Objectives: To provide a standardized metric for the assessment of depression severity to enable comparability among results of established depression measures. Study Design and Setting: A common metric for 11 depression questionnaires was developed applying item response theory (IRT) methods. Data of 33,844 adults were used for secondary analysis including routine assessments of 23,817 in- and outpatients with mental and/or medical conditions (46% with depressive disorders) and a general population sample of 10,027 randomly selected participants from three representative German household surveys. Results: A standardized metric for depression severity was defined by 143 items, and scores were normed to a general population mean of 50 (standard deviation = 10) for easy interpretability. It covers the entire range of depression severity assessed by established instruments. The metric allows comparisons among included measures. Large differences were found in their measurement precision and range, providing a rationale for instrument selection. Published scale-specific threshold scores of depression severity showed remarkable consistencies across different questionnaires. Conclusion: An IRT-based instrument-independent metric for depression severity enables direct comparisons among established measures. The ‘common ruler’ simplifies the interpretation of depression assessment by identifying key thresholds for clinical and epidemiologic decision making and facilitates integrative psychometric research across studies, including meta-analysis.