The idea behind Coast FIRE
Full FIRE means saving enough to live off your investments now. Coast FIRE is a gentler milestone: the point where your current portfolio, left alone to compound, will grow into a full retirement nest egg by traditional retirement age — without any further contributions. Once you "coast," you only need to earn enough to cover today's expenses; your retirement is already handled by compounding.
The math
Work backwards from your retirement target using compound growth: Coast number = target ÷ (1 + return)years. A 30-year-old wanting $1,000,000 at 65, assuming a 5% real return, needs only about $181,000 invested today to coast. That figure is far smaller than the final target because 35 years of compounding does the heavy lifting.
Why it's liberating
- It reframes the finish line. Reaching Coast FIRE early (often in your 30s) means you can downshift to lower-paying but more enjoyable work, take career risks, or go part-time — because you no longer need to save for old age.
- It rewards starting young. The earlier you front-load savings, the longer compounding runs and the smaller your coast number. This is the strongest possible argument for investing aggressively in your 20s.
- Use real returns. Because the target is in future dollars, use an inflation-adjusted (real) return like 5% rather than a nominal 7–8%, so the answer stays in today's purchasing power.
Coast FIRE pairs naturally with the full FIRE number calculator: reach Coast FIRE first, then decide whether to keep pushing for full independence or simply enjoy the freedom of no longer having to save.