CoastFIRE Calculator

How to use the CoastFIRE calculator

1. Enter your ages and what you have invested. Use the total across retirement and brokerage accounts. Leave out your emergency fund and the house you live in, neither is going to fund your retirement spending.
2. Say what a year of retirement costs in today’s money, and pick a withdrawal rate. 4% is the usual starting point; 3.5% is the cautious version.
3. Add Social Security, a pension or an annuity if you expect one, with the age it starts. If it starts after you retire, the tool works out what your portfolio needs to cover the years in between.
4. Read the three boxes. What you need at retirement, what you are on track for if you never save another penny, and the gap. Open “Show the working” to see every step of the arithmetic.

What CoastFIRE actually means

CoastFIRE is the point where the money you have already invested will grow into enough to retire on, without another contribution. You still work. You still need to cover rent, food and everything else. But retirement is handled, compounding does the rest of the job on its own.

It is the first milestone on the way to financial independence, and it usually arrives decades before the last one. A 30-year-old who wants $40,000 a year at 67 needs $1,000,000 by then at a 4% withdrawal rate. To coast there, they need $244,218 today: under a quarter of the target, because 37 years of compounding does the other three quarters.

The practical difference is what it unlocks. FIRE lets you stop working. CoastFIRE lets you take the lower-paid job, cut to four days, or move to the role you actually want, because the savings rate that job can’t support no longer matters.

The CoastFIRE formula

Two steps. First, what you need at retirement:

FIRE number = annual spending ÷ withdrawal rate

$40,000 ÷ 0.04 = $1,000,000. Then discount that back to today at your return after inflation:

CoastFIRE number = FIRE number ÷ (1 + real return)^years

$1,000,000 ÷ 1.038835³⁷ = $244,218.

The real return is a division, not a subtraction

This is where most CoastFIRE calculators are wrong, and it is worth a paragraph because it changes the answer.

With 7% growth and 3% inflation, the real return is not 4%. It is:

(1 + 0.07) ÷ (1 + 0.03) − 1 = 0.038835 = 3.88%

Subtracting inflation always overstates the real return, and an overstated return always understates what you need today. Over 37 years the gap compounds into real money. On one popular CoastFIRE calculator’s own default figures, $750,000 needed, 37 years, 7% and 3%, it reports $175,723 where the correct answer is $183,163. It is 4.1% light, and the error grows with the time horizon.

Each column: $1,000,000 discounted back at that rate. This calculator uses the exact figure.

Social Security and pensions change the number a lot

Every dollar of guaranteed retirement income is a dollar your portfolio doesn’t have to produce. Most CoastFIRE calculators have no input for it at all, which means they quote a number for someone with no Social Security, a rare situation in the US.

Take a 40-year-old retiring at 60 who wants $60,000 a year:

That third row is the real-world one, and it is the row most calculators get wrong. Social Security doesn’t start when you retire at 60, it starts at 62 at the earliest, and full retirement age is 67 for anyone born in 1960 or later. So the portfolio has to pay for everything for seven years, then drop to covering the $36,000 that Social Security doesn’t.

The maths splits in two:

• The steady state: ($60,000 − $24,000) ÷ 0.04 = $900,000, funding the gap between spending and benefit forever.
• The bridge: $24,000 a year for the seven years before the benefit starts, discounted at the real return, is $144,672.

Add them: $1,044,672. It is 30% less than the no-benefit figure and 16% more than pretending the benefit starts on day one. We checked the four calculators ranking for this term in September 2026: three have no input for other retirement income at all, and the fourth discards the benefit entirely whenever it starts after your retirement age, it returns the same $1,500,000 as having no benefit at all.

A worked example, start to finish

Alex is 30, has $100,000 invested, saves $800 a month, and wants $40,000 a year from 67.

1. Real return. (1.07 ÷ 1.03) − 1 = 3.88% a year.
2. Needed at 67. $40,000 ÷ 0.04 = $1,000,000.
3. Discount 37 years. $1,000,000 ÷ 1.038835³⁷ = $244,218. That is the CoastFIRE number.
4. Compare. Alex has $100,000, so they are $144,218 short. Not coasting yet.
5. Project. $100,000 left alone grows to $409,470 by 67, about 41% of the target.
6. When? At $800 a month, Alex crosses the line at age 53.

Age 53 is the answer that matters, and it is not obvious, because the CoastFIRE line is a moving target. Every year Alex waits, there is one less year of compounding ahead, so the number they need goes up: $244,218 at 30, $357,473 at 40, $523,249 at 50. The chart shows both lines, the rising requirement and the growing balance, the crossing point is where they meet.

This is why “how much do I need to save to hit my CoastFIRE number” has no clean answer. Answering it as a fixed target gives you the savings rate to reach full FIRE, not CoastFIRE. The crossing age is the honest version.

What the calculator assumes

Where this estimate breaks down

A smooth 3.88% every year is a modelling convenience, not a forecast. Four things the model can’t see:

• Sequence of returns. The same average return delivered in a different order gives a different result. A bad first decade leaves you behind even if the long-run average holds exactly.
• Your spending estimate will move. Guessing annual spending 30 years out is the largest source of error here, and it is much larger than the return assumption.
• Tax. $1,000,000 in a Roth and $1,000,000 in a traditional 401(k) do not fund the same retirement.
• Benefits are not guaranteed. The Social Security trustees project the trust fund reaches depletion in the mid-2030s, after which scheduled payroll taxes cover roughly three-quarters of benefits absent a change in law. If that worries you, enter 75% of your estimate and see what happens.

Treat CoastFIRE as a milestone, not a switch. Plenty of people who reach it keep contributing anyway, and recheck the number once a year. This tool gives you an estimate, not financial advice, talk to a regulated adviser before making decisions about your retirement.

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