Our methodology
Every calculator here is built to be correct first, pretty second. Formulas are cross-referenced against the community-standard math used across the FIRE ecosystem — the 4% rule from the Trinity Study and William Bengen’s research, the 25×-expenses target, and the compound-growth and annuity formulas that underpin FIRE, Coast FIRE, and Barista FIRE numbers.
Where a tool uses historical market data — such as our S&P 500 backtest — we use real annual total returns (with dividends reinvested) rather than round-number assumptions, and we cite the source. We deliberately handle the tricky edge cases many online calculators get wrong: Barista FIRE, for instance, behaves differently depending on whether your part-time income is below, equal to, or above your expenses — and our tool models all three.
What we believe
- Free and open. No email gate, no “unlock the results” friction. Every tool works fully, immediately.
- Your data stays yours. All calculations run entirely in your browser. We don’t send your numbers to a server — the optional “remember my inputs” feature uses your device’s local storage only.
- Education, not advice. These are planning tools to help you think clearly, not personalized financial, tax, or investment advice.
- Show the work. Each tool explains its formula and assumptions so you can check the logic yourself.
- Depth over breadth. We’d rather build a handful of genuinely useful calculators than a hundred thin ones.
Who builds this
Financial Depth is built and maintained by a solo software developer — writing under the site's name here. I'm not a financial advisor, and nothing on this site is advice. What I am is the person who writes every formula, so I care a great deal about one thing: that the math is correct and that you can see exactly how each number was produced.
This project started from a simple frustration. Most online finance calculators are black boxes — you type numbers in, a result comes out, and you have no way to check whether the logic is sound. Several get the tricky cases quietly wrong. Barista FIRE is the classic example: the math changes depending on whether your part-time income is below, equal to, or above your expenses, and many tools only handle one of those cases. I built these to handle all three, and to show their working rather than hide it.
Every calculator here runs entirely in your browser. There's no signup, no email wall, no account, and nothing you enter is sent to a server — your numbers stay on your device. That's a deliberate engineering choice, not a marketing one: financial inputs are private, and they should stay that way.
Where a tool relies on historical data, I use real figures and cite the source rather than round-number assumptions, and I document the formula behind each result in plain terms. If you spot an error, have a question about how a calculation works, or want a tool that doesn't exist yet, I genuinely want to hear it — corrections are how these get better. You can reach me at hello@financialdepth.com.
Data sources
- S&P 500 historical returns — annual total returns compiled from Slickcharts and cross-referenced with the NYU Stern (Damodaran) historical dataset.
- Safe withdrawal rate — the 4% rule, based on the Trinity Study and William Bengen’s original 1994 research.
- Core FIRE formulas — the 25×-expenses rule and compound-growth math standard across the financial independence community.
Disclaimer
Financial Depth is for educational and informational purposes only and does not constitute financial, investment, tax, or legal advice. Calculators use simplified models and assumptions that may not reflect your personal circumstances or future market conditions. Always consult a qualified professional before making financial decisions.