The Math Behind Early Retirement (FIRE) And What Most Calculators Get Wrong

You've probably seen the FIRE pitch: save aggressively, invest, retire at 40. The subreddits are packed with spreadsheets and projections. But most back-of-napkin FIRE math makes the same mistakes, and they compound badly over a 20-year horizon. Let's break down the actual math, where the popular shortcuts fail, and how to run the numbers properly. The Core FIRE Equation FIRE boils down to one question: how many years until my investment portfolio covers my annual spending? The classic rule of thumb is the 4% rule (from the 1998 Trinity Study). If your annual expenses are X, you need 25X saved. That's it. Simple division. FIRE number = Annual expenses / 0.04 Spending 30,000/year? You need 750,000. But this formula hides three assumptions that quietly wreck projections. Mistake #1: Ignoring Inflation During Accumulation The 4% rule accounts for inflation-adjusted withdrawals in retirement , but most people forget inflation during the savings phase . If you earn 60K and spend 30K today,