Lean FIRE vs fat FIRE: the same savings, two very different retirement dates

Every FI number is a multiple of spending, so the fastest way to change the number is to change the spending it is built on. Lean FIRE plans a retirement that costs less than your life does today, typically 60 to 80 percent of it. Fat FIRE plans one that costs more, one and a half to two times, because you want the travel, the second home, or simply no constraint at all. Neither is right. But the gap between them is not a matter of taste; it is a matter of years.

Why the gap is bigger than the multiplier

Cut retirement spending by 30 percent and the FI number falls by 30 percent. That part is obvious. What is not obvious is what it does to the date: the last third of any FI number is the slowest third to accumulate in years, because it is the part contributions barely touch and compounding has to do alone. Shave the target and you skip the slowest years. Raise it by half and you add the slowest years twice over. The calculator below makes this visible: three targets, one set of savings, and dates that are not evenly spaced.

The horizon effect cuts the other way

Retiring earlier means the money must last longer, so Reign uses a lower withdrawal rate for an earlier retirement, which raises the multiple. A lean plan that lands at 45 needs 29 times a year's spending, not 25. So lean FIRE gets part of its advantage back taken away, and fat FIRE, landing later, gets a small discount. The table shows the FI number at the age each plan reaches it, so that adjustment is already inside the figures.

Run your own numbers

The one input that matters most is the one people guess at: what a month actually costs today. Reign's free Snapshot starts from what you state and, once accounts are connected, shows what they say beside it. The calculator also lives at /tools/lean-vs-fat-fire.