The Interest Rate Solver solves the common personal and business finance question: back-solve rate from emi. Enter your numbers on the left, the answer updates instantly on the right — no submit button, no signup.

Yes. Every calculator on CalcMaster is free, has no usage caps, requires no signup, and shows no ads. The site is open-source-friendly and supported entirely by the author.

Yes. CalcMaster is fully responsive and installable as a PWA — on Android tap the browser menu → "Add to Home Screen"; on iOS Safari → Share → "Add to Home Screen". After installing, the Interest Rate Solver works offline.

Nowhere by default. Your inputs live in your browser's memory while you're on the page; a copy of your recent calculations is saved to localStorage on your device so the History page works. Nothing is sent to a server unless you explicitly enable cloud sync.

The math is sourced from peer-reviewed and standard public formulas; you can read the formula in the result card. For decisions involving real money or health, always cross-verify with a qualified professional — calculators are educational, not advice.