The problem it answers
Quantum roadmaps talk in logical qubits; hardware ships physical ones. The surface code bridges the two at a steep price: one logical qubit costs roughly 2d² physical qubits at code distance d, and every logical operation costs time in code cycles. This calculator turns an algorithm's requirements into that physical bill.
Design
Pure Python, zero dependencies, a full test suite, and a citable release on Zenodo. It is deliberately small enough to audit in one sitting, which is the point for a tool whose job is to be trusted.