Meet the FI Number Calculator: The Simplest, Most Important Number in Personal Finance

Alistair TeamJuly 1, 20265 min read
MicrotoolsFI numberfinancial independenceFIREmicrotools

There are a lot of complicated numbers in personal finance. Your FI number isn't one of them. It's your annual expenses multiplied by 25. That's it. That's the number you need invested to stop working forever, based on the 4% rule.

But "enough to stop working" isn't the same for everyone. Some people want a bare-bones exit. Some want to maintain their current lifestyle. Some want to upgrade. Alistair's FI Number Calculator gives you all three.

The Three FI Numbers

Lean FI

Your Lean FI number covers essential expenses only: housing, food, utilities, transportation, healthcare. No dining out, no travel, no entertainment — just the costs of staying alive and housed. It's the minimum viable financial independence number.

For someone spending $24,000/year on essentials: Lean FI = $600,000 (25 × $24,000)

At Lean FI, you can quit your job — but you're living frugally. No frills. No margin for error. It's the "I'm done with this career and I'll figure out income later" number.

Standard FI

Your Standard FI number covers your full current lifestyle — everything you spend today. If you spend $60,000/year: Standard FI = $1,500,000 (25 × $60,000)

This is the classic FIRE number. Quit your job, maintain your lifestyle, adjust for inflation each year. Based on the Trinity Study's 4% safe withdrawal rate for a 30-year retirement.

Fat FI

Your Fat FI number includes a lifestyle upgrade: more travel, better housing, hobbies, generosity. Pick your dream annual spend. If it's $120,000/year: Fat FI = $3,000,000 (25 × $120,000)

Fat FI is financial independence with cushion. You're not just surviving or maintaining — you're thriving.

What the 25x Rule Means

The 25x multiplier comes from the 4% rule: $1,000,000 invested, withdrawing 4% ($40,000) per year, adjusted for inflation, has historically lasted 30 years with a balanced portfolio. The inverse of 4% is 25.

For a longer retirement (40+ years), some planners suggest 30x or 33x. The tool lets you explore different multipliers if you're retiring very early.

Real Numbers: Three People, Three FI Numbers

  • Alex (age 28): Essential expenses $24,000/year → Lean FI = $600,000 → saving $30,000/year → reaches Lean FI in ~12 years at 7% returns
  • Jordan (age 34): Current expenses $60,000/year → Standard FI = $1,500,000 → saving $40,000/year with $200K already → reaches Standard FI in ~16 years
  • Taylor (age 45): Dream expenses $120,000/year → Fat FI = $3,000,000 → saving $80,000/year with $800K already → reaches Fat FI in ~12 years

Each person's number is different. Each person's timeline is different. But the formula is the same: expenses × 25 = freedom.

Why This Is the Starting Point

Before you optimize your portfolio, debate Roth vs. Traditional, or analyze tax-loss harvesting — you need to know your number. Everything in financial planning flows from this: how much you need to save, what return you need, how long you need to work.

Alistair gives you all three FI numbers in one screen so you know what's enough for you.

That's what this microtool does. Know your number — then build the plan to reach it.