Worked example
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 (versus $197,051 without adjusting for inflation) 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. A common mistake is skipping the inflation adjustment, which understates the real number needed.
Coast FIRE: Letting Compounding Finish the Job
The point where you can stop saving
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.
Discounting the target back to today, in real terms
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. Because compounding is exponential, a modest sum invested early can reach a large target — which is exactly why younger savers have much lower Coast FIRE numbers than people closer to retirement chasing the same goal. Adjusting for inflation matters: a target left in nominal terms understates what today’s dollars actually need to become.
Turning $462,478 into $1,500,000
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. Dividing $1,500,000 by 3.243 gives roughly $462,478. That is meaningfully more than the $197,051 an inflation-blind version of the same calculation would show — because 4% real growth compounds far more slowly than the 7% nominal figure most calculators quote by default.
Are you already coasting?
The calculator compares the Coast FIRE number with what you already have invested and shows the gap directly. At or above the Coast FIRE number, the result reads as a surplus — you have technically reached Coast FIRE and could redirect new savings elsewhere while work covers current living costs. Below it, the gap is exactly how much more you need before compounding can take over the job on its own.
The danger of banking on the return
The main pitfall is treating the assumed real return as certain. 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. Forgetting taxes and fees, and reading a projection as a locked-in outcome, compound the risk.
Buying flexibility before full independence
Coast FIRE appeals 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.