Why Is My Coast FIRE Number Lower Than Full FIRE?

Because it is a different target, not a smaller version of the same one. Full FIRE requires the whole nest egg now; Coast FIRE requires just enough today that compounding finishes the job by your retirement date. The further out that date is, the lower the Coast number goes.

The questions below walk through the math and the assumptions.

Why is my Coast FIRE number lower than my full FIRE number?
Because it is not the same target. Full FIRE needs enough invested to cover spending forever at withdrawal; Coast FIRE only needs enough to grow into that number by your planned retirement date. Compound growth does the remaining work, so the required principal today is smaller — sometimes much smaller.

What exactly is the Coast FIRE number?
It is the current lump sum that, left untouched and compounding at your assumed real return, reaches your full FIRE number by your target age. You calculate it backwards: take the FIRE number and discount it to today using the expected growth over the years left.

How do I calculate my Coast FIRE number?
Decide your annual spending and safe withdrawal rate to get the full FIRE number (spending divided by withdrawal rate). Then discount that number to today: FIRE number divided by (1 + expected real return) raised to the years until retirement. That quotient is your Coast number.

Why does the gap grow so fast as I get younger?
Compounding is exponential. Each extra decade of growth roughly multiplies the discount factor by the growth rate, and the divisor lands on the whole nest egg. At 25 with a 7% real return and retirement at 60, a $1M FIRE number needs roughly $185k today; at 45 it needs roughly $640k.

Does a lower Coast number mean I can stop saving now?
It means you can stop contributing to retirement savings and let the existing pot grow — that is the definition. Your spending still needs funding until retirement, and the assumption only holds if the invested money actually compounds at the expected rate without withdrawals.

What assumptions does the Coast number depend on?
Three: the expected real return on the portfolio, the withdrawal rate you plan to use in retirement, and the number of years to your target age. Change any of them and the number moves a lot. A 1% lower return assumption raises the Coast number noticeably.

Should I use the Coast number or the full FIRE number as my goal?
They answer different questions. Full FIRE is the savings target for total freedom. Coast FIRE is the 'safe to relax contributions' checkpoint. Most people use both: aim for the Coast number first, then keep contributing what they can while the pot compounds.

Is a lower number always better?
No. A lower Coast number reflects a bigger reliance on future growth and a longer wait. If returns underperform, the Coast assumption quietly breaks. Treat the lower number as permission to ease off, not as a license to stop planning.

Want the number for your own situation? Run the Coast FIRE calculator here and compare it side by side with your full FIRE target.