • Full FIRE: your investments are already large enough to cover your living costs now. Work is optional today.
  • Coast FIRE: your investments are large enough to grow into a full retirement fund later, without new contributions. You still work to pay current bills, but you do not draw on the portfolio.
  • Barista FIRE: you have saved enough that part-time income plus withdrawals from your portfolio can cover your expenses now. ProjectionLab notes the key difference: with Barista FIRE you are actively drawing down the portfolio, while with Coast FIRE you are not touching it.

Coast FIRE can therefore be a waypoint. Some people reach it, keep saving anyway, and move on to Barista FIRE or full FIRE sooner.

The assumptions that can make your number wrong

A Coast FIRE number is only as good as the inputs behind it. Before you stop saving, it is worth looking hard at each one.

Market returns are not smooth. The formula assumes the same average return every year. Real markets rise and fall. ProjectionLab points out that a long downturn early in your coasting years can significantly delay when your portfolio reaches its target. Running a range of return assumptions, rather than one, gives a more honest picture.

Inflation can surprise you. The formula uses inflation-adjusted returns, but actual inflation can run higher than expected for long stretches, which reduces what your money will buy.

Your spending will change. The yearly spending figure you choose at 30 may not hold at 50. Children, health costs, housing choices and lifestyle changes can all raise your real retirement target.

Accessing the money early has costs. If you plan to leave work before the usual retirement ages, remember that many retirement accounts have rules about when you can take money out. The IRS explains that a 10% additional tax generally applies to certain early distributions from retirement plans taken before age 59 1/2, with some exceptions. That can affect how and where you hold your Coast FIRE savings.

Other income is left out. The basic formula ignores Social Security, pensions, and any part-time income in retirement. ProjectionLab suggests one rough adjustment: subtract an expected yearly benefit from your retirement spending before multiplying by 25. It also notes that this shortcut is imperfect if you retire before benefits begin.

How to use Coast FIRE sensibly

If the idea appeals to you, a few habits can keep it grounded:

  • Run several scenarios. Try a cautious, a middle and an optimistic return. Plan around the cautious one.
  • Recheck every year. Update your balance, your expected spending and your timeline. Coast FIRE is a moving target, not a one-time test.
  • Keep an emergency fund. Coasting only works if a job loss or big bill does not force you to raid your retirement accounts. Our guide to emergency funds covers how much people typically aim to keep.
  • Do not give up an employer match lightly. If your employer matches retirement contributions, stopping your own contributions may mean leaving that match behind. Check your plan's rules before you change anything.
  • Consider keeping a small contribution going. Even modest ongoing savings build a buffer against weaker-than-expected returns.
  • Get personalised help for big decisions. Before cutting your hours or changing careers on the strength of a Coast FIRE calculation, it can be worth talking to a fee-only financial planner or tax professional.

Bottom line

Coast FIRE is a simple idea with powerful math behind it: save hard early, then let compound growth carry your retirement savings while your paycheck covers today. The formula is easy (retirement target divided by growth over the years you have left), but the answer depends heavily on assumptions about returns, inflation and spending. Treat your Coast FIRE number as an estimate to revisit, not a finish line, and it can be a useful way to see how much flexibility your past saving has already bought you.

Sources

Related Articles