I built a free FIRE calculator that runs in a single HTML file

The FIRE calculator space has a problem. Every tool I found wanted my email, showed ads, or tried to sell me a "personalized retirement plan" for $29/month. I just wanted the math.

So I built one. Single HTML file, 32KB, zero dependencies. MIT license.

The core calculation: if you're 30, spend $40K/year, and want to retire at 65 — you need $93,676 invested right now. After that, compound interest does the work. You never save another dollar.

That number comes from two formulas:

- FIRE Number = Annual Spending / 4% (Trinity Study)

- Coast FIRE Number = FIRE Number / (1.07)^years_to_retirement

It grew into a full Next.js app with 5 calculator types (Coast, Barista, Fat, Lean, Grid) and a Chrome extension. But the standalone HTML still works — download it, open it, run offline.

Site: coastfirehub.com

Source: github.com/coastfirehub/fire-calculator

Revenue model: none. It's free. I built it because the existing options were annoying.

Would love feedback from anyone in the FIRE/personal finance space.