Coast FIRE Calculator

How the real-return Coast FIRE formula works

Coast FIRE discounts your target retirement corpus back to today using a real (inflation-adjusted) return, then compares that figure with what you already have invested. Real return is approximated as (expected return − inflation), the convention most Coast FIRE calculators use, not the compounded Fisher equation.

Turning $462,478 into $1,500,000 with nothing added

Age 30, retiring at 60, $100,000 already invested, a $1,500,000 target (today’s dollars), a 7% expected return, and 3% inflation.

• Years to retirement: 60 − 30 = 30
• Real return: 7% − 3% = 4%
• Growth factor: (1.04)^30 ≈ 3.243
• Coast FIRE number: $1,500,000 / 3.243 ≈ $462,478
• Gap to Coast FIRE: $462,478 − $100,000 = $362,478 still needed
• Projected value at 60: $462,478 × 3.243 ≈ $1,500,000

About $462,478 invested today is projected to grow into a $1,500,000 target in today’s dollars by age 60, assuming a steady 4% real return. With $100,000 already invested, this saver still needs about $362,478 more before compounding alone can finish the job — they have not yet reached Coast FIRE. From the number onward, no further contributions are added: that is exactly what the projection assumes, and it is the whole point of the milestone.

The $265,427 the inflation adjustment adds to your number

Coast FIRE is the present value of your target corpus, calculated with a real (inflation-adjusted) return. Subtract your assumed inflation rate from your expected return, then divide the target by the growth factor (1 + real return) raised to the years until retirement. For a $1,500,000 target 30 years out at a 7% expected return and 3% inflation, the real return is 4%, so the growth factor is (1.04)^30, about 3.243 — and dividing $1,500,000 by 3.243 gives roughly $462,478.

Run the same scenario with the inflation line switched off — a flat 7% nominal return, growth factor 7.612 — and the number needed today collapses to $197,051. The inflation adjustment is worth $265,427 of required capital in this one scenario, because 4% real growth compounds far more slowly than the 7% nominal figure most calculators quote by default. Adjusting for inflation matters: a target left in nominal terms understates what today’s dollars actually need to become.

Subtracting inflation converts your nominal (stated) return into a real return, so the result is expressed in today’s purchasing power rather than inflated future dollars. Skipping this step understates how much you actually need, because it ignores that $1,500,000 in 30 years buys less than $1,500,000 today. The target corpus is treated as already expressed in today’s purchasing power, and growth is calculated in real terms throughout. The most common mistake made with this figure is running it nominal anyway, which understates the real number needed by exactly that $265,427.

The gap reads $362,478 short, or it reads surplus

This calculator finds your Coast FIRE number — the sum that, invested today, is projected to grow into your retirement target by the age you choose with no further contributions. It answers a motivating question: how much do I need invested before I can stop saving and let growth do the rest? The output is a milestone, expressed in today’s money, and it is reported against what you already hold.

The calculator compares the Coast FIRE number with what you already have invested and shows the gap directly, so the reading is one subtraction rather than a projection you have to interpret. Below the number, the gap is exactly how much more you need before compounding can take over the job on its own: $362,478 in the scenario above, on $100,000 held against a $462,478 requirement. If what you already have invested is equal to or above that number, the result reads as a surplus instead of a gap — you have technically already reached Coast FIRE, growth alone is projected to carry you to your target, and you could redirect new savings elsewhere while work covers current living costs. One condition sits under both readings: the gap figure assumes your current invested amount is available for long-term growth and is not otherwise earmarked.

The same $1,500,000 target costs a 45-year-old $832,897 today

The more years until retirement, the longer compounding works, so you need less today: a younger person needs a smaller Coast FIRE number than someone closer to retirement aiming for the same target. Because compounding is exponential, a modest sum invested early can reach a large one — which is exactly why younger savers have much lower numbers than people chasing the same goal from nearer the finish.

Hold the $1,500,000 goal and the 4% real return fixed and move the starting age from 30 to 45: the growth factor falls from 3.243 over 30 years to 1.801 over 15, and the amount needed today rises from $462,478 to about $832,897. Fifteen years of delay adds $370,419 to the bill. The two age fields are not a detail — they set the exponent, and the number they produce gets harder to reach every year it is left alone. The decision rule falls out of that: at 45 with less than $832,897 invested, contributions still have to do part of the work, and only the years you have left decide how much.

Why a steady 4% real return is the assumption that can break the plan

The main pitfall is treating the assumed real return as certain. A constant annual return and inflation rate are assumed for the whole period; real returns vary and are not guaranteed, and a single return and inflation figure cannot capture the volatility of real markets. Markets are volatile — the SEC notes that no asset category performs well every year — and a long stretch of weak returns, or inflation running hotter than assumed, can leave a coaster short of the target with little time to fix it. The target corpus itself usually leans on the four percent rule, which Corporate Finance Institute treats as a guideline rather than a guarantee.

The calculator has no input for Social Security, a pension or other retirement income. To allow for one, lower the Target Retirement Corpus by the savings that income replaces: roughly the annual income divided by your withdrawal rate, so $20,000 a year at a 4% withdrawal rate stands in for about $500,000. For a couple, enter one combined target and one combined invested amount.

Many people use a long-run estimate for a diversified portfolio, but any figure is an assumption, not a promise. A more conservative rate raises the amount you need today and provides a safety margin — the cheapest hedge this model offers, since coasting relies entirely on the assumed real return materializing, and poor markets or higher-than-expected inflation can leave you short.

So stopping contributions at the number is a plan, not a guarantee. If returns disappoint or inflation runs hotter than assumed, you could fall short, which is why many people treat the Coast FIRE number as a milestone and keep contributing, or hold a buffer, to manage that risk. The clearest limitation is what this does not model: contributions, account types, and sequence-of-returns risk. It also ignores taxes and fees unless you build them into your return assumption. Forgetting those, and reading a projection as a locked-in outcome, compound the risk — this is for educational and general planning use only.

Hitting the number buys the right to stop saving, not to stop working

Coast FIRE is the point where your invested savings are large enough to grow into your retirement target on their own, without any new contributions. From then on you only need to earn enough to cover current expenses — you can coast to retirement on compounding alone. Regular FIRE is the other finish line: it means you have the full corpus and can stop working entirely. Coast FIRE means you have enough invested to reach that corpus by retirement age through growth alone, so you can stop saving but still need income for current costs. It is a step on the way to full FIRE, not the same milestone.

That is what makes it appeal to people who want options — to move to lower-paying work, take a career break, or stop aggressive saving once growth can finish the job. It pairs naturally with a full Retirement Calculator or 401(k) Calculator for building the target corpus, and a Retirement Withdrawal Calculator for what happens once you get there.

Sources and methodology

The four percent rule behind FIRE corpus targets, and general diversification guidance, come from these publishers.

• Corporate Finance Institute — The Four Percent Rule
• U.S. SEC (Investor.gov) — Beginner's Guide to Asset Allocation, Diversification, and Rebalancing

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