Guides
Long-form explainers for the topics our calculators and developer tools cover. Each guide is grounded in the formula or specification that the matching tool implements, and links back to that tool with a worked example.
Finance & mortgages
What is PITI?
Principal, interest, taxes, and insurance — the four parts of a real monthly mortgage payment, and what each one actually pays for.
15-year vs 30-year mortgage
The monthly payment difference is small; the lifetime interest difference is enormous. A worked example with real numbers.
Compound interest, explained
Why a 25-year-old who saves $200/month catches a 35-year-old who saves $400/month, and why 10 years matters more than the rate.
Health & body composition
Developer
JSON vs JSON5 vs JSONC
Why your .json file with comments isn't actually JSON, and which dialects which tools accept.
How to decode a JWT
The three-part structure, what each part contains, and why decoding the payload tells you nothing about whether to trust it.
Common regex patterns
Email, URL, phone, IPv4, ISO date — the patterns you keep half-remembering, with the gotchas spelled out.
Base64 encoding, explained
What Base64 is, why it's not encryption, and the URL-safe variant you've probably seen but never had spelled out.