Guides

The calculators on this site give you a number quickly. These guides are for when you want the reasoning behind that number — the research a formula is based on, the assumptions baked into it, and the parts of a decision that a calculator alone can't fully capture. New guides get added as they're written.

Most financial and planning tools online present a single formula as though it's the whole answer — enter your numbers, get a result, done. In practice, almost every useful formula in personal finance rests on assumptions that matter: a withdrawal rate backtested against a specific stretch of market history, a loan comparison that only tells half the story without accounting for what you'd do with the difference, a "rule of thumb" that got repeated so often it started sounding like a law rather than a starting point. These guides exist to make those assumptions visible rather than hidden.

Each guide is written to stand on its own — you don't need to have used the related calculator first, and reading a guide doesn't require running any numbers if you just want the context. Where a guide connects directly to a specific tool on this site, it links to it, so you can move from the reasoning to your own actual numbers when you're ready.

If there's a specific formula or rule of thumb you'd like explained this way — where it came from, what it assumes, and where it breaks down — the contact page is open for suggestions, and it's a genuinely useful way to influence what gets written next.