Lean FIRE vs fat FIRE: same savings, different dates

This post is the explainer: what lean, chubby and fat FIRE mean, and what moves the years between them. Use the lean, chubby and fat FIRE calculator →

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. Between them sits chubby FIRE, a retirement that costs about a quarter more than your life does today: comfortable, not extravagant. None of these is right. But the gaps between them are not a matter of taste; they are a matter of years.

How far apart the dates are

Cut retirement spending by 30 percent and, at the same retirement age, the FI number falls by 30 percent. What it does to the date is less obvious. At one retirement age, each extra dollar of target takes less time to reach than the one before, because your balance grows on its own growth as well as your contributions and builds faster the larger it gets. The multiple adds a one-off step: a plan that lands past 55 is measured against 27 times a year's spending instead of 29, which pulls it earlier than it would otherwise land. Together they make the dates unevenly spaced, and how far apart they sit depends on how much you have already saved, which is why the calculator below shows all four from one set of savings: lean at 70 percent of today's spending, as-is at 100, chubby at 125 and fat at 150.

The horizon effect is already in the figures

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 input people most often guess at is what a month actually costs today. Reign's free check 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.