Many Ubicomp appliances require the recognition of context. Existing context systems do not provide information about the quality of the context recognizied to the appliance at runtime. In this paper we propose the first context quality system which gives quantitave measures, the Context Quality Measure (CQM), in real time. The CQM can be used by application to improve decision quality when interpreting context values. Our Fuzzy Inference System based approach considers the context detection algorithm as a black-box. It is therefore able to give generalized independet context quality measures and is applicable as an add-on to any context recognition system. A first practical implementation shows a gain of 33% in context detection quality in tested application scenarios.