Coast FIRE Calculator Explained: Formula and Three Worked Examples

Quick answer. Coast FIRE is the amount invested today that will grow to cover a traditional retirement without any further contributions. The formula is Coast FIRE Number = Retirement Target / (1 + r)^n, where r is the inflation-adjusted annual return and n is years until retirement. For a 35 year old targeting $1.25M at age 65 at 7 percent real return, the Coast FIRE number is about $164,000.

The Coast FIRE concept is simple, but the math behind it gets glossed over in most explainers. This post publishes the actual formula, walks three worked examples at different ages and income levels, and links to our free FIRE Financial Freedom Calculator so you can plug in your own numbers.

The formula

The Coast FIRE number is the present value of your retirement target, discounted by the rate of return over the years until retirement.

Coast FIRE Number = FV / (1 + r)^n

Where:

• FV is your retirement target (typically 25 times your annual retirement spending, per the 4 percent rule)
• r is your assumed real annual return (commonly 5 to 7 percent after inflation)
• n is the number of years from today until retirement

In plain English: how much money do I need invested right now so that, with no further contributions, compounding grows it to my retirement target by the time I want to retire?

Hit that number and you’ve reached “coast.” You can stop saving for retirement and let the existing balance grow on its own. You still have to cover your current living expenses, but the future is funded.

Why this matters

There are two related questions in retirement planning, and most calculators only answer one.

Question 1: How much do I need to retire? (Answered by 25x annual expenses, or whatever variation of the 4 percent rule you prefer.)

Question 2: How much do I need today so that compounding alone gets me there? (This is Coast FIRE.)

The second question is more useful for younger savers because it converts a distant intimidating number ($1.5M at 65) into a near-term concrete one ($150K at 35). Hitting $150K feels possible. Hitting $1.5M feels mythical. Coast FIRE is just a different framing of the same problem, and the framing changes behavior.

Three worked examples

Same retirement target ($1.25M, based on $50K annual spending at the 4 percent rule), three different ages, same 7 percent real return assumption.

Example 1: 28 year old

• FV = $1,250,000
• r = 0.07
• n = 37 (retire at 65)

(1.07)^37 = 12.78

Coast FIRE = 1,250,000 / 12.78 = $97,810

A 28 year old with $97,810 invested in a low-cost diversified portfolio, who never adds another dollar to retirement accounts, will have $1.25M at 65 if real returns average 7 percent.

Example 2: 35 year old

• FV = $1,250,000
• r = 0.07
• n = 30

(1.07)^30 = 7.61

Coast FIRE = 1,250,000 / 7.61 = $164,260

The same target, seven years later. The Coast FIRE number jumped 68 percent because compounding has fewer years to work.

Example 3: 45 year old

• FV = $1,250,000
• r = 0.07
• n = 20

(1.07)^20 = 3.87

Coast FIRE = 1,250,000 / 3.87 = $322,990

By 45, the Coast FIRE number is more than triple what it was at 28. This is the cost of waiting, expressed as a single number.

What changes if you use different assumptions

The 7 percent real return is the historical US stock market average over long periods, after inflation. It’s a reasonable default but not the only choice.

A 1 percent change in your return assumption shifts the number by roughly 25 percent. This is the single biggest source of variance in Coast FIRE estimates. People who use 5 percent are being conservative; people who use 8 percent are being aggressive. The honest answer is somewhere in the middle and you don’t get to know in advance.

The same sensitivity applies to your retirement spending estimate. $50K annually means a $1.25M target. $60K means $1.5M (and a 20 percent higher Coast FIRE number). Be honest about your spending; lowballing it is the most common error and the most expensive one to discover at 60.

What Coast FIRE doesn’t tell you

It’s a forecast, not a promise. The 7 percent real return assumption smooths over 30 years of market volatility that, in any given decade, can mean negative real returns. Sequence-of-returns risk is real: a 50 percent crash in years 1-3 changes the picture even if the long-run average works out.

Three caveats worth knowing:

It assumes static spending. If your retirement lifestyle inflates with your income (a common pattern), the target grows with you and you might find yourself behind on a moving target.

It doesn’t account for healthcare or unusual events. A long-term care need or a major medical expense in retirement can shift the target meaningfully. The base 25x rule is for a “typical” retirement.

It assumes the money is invested in something close to a market portfolio. Coast FIRE on $164K in a checking account doesn’t work. The math depends on actual investment returns, which require actually being invested.

For a more cautious view, drop your real return assumption to 5 percent and increase your target by 25 percent. If the resulting Coast FIRE number still feels reachable, you have meaningful margin.

How to use the free calculator

The on-site Coast FIRE Calculator runs the math in your browser. Inputs:

1. Current age
2. Retirement age (default 65)
3. Annual retirement spending in today’s dollars
4. Expected real return (default 7 percent)
5. Current invested balance

Output: your Coast FIRE number and a year-by-year projection chart.

For the general FIRE number (how much you need to retire outright, not just coast), use the free FIRE Financial Freedom Calculator spreadsheet. If you want a fully built retirement projection (multiple accounts, contribution schedule, Social Security, withdrawal phase), our paid Retirement Financial Planning Projections template covers that. Coast FIRE is a single input on a much larger picture; the on-site calculator handles the standalone question.

Coast FIRE vs other FIRE flavors

Quick definitions because the terminology trips people up.

For more on the distinctions and what each implies for your savings rate, see Lean FIRE vs Fat FIRE vs Coast FIRE: What the Numbers Actually Are.

A practical takeaway

If you’re under 40 and your invested balance is already past your Coast FIRE number for a reasonable target, you’ve earned the option to redirect future income. That doesn’t mean you should stop saving. It means you no longer need to.

For a 28 year old who hits Coast FIRE at $98K, the next decade of savings becomes optional. They might invest it anyway (more cushion), they might use it for a career pivot, or they might shift to a lower-paying job that fits better. The math gives them the choice.

That’s the whole point of running the number.

Get the template

• FIRE Financial Freedom Calculator — FIRE and Coast FIRE numbers for your inputs. Free, no signup.
• Retirement Financial Planning Projections — 40-year accumulation and withdrawal projection with configurable assumptions.
• Retirement Financial Planning Projections — 40-year accumulation and withdrawal projection with configurable assumptions.
• FIRE Calculator Ultimate ($19) — Multi-scenario FIRE planning with Lean/Regular/Fat/Coast views and year-by-year charts.

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