What Is Coast FIRE?

How to Calculate Your Coast FIRE Number

Coast FIRE is the amount you need today so that, left to compound at an assumed return, your portfolio reaches your retirement target by the age you plan to retire. The math:

Coast number = target ÷ (1 + rate)^{(retire age − current age)}

1. Set your retirement target. Estimate the portfolio you want by retirement.
2. Choose your retirement age and return assumption. Use a range rather than treating one rate as guaranteed.
3. Discount the target back to today. Divide the target by (1 + return rate) raised to the years until retirement.

Here is what that looks like for a $1.5M target at age 65, assuming a hypothetical 7% return:

Notice the pattern: starting 10 years later roughly doubles the coast number. Time is the cheapest input. Run your own coast number →

Coast FIRE does not mean retiring today

Reaching the modeled coast number means your current investments could reach the retirement target without further contributions under the chosen assumptions. It does not mean you can stop working, or that future retirement saving is unnecessary regardless of what happens. Most people still earn to pay current living costs while the portfolio compounds. Coast FIRE is a milestone on the way to full FIRE, not the finish line. You can think of it as "retirement savings: done, paycheck: still needed for now."

How much does the return assumption change your Coast FIRE number?

The same $1.5M retirement target can require very different amounts today. These are hypothetical constant nominal returns, not forecasts or recommended rates. All scenarios retire at age 65 with no further contributions, taxes or fees.

At age 30, assuming 5% instead of 7% nearly doubles the modeled amount required today. A smaller coast number based on a higher assumed return is not evidence that you are more financially secure.

Open the same age-30 scenario in the calculator with 5% assumed return · 7% assumed return · 9% assumed return. Change the ages and target to compare your own assumptions.

Is your retirement target in today’s dollars or future dollars?

The base calculator pairs a nominal annual return with a target in future dollars. A $1.5M balance in 35 years will not necessarily buy what $1.5M buys today. For an illustrative 3% annual inflation assumption, matching $1.5M of today’s purchasing power in 35 years would require a future target of about $4,220,794.

Future-dollar target = today’s-dollar target × (1 + inflation)^years

With a 7% nominal return and that inflation-adjusted future target, the age-30 coast amount is about $395,332, rather than $140,494. Open the future-dollar target scenario →

An equivalent approach uses a today’s-dollar target and a real return of (1 + nominal return) ÷ (1 + inflation) − 1. Keep the target and return on the same basis; do not adjust both for inflation a second time. These constant-rate examples exclude taxes and fees. Explore purchasing power →

Risk & limitations

• 7% is a hypothetical long-run assumption — not a guarantee; returns vary.
• The model ignores inflation adjustments, taxes and fees.
• Reaching the coast number assumes you can leave the money invested until retirement age.
• Educational projection only, not personalized financial advice.

Sources & assumptions: Coast FIRE values on this page are Caspenda scenario calculations using user-selected return assumptions, not forecasts. The SEC's Investor.gov compound-interest calculator likewise treats the interest rate as an estimated input and supports testing a variance range. Long-run historical return context is available from NYU Stern. Purchasing-power definitions are explained by Investor.gov. Educational projection only.