Practical use and limits
Use it for: Treat Coast FIRE as a sensitivity exercise: change target age, spending, inflation, real return, and current balance. The useful output is a range and a work choice, not a finish line.
Limits: Illness, dependents, unemployment, taxes, and a long downturn can interrupt the plan. Revisit assumptions and keep an emergency reserve outside the target.
The idea in one sentence
You have reached Coast FIRE when your invested assets, left to grow for a chosen period, could theoretically fund a future target without additional contributions. Current income still needs to cover living costs. For many people, the value is psychological and practical: work can be selected for learning, flexibility, or health rather than maximum salary.
The calculation
A simple estimate grows today's portfolio by an assumed real annual return for the years until the target date. The result is then compared with the future value of the portfolio needed to support planned spending. The calculation is sensitive to every input: target spending, withdrawal assumptions, inflation, taxes, fees, asset allocation, and the time horizon. Small changes can move the date by years.
Why real returns matter
If spending is measured in today's money, the growth assumption should be expressed after inflation. A nominal return can look generous while purchasing power grows more slowly than expected. Even a real-return model is only a scenario: actual returns arrive unevenly, and the path matters when withdrawals begin.
A worked example with a range
Suppose a plan needs $40,000 a year in today's money at age 60. Using a 4% planning withdrawal rate implies a $1,000,000 target. Someone age 35 with $250,000 invested would reach about $811,000 after 25 years at a 4.8% real return, but only about $523,000 at 3%. The first case is still below the target and the second is far below it, so declaring Coast FIRE from one optimistic assumption would be misleading. Contributions, a later date, lower spending, or other income can close the gap.
Stress-test the decision, not only the balance
Model at least a lower real return, a higher spending case, and a target date five years earlier or later. Then translate each result into a work decision: how much income must continue, which contributions can safely change, and what event would trigger a review? A threshold is more useful when paired with guardrails such as keeping an emergency fund, avoiding new high-interest debt, and resuming contributions after income recovers. The calculation should support a reversible choice rather than justify an irreversible leap.
What Coast FIRE does not solve
It does not create an emergency fund, health coverage, a home, or a plan for dependents. It does not guarantee that a portfolio will reach its target. It also does not mean contributions can never resume. A flexible plan can reduce contributions during a difficult period and increase them later when income improves.
Use Coast FIRE as a decision tool
Calculate a range rather than a single threshold, then ask what work would become possible at each stage. You might move to four days a week, take a lower-paid role with better health, or spend a year building a product. Revisit the plan when spending, family responsibilities, or market conditions change. The point is to buy options carefully, not to declare a finish line early.
Frequently asked questions
Can I stop investing after reaching Coast FIRE?
You can choose to reduce or stop contributions, but the plan remains exposed to investment performance, spending changes, inflation, taxes, and emergencies. Keep reviewing it and maintain a cash reserve.
How often should I recalculate Coast FIRE?
Review it at least annually and after a material change to income, spending, family obligations, asset allocation, or target date. Use the same assumptions when comparing progress so a changing model does not create false improvement.