Practical guides

Guides

Plain-language articles that explain the formulas, assumptions and real-world factors behind the calculators.

Every guide on this page exists because a calculator result on its own only tells you half the story. A number without context is easy to misread — a "40% savings rate" or a "9-year solar payback" means very little until you know what went into it and what it leaves out. These guides walk through the reasoning behind each calculator: what the formula actually does, which assumptions it makes on your behalf, where those assumptions tend to break down in real life, and what to check before you rely on the result for a real decision.

They are written to be read on their own, not just as an appendix to a calculator. Start with the topic closest to the decision in front of you — a monthly budget, a loan you are comparing, a rent-versus-buy question, a car purchase, an electricity bill, a solar quote, a university cost, a trip, or a freelance rate — and use the linked calculator once you understand what it can and cannot tell you.

How to get the most out of these guides

Read the "Assumptions" and "Limitations" sections in each guide before trusting a number, especially for anything with a multi-year horizon like a mortgage, a degree or a solar system — small changes in an interest rate or an appreciation figure compound a lot over five or ten years. If two guides overlap, for example loan interest and rent versus buy, that overlap is intentional: real decisions rarely involve only one variable, and understanding both gives you a fuller picture than either guide alone.