Introduction: Injury Severity Score (ISS) is used to describe anatomical lesions. ISS is traditionally determined through medical record review (standard ISS), which requires specific training and may be time-consuming. An alternative way to obtain ISS is by use of ICD-9/10 injury diagnoses, and several conversion tools exist. We sought to evaluate the agreement between standard ISS and ISS obtained with two tools converting ICD-10 diagnoses. Methods: Our cohort consisted of trauma patients ≥18 years admitted to Rigshospitalet between 1999 and 2016. The included patients had standard ISS recorded in the Trauma Audit and Research Network (TARN) database (ISS-TARN), and ICD-10 injury diagnoses for the trauma contact were recorded in the Danish National Patient Registry. We used the tools ICDPIC-R and ICD-AIS map to calculate ISS based on ICD-10 diagnoses. ICDPIC-R provided two ISSs: ISS-TQIP and ISS-NIS. The ICD-AIS map resulted in one ISS: ISS-map. The ISS-TARN was compared to the conversion tool ISSs using Bland-Altman plots. The agreement between ISS-TARN and the conversion tool ISSs for ISS above 15 was assessed using kappa statistics (κ). Results: We included 4308 trauma patients. The median age was 44 years, 70% were male, and 92% had a blunt injury mechanism. The median ISS-TARN was 16 [IQR: 9–25], and the median conversion tool ISSs were 10 [2–25] (ISS-TQIP), 17 [5–26] (ISS-NIS), and 9 [4–16] (ISS-map). The Bland-Altman plots all showed increasing difference in ISS with increasing mean ISS. Bias ranged from −7.3 to 1.1 and limits of agreement ranged between −28.0 and 25.7. The agreement for ISS above 15 was fair to moderate (κ = 0.43 (ISS-TQIP), 0.44 (ISS-NIS), and 0.29 (ISS-map)). Conclusion: Using ICDPIC-R or ICD-AIS map to determine ISS is feasible, but limits of agreement were unacceptably wide. The agreement between ISS-TARN and ICDPIC-R was moderate for ISS above 15.