Your Coast FIRE Number by Age: 30 vs 40 vs 50

The number that made Coast FIRE click for me was not a formula, it was a table row: age 30, $60,000 a year of retirement spending, Coast FIRE number about $140,000. Twenty years later, age 50, the same spending needs about $544,000. Nothing about the goal changed. Only the calendar did. Here is your Coast FIRE number by age, and why the calendar matters more than your income.

Coast FIRE number by age: the table

Assumptions: 7% real (inflation-adjusted) return, retirement at 65, full FIRE number at 25x annual spending. The figures follow the standard formula, FIRE number divided by 1.07 raised to the years until retirement, and line up with published Coast FIRE calculators.

Read the $40k column diagonally: $93,657 at 30, $184,249 at 40, $362,449 at 50. Roughly doubling every decade. That is not a coincidence; it is the math in the next section.

Why the number doubles every decade

The formula is your FIRE number divided by (1 + return) raised to the years until retirement. At 7% real, 1.07 to the 10th power is about 1.97, so every decade you wait roughly doubles the capital you need banked today. That is the entire argument for front-loading savings in your 20s and 30s. Time does the compounding, and time is the one input you cannot buy back later at any price.

Two ways this table can lie to you

The return assumption. Seven percent real is the long-run stock market estimate, not a promise. Run the same table at 5% real and the age-50 number for $40,000 of spending jumps from about $362,000 to about $481,000. Small assumption changes compound into life-sized differences over 35 years. If your plan only works at 7%, it doesn't work.

Your spending number. The table scales linearly with spending, which makes lifestyle the biggest lever you control. The difference between $40k and $60k of annual spending at age 40 is about $92,000 of required capital today. Every honest Coast FIRE plan starts with an honest spending estimate, in today's dollars, for the life you will actually live.

My take

Run your numbers at 5% real, not just 7%. If you are coast at 5%, you are actually coast. Then keep the employer match anyway, it is free money, and recheck yearly, because the formula is exact and your life is not. The table is a direction, not a destination.

Find your number: plug your spending, age, and return assumption into the Coast FIRE Calculator and see your Coast FIRE number and the gap left to close.

The 5% stress test

Seven percent real is the optimistic case. Here is the same table at 5% real, the stress test, for $40,000 a year of spending ($1 million FIRE number), retiring at 65:

At 50, the stress test demands about $119,000 more than the headline number. That gap is the price of humility about future returns. My rule: if you are coast at 5%, you are actually coast. If you are only coast at 7%, you have a plan, not a position.

What moves your number most

Three inputs, in order of leverage. Spending is linear and entirely yours: every $10,000 of annual spending is $250,000 of FIRE number, which at age 40 and 7% real is about $46,000 of Coast number today. Retirement age is the quiet one: a 40-year-old targeting 60 instead of 65 needs about $258,000 instead of $184,000 for the same $40k spending, because five fewer compounding years cost real money. Return assumption, you saw above. Notice what isn't on the list: your income. Coast FIRE is a capital-and-time equation; income only matters through what it lets you bank.

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