FIRE vs. Coast FIRE: Which Retirement Number Are You Solving For?

Start with one spending-based FIRE number

Divide annual spending in today's dollars by the entered withdrawal-rate assumption to create the current FIRE number. A $60,000 spending target at 4% produces $1.5 million in today's dollars. That arithmetic is a planning convention, not evidence that 4% is appropriate for every retirement length, portfolio, tax situation, or market path.

FIRE searches for the first modeled crossing month

The FIRE Calculator grows the current portfolio, adds the same nominal contribution each month, and grows the target with inflation. It reports the first whole month when the unrounded portfolio reaches the unrounded target, plus the prior month's values so the threshold can be checked.

Coast FIRE removes future contributions

Coast FIRE discounts the inflation-grown target at the entered return for the years remaining until the target age. The result is the balance that would need to be invested now if no additional contributions were made. If the current balance is below it, the gap is a present-balance gap, not a monthly contribution recommendation.

Keep nominal and today's-dollar values labeled

A future portfolio can look larger because prices also rise. These tools retain a spending target in today's dollars, inflate it to the target date, and label future nominal balances separately. Comparing unlike dollar bases can create a false sense of progress.

A smooth return hides sequence risk

Both calculators use one constant return and one constant inflation rate. Real markets and inflation move unevenly, and losses near a withdrawal date can matter more than the same average loss earlier. Run conservative return, inflation, and spending cases and keep a margin rather than planning to the exact crossing month.

Use the result as a scenario checkpoint

A modeled FIRE date can help compare saving rates; a Coast FIRE balance can show whether future contributions remain necessary under one scenario. Neither result decides whether to leave work. Benefits, healthcare, taxes, debt, cash reserves, withdrawal flexibility, and nonfinancial goals require separate review.

Comparison table

Real examples

• At $60,000 of annual spending and a 4% entered withdrawal assumption, the current FIRE number is $1.5 million.
• Starting at age 35 with $200,000 and adding $2,500 monthly, a constant 7% return and 2.5% inflation first cross the modeled target after 280 months, or age 58 years 4 months.
• With the same spending, return, inflation, and a target age of 65, the modeled Coast FIRE balance needed at age 35 is about $413,327 when no future contributions are included.

Mistakes to avoid

• Calling the FIRE number a guaranteed safe balance.
• Using continued contributions in a Coast FIRE calculation.
• Comparing a future nominal portfolio with a target stated in today's dollars.
• Treating one smooth return as a market forecast.
• Ignoring taxes, fees, healthcare, benefit timing, debt, and withdrawal flexibility.

When this estimate is not enough

• The plan depends on a detailed withdrawal schedule or changing retirement income.
• You need to model taxes, account order, healthcare, benefits, or sequence-of-returns risk.
• Contributions, spending, or work income change on a known schedule.
• Leaving work would depend on legal, tax, investment, insurance, or benefit advice.

Source notes

• The Department of Labor retirement materials emphasize estimating needs, planning contributions, and revisiting assumptions.
• Investor.gov's compound-interest calculator demonstrates the sensitivity of hypothetical growth to time, rate, and additions; it does not predict returns.

Sources

Source boxes list references used for factual claims, safety notes, energy rates, product-sizing conventions, or official data points.

• U.S. Department of Labor Preparing for Retirement
• U.S. Department of Labor Savings Fitness
• Investor.gov compound interest calculator