Numi is a beloved indie macOS calculator that treats arithmetic like writing. You type 15% of 80 EUR in USD or 2 hours and 30 minutes - 45 minutes and the answer appears inline, right next to your prose. Variables, multi-line notebooks, and live currency / unit conversions are built in, so it doubles as a back-of-the-envelope scratchpad rather than a button-based calculator.

Numi has been around for years, with a public GitHub repo of the underlying engine, transparent paid pricing (free trial, then a license), and a small dedicated audience that includes designers, devs, and engineers who keep it open on a second monitor. It runs as a macOS-only desktop app with a global hotkey for quick access.