The Problem
FIRE (Financial Independence, Retire Early) is one of those concepts that sounds extreme until you actually read about it. The basic idea is: accumulate enough that your investment returns cover your expenses indefinitely. You don’t have to retire at 35, but knowing the number gives you options.
I got curious about my own numbers and started looking at calculators online. There are quite a few good ones, actually. But most limit the assumptions you can make and are not easy to customise.
I wanted something I can control and customise. So I started building a spreadsheet. This lacked a few things:
- It was difficult to add assets/incomes/expenses to model different scenarios
- It was unusable on my phone
- Adding new features was becoming tricky
Once I had the basics in the spreadsheet, I started thinking about turning it into a small Web app.
What the Tool Does
The FIRE Calculator runs a year-by-year simulation of your financial life. Rather than a single input box, it lets you model your situation in full:
- Assets: add as many as you need (investment portfolios, rental properties, pension pots). Each has its own starting value, expected appreciation rate, yearly income (fixed or as a percentage of value), and yearly contributions drawn from your cash balance. You can set a start and end age, mark an asset as non-liquid, and transfer its balance to another asset when it ends.
- Cash Balance: a special liquid account where all income lands and all expenses are paid from. It earns its own interest rate and can go negative to represent debt.
- Income streams: salary, freelance work, pension, rental income, or anything else. Each has an annual amount, a yearly growth rate, and start/end ages. When a stream ends, you can optionally transfer its final value (e.g. a pension lump sum) into an asset.
- Expenses: living costs, mortgage repayments, school fees, or any recurring outgoing. Each grows at its own rate and runs between the ages you specify.
Each year the simulation pays expenses, applies appreciation, collects income, and checks whether your safe withdrawal amount covers your expenses - that’s the FIRE condition. The results table shows every year broken down by asset, income stream, and expense, with totals for net cash flow and safe withdrawal. Summary cards tell you when (or whether) FIRE is achieved, and when (or whether) the portfolio depletes.
If you want to understand the concepts behind the numbers; compound growth, the 4% rule (Trinity Study), safe withdrawal rates, or the FIRE movement in general. These are good starting points: Investopedia on FIRE, Mr. Money Mustache on the maths, and Early Retirement Now’s deep dive on safe withdrawal rates.
The most important thing I tried to build in is honesty. The results depend entirely on your inputs: expected return, how consistent your contributions are, how stable your expenses really are. Real life throws curveballs. What I hope people get from the FIRE Calculatorisn’t a precise retirement date, it’s more of a direction. A sense of whether you’re on track, whether a small change in savings rate makes a big difference, or whether you need to rethink something entirely. That’s worth something, even without exact numbers.