Calculators that show their formula and where they break.
244 free calculators. Each page shows the formula, a worked example, the source it was checked against with the date, and the cases where the answer stops being right.
Started in September 2026 as a calculator site that shows its working. Every calculator needs a named source and a check date before the build will publish it, and a file of hand-worked values pins its output. Checking each calculator against its source in October found 40 faults, from an inverted baking-pan converter to state tax deductions the paycheck engine ignored.
Our stack
Next.js 15 static export and TypeScript on Cloudflare Workers, with D1, KV and Durable Objects. One node:test golden-value file per calculator, a post-build SEO check in CI, Dodo Payments for the paid embed plan and sponsor slots, Resend for email.
