A FIRE calculator has one job: turn what you hold and what you earn into an FI number and a date. We took the forecast engine out of the app and put it on the public site as a free one: krosos.com/forecast. It runs entirely in your browser, needs no account, and stores nothing.
The same engine, not a toy version
This is not a simplified marketing calculator. The FIRE calculator ships the actual engine file from the product, byte for byte, so the numbers it produces are the numbers a customer sees. It models, month by month:
- Your portfolio by asset class: cash, money-market funds, stocks, pension, property, commodities, valuables and crypto, each with its own expected return and volatility.
- Income and spending: salary with annual growth, expenses indexed to inflation, a separate retirement budget from your chosen retirement month, and what you invest into each asset class every month.
- A mortgage or loan: balance, rate, monthly payment and years left, so the repayment sits in the plan until it ends.
- Taxes, two ways: a simple flat haircut on growth, or a realistic model that taxes gains only when they are realized, with an annual exemption and a separate rate on interest-type yield.
- Retirement mechanics: a pension income with its own start date, a pension pot that stays locked until an unlock month, and three drawdown strategies (sell as needed, cash out everything, or glide to a target mix).
- Uncertainty: a Monte Carlo mode that runs 500 simulated markets and shows the 10 to 90 percent range around the median, plus how often the plan survives.
The result is an FI number, the earliest date you could stop working with your age next to it, how long the money lasts, and a chart you can interrogate month by month.
We used it to run one European household under Belgian, German and French capital gains rules, and the stop date came out more than three years apart: what the 4% rule leaves out in Europe.
The link is the save button
Nothing you type leaves your browser. Instead, every number lives in the page address: change a value and the URL updates. Bookmark it and your plan is there next time; send it to someone and they see exactly your scenario, editable. That is the whole persistence model, and it means we could make this free without accounts, cookies or stored data.
What it deliberately leaves out
The FIRE calculator stops where typed-in numbers stop making sense. The app computes the same forecast from your real ledger: balances synced read-only from banks, brokers and exchanges, several loans rather than one, recurring purchases that actually execute, and cost basis tracked per lot so the tax model works on your actual gains rather than an estimate. If the calculator's answer matters to you, the 14-day trial runs it on the real thing, and the live demo shows the full app on example data.
As always: these are estimates from your own assumptions, for planning. Not financial advice.