Reasoning about quantum programs remains a fundamental challenge, regardless of the programming model or computational paradigm. Existing verification techniques are insufficient – even for quantum circuits, a deliberately restricted model that lacks classical control, but still underpins many current quantum algorithms. Many existing formal methods require exponential time and space to represent and manipulate (representations of) assertions and judgments, making them impractical for quantum circuits with many qubits. This paper presents SAQR-QC, a logic for Scalable but Approximate Quantitative Reasoning about Quantum Circuits. SAQR-QC has three characteristics: (i) some deliberate loss of precision is built into it; (ii) it has a mechanism to help the accumulated loss of precision during a sequence of reasoning steps remain small; and (iii) every reasoning step is local – involving just a small number of qubits – making reasoning scalable. We demonstrate the effectiveness of SAQR-QC via two case studies: the verification of GHZ circuits involving non-Clifford gates, and the analysis of quantum phase estimation – a core subroutine in Shor's factoring algorithm.