What is Coast FIRE? How It Works, the Math, and Your Number by Age

Most people picture financial independence as a single finish line. Hit your number, retire, done. Coast FIRE says there’s an earlier finish line worth aiming for, and once you cross it, the math finishes the job for you.

Below: what Coast FIRE means, the one-line equation behind it, and what your number probably looks like at your age. If you’d rather skip ahead, the Coast FIRE Calculator does the math for you.

What it means

Coast FIRE is the portfolio balance at which, if you stopped contributing tomorrow, compound growth alone would carry you to your full FIRE number by retirement age.

Said differently: the point where you can stop saving for retirement and still retire on time.

You’re not retired at Coast FIRE. You still need a job (or freelance income, or some other paycheck) to cover today’s bills. But the retirement-savings pressure is off. Every dollar you earn from then on is for now, not for your 65-year-old self.

The name comes from the idea that you can coast the rest of the way. Your portfolio has enough momentum to reach the finish line on its own.

The math

Coast FIRE is one line. Just compound interest, rearranged.

If you need FIRE dollars at retirement, you have N years to get there, and your portfolio grows at a real (after-inflation) rate of r, the amount you need invested today is:

Coast FIRE number = FIRE number / (1 + r)^N

Pick a retirement age, pick an expected real return, plug in your full FIRE number. You get a single dollar figure to aim at right now.

For the FIRE number itself, most planners use the 4% safe withdrawal rate, which means 25× your annual retirement spending:

FIRE number = annual retirement spending × 25

So $60K a year in retirement is a $1.5M FIRE number.

For real returns, 5% to 7% is the usual range, based on US stock market history minus inflation. The tables below use 7%.

Coast FIRE number by age

Your Coast FIRE number is your full FIRE number discounted back to today by the compounding years you have left. Written out in one line:

Coast FIRE number = (annual spending / withdrawal rate) / (1 + real return)^(years until retirement)

Worked example, a 35-year-old retiring at 65:

1. Annual retirement spending: $60,000. Withdrawal rate: 4%. FIRE number = $60,000 / 0.04 = $1,500,000.
2. Years until retirement: 65 - 35 = 30. Real return: 7%, so the growth factor is 1.07^30 ≈ 7.61.
3. Coast FIRE number = $1,500,000 / 7.61 ≈ $197,000.

If that 35-year-old has $197,000 invested today, compounding alone carries them to $1.5M by 65 with zero further contributions.

Here is the same math at every age. Assumptions: $60,000 annual retirement spending, 4% withdrawal rate ($1.5M FIRE number), 7% real returns, retirement at 65. Rounded to the nearest $100.

Your own numbers scale linearly with spending: at $80K of retirement spending (a $2M FIRE number), multiply every figure by 1.33. To run the table with your exact spending, return, and retirement age, use the Coast FIRE Calculator.

A 25-year-old who hits $100K invested and never contributes another retirement dollar will land on $1.5M at 65. The 50-year-old needs five times as much, because they have five fewer doublings ahead of them.

This is why the FIRE community is obsessed with the early years of saving. At 7% real, money roughly doubles every 10 years, so each decade of delay halves what every dollar can become.

Coast FIRE vs full FIRE vs the rest

Quick clarification on the variants since the terms get blurred together:

• Full FIRE: portfolio is big enough to fund all your expenses indefinitely. You can stop working entirely.
• Coast FIRE: portfolio is big enough that, left alone until retirement age, it’ll reach your full FIRE number on its own. You still have to work for today’s bills.
• Barista FIRE: in between. Portfolio covers most of your expenses, you work part-time for the rest.
• Lean / Fat FIRE: variations on full FIRE based on lifestyle. Lean is around $25K-40K annual spending, Fat is $100K+.

Full breakdown in every FIRE variant explained. For a direct side-by-side of the two partial milestones, see Coast FIRE vs Barista FIRE.

Why it matters

Once you hit Coast FIRE, three things shift.

Your career options open up. The six-figure job you tolerate is now optional. So is the 60-hour week. So is putting up with a bad manager because you can’t afford to leave. You can take a pay cut to do work you actually like, because that income no longer has to fund retirement.

Your savings rate becomes a choice. Want to keep stacking? Great. Want to spend more on travel, hobbies, or family? Also great. Either way your retirement is on track.

Time becomes your friend. Before Coast FIRE, every year of low savings hurts you forever. After Coast FIRE, time is doing the work for you instead of against you.

Most people aim for full FIRE, which usually takes 20 to 30 years. Coast FIRE is typically 10 to 15 years out if you start in your 20s or 30s. The gap in life flexibility between those two horizons is huge.

How to find your number

Cleanest way is to use a calculator that handles your current portfolio, ongoing contributions, target spending, and assumed return. The Coast FIRE Calculator does it and tells you how many years out you are at your current pace.

By hand:

1. Decide your annual retirement spending in today’s dollars.
2. Multiply by 25 for your full FIRE number.
3. Pick a retirement age, subtract your current age for N.
4. Pick a real return (5% conservative, 7% historical).
5. Divide your FIRE number by (1 + r)^N.

If your current portfolio is above that number, you’ve already hit Coast FIRE.

Common questions

Does Coast FIRE assume I never save again? That’s the definition. In practice most people who hit it keep saving anyway, because it lets them retire earlier or fund a bigger lifestyle. The point is you no longer have to.

What return rate should I use? US stocks have averaged about 7% real (after inflation) over long periods. Some planners use 5% to be safe. The choice matters more for younger ages because compounding amplifies small differences over decades.

What about Social Security? Most FIRE math ignores it as a margin of safety. If you include it, your FIRE number drops, and so does your Coast FIRE number.

Does inflation break the math? Not if you use real (inflation-adjusted) returns. The 7% figure already nets out inflation, so the dollar amounts above are in today’s purchasing power.

What if I want to retire before 65? Use whatever target retirement age you actually want. Coast FIRE at 55 needs more today than Coast FIRE at 65, because there’s less compounding time ahead.

Track your progress

Math is the easy part. Keeping a real read on where your portfolio actually sits month to month is harder. Bonsave hooks into your investment accounts, tracks your net worth automatically, and shows your progress toward FIRE and Coast FIRE in one place.

Run the Coast FIRE Calculator to get your number, then sign up for Bonsave to watch the trajectory.