Clinicians need to provide accurate and useful mental health assessments, but existing scales often fail to meet expectations. The most urgent assessment problem may be in calculating suicide risk. We used a multi-model approach to develop a highly valid, ‘free culture’ license, instrument – the Suicidality Scale (SS). Diverse samples, aged 13-83, included 6,555 participants from three studies. An item pool and modified SS versions were critically examined through item response theory, hierarchical cluster, factor and bifactor analysis. SS psychometric properties were high (loadings > .77; a > 2.2; test-retest r = .87; internal consistency, ω = .96). Invariance checks were satisfied for demographics, self-reported psychiatric diagnosis, and suicide attempt history. The SS showed stronger psychometrics and significant differences in bivariate associations compared with included suicide measures. The ‘open source’ Suicidality Scale represents a step forward in accurate suicide risk assessment for adolescents, adults, and diverse populations.