Coast FIRE calculator

Find the age your current savings, with no further contributions, compound into your full financial-independence number — and whether you've already coasted.

How the math works

Coast FIRE runs the standard compound-growth formula twice. First it sets your full financial-independence number — the portfolio that funds a year of spending indefinitely at your withdrawal rate. That is the FIRE number, annual spending divided by the withdrawal rate.

Where r is the expected annual real return and t is the years until your target retirement age. The coast number is the balance you would need invested today to reach the FI number on its own, with zero new contributions. The coast age inverts the same equation: it solves for the age at which the money you already hold compounds into the full FI number by itself.

Because the FI number is stated in today's dollars, the return you enter has to be a real return — one already net of inflation. That single choice does more to the answer than any other input, and the next section shows by how much.

Worked example

Take a 30-year-old with $150,000 already invested, planning to retire at 67 — the full Social Security retirement age for anyone born in 1960 or later. They expect to spend $48,000 a year in retirement and use the 4% rule, so the full FI number is $48,000 / 4% = $1,200,000.

At the S&P 500's long-run real return of 7.0%, the coast number — the balance needed today to reach $1.2M by 67 with no further saving — works out to about $98,171. This saver already holds $150,000, a surplus of roughly $51,829. Their money alone compounds to the full $1.2M by about age 61, six years ahead of the retirement date, and keeps going to around $1.83M by 67 if left untouched.

That is the whole idea of Coast FIRE: once the balance crosses the coast number, the heavy lifting is done by compounding, not by new contributions. It does not mean the saving was wasted — more invested means more at the end — it means further contributions are now optional rather than required.

When this calculator is wrong

The number this tool produces is clean. The decision it implies is not, because two things the calculator cannot see usually decide whether coasting actually works.

• Coast FIRE stops your saving, not your spending. Hitting the coast number means you can stop adding to the portfolio. It does not mean you can stop earning. Every year between now and retirement, you still have to cover current living costs — rent or mortgage, food, and, before 65, your own health insurance rather than Medicare — out of income the calculator never asks about. "Reached Coast FIRE" is a headline about your retirement account, not a notice that you can quit.
• Real and nominal returns give wildly different answers. The FI number here is in today's dollars, so the return has to be real. Enter the S&P 500's 10.2% long-run nominal return instead of its 7.0% real return, and the same $1.2M target over 37 years discounts to roughly $32,996 rather than $98,171 — a coast number three times too low. Most calculators don't tell you which they assume. The gap between the two is inflation, running at a post-WWII average of 3.3% a year.
• It assumes the return shows up every year. The 7.0% real return is a long-run average, not a schedule. With no new contributions during the coast years, a weak decade early on can't be bought back into at lower prices — there is no new money going in. A balance that looks coasted at 35 can arrive at the retirement date short.

The 4% rule is a starting point, not a guarantee. The Trinity Study (1998) found 4% withdrawals held up across most historical 30-year periods for stock-heavy portfolios — not all of them, and not across the 40- and 50-year horizons an early coaster is really planning for.

What to do with the result

If the calculator says your current savings coast to the FI number before your target age, the practical move is not to stop saving outright — it is to stop saving under pressure. The portfolio no longer needs new money to hit the target, so you can redirect contributions toward a shorter career, a lower-stress job, or spending you'd been deferring, and let compounding finish the job.

If you're short of the coast number, the gap shrinks fastest the earlier you close it, because every dollar added young has the most years to compound. A useful cross-check is the Fidelity milestone of 10× your salary saved by age 67: if your coast age lands well before then on a stock-heavy portfolio, you are ahead of the standard benchmark. Note too that Social Security replaces about 40% of pre-retirement earnings for a medium earner, which this calculator ignores — counting it would lower the portfolio you actually need.

Common questions

What is Coast FIRE?

It's the point where your invested balance is large enough to grow into your full financial-independence number by retirement with no further contributions. You still work to cover current expenses, but you no longer have to save for retirement — compounding does it.

How is Coast FIRE different from the FIRE number?

The FIRE number is the portfolio you need to live off at retirement — annual spending divided by your withdrawal rate. The coast number is that figure discounted back to today at your expected return. Coast is always the smaller number, and the gap between them widens the younger you are.

What return should I use?

Use a real (inflation-adjusted) return, because the FI number is in today's dollars. The S&P 500's long-run real return has averaged 7.0%. Entering the 10.2% nominal figure instead will understate the balance you actually need, sometimes by a factor of three over a long horizon.

Does reaching Coast FIRE mean I can retire?

No. It means you can stop contributing to retirement savings. You still need earned income to cover living costs — and your own health insurance before Medicare at 65 — for every year between the coast date and your actual retirement.

Should I keep investing after I hit my coast number?

You don't have to, but more invested means more at the end and a buffer against a weak run of returns. The 7.0% figure is a long-run average; with no new contributions, a flat early decade can leave you short, so many coasters keep saving at a reduced rate rather than stopping cold.