What is "FU money"?
FU money is enough savings and investments that you no longer have to accept any particular job or situation for the money. It isn't necessarily "never work again" wealth — it's the cushion that gives you the freedom to say no. Your FU number is the size of that cushion.
The 4% rule (and 25× spending)
A widely cited guideline suggests that if you withdraw about 4% of an invested portfolio in your first year of independence and adjust for inflation after that, the portfolio can reasonably be expected to last for decades. Flip it around and 4% means you need roughly 25× your annual spending invested. Lower the withdrawal rate to 3% and you need more (about 33×); raise it to 5% and you need less (20×). It's a starting framework, not a guarantee.
Why your savings rate matters most
The single biggest lever on how fast you reach independence is the gap between what you earn and what you spend — your savings rate. A higher savings rate shrinks the number you need and grows the pile faster at the same time, which is why it compounds so powerfully.
Freedom runway vs. full independence
You don't need your full FU number to benefit. Your freedom runway — how long your current savings could cover your costs — is what buys you time to change careers, take a break, or take a risk. Independence is the destination; runway is the freedom you have along the way.