Injury Severity Score (ISS) Calculator — Anatomical Trauma Severity

How to Use This Calculator

1. Select the highest AIS score for each of the three most injured body regions.
2. ISS, injury severity category, and approximate mortality display instantly.
3. Use the full 6-region ISS Calculator tab for complete anatomical scoring.
4. Use the NISS tab if the patient has multiple severe injuries in the same region.
5. Professional tier calculates both ISS and NISS simultaneously.

Formula

Example

Frequently Asked Questions

• What is the Injury Severity Score and how is it calculated? ▼
  The Injury Severity Score (ISS) is an anatomical scoring system that quantifies overall injury severity in trauma patients with injuries to multiple body regions. Introduced by Baker et al. in 1974, ISS is calculated from the Abbreviated Injury Scale (AIS), which grades individual injuries in six body regions — head/neck, face, chest, abdomen, extremities/pelvis, and external — on a scale of 1 (minor) to 6 (unsurvivable). ISS is calculated by taking the highest AIS scores from the three most severely injured body regions, squaring each, and summing them. For example, AIS scores of 4 (chest), 3 (abdomen), and 2 (extremities) give ISS = 16 + 9 + 4 = 29. If any region has an AIS of 6, ISS is automatically set to 75 (the maximum). An ISS of 15 or above is the conventional threshold for major trauma.
• What does the ISS score mean in terms of injury severity? ▼
  ISS scores are grouped into severity categories: 1–8 represents minor injury with mortality under 1%; 9–15 represents moderate injury with approximately 1–2% mortality; 16–24 represents serious injury with about 5–10% mortality; 25–40 represents severe injury with approximately 20–40% mortality; and 41–75 (including the maximum of 75 for any AIS 6 injury) represents critical or maximal injury with 50–80% or higher mortality. An ISS of 15 or above is the standard threshold used in most trauma registries and research to define major trauma, triggering level I or II trauma centre care recommendations. These mortality estimates are population averages; individual patient outcomes depend on age, physiological response, access to trauma care, and many other factors.
• What is the difference between ISS and NISS? ▼
  ISS selects the highest AIS score from the three most severely injured body regions — meaning no region can contribute more than one injury to the final score, even if a patient has two severe chest injuries. This limitation led to the development of the New ISS (NISS), introduced by Osler et al. in 1997. NISS takes the three highest AIS scores from any body region, regardless of anatomical location. For a patient with AIS 4 haemothorax, AIS 4 pneumothorax, and AIS 3 liver laceration: ISS = 4² + 3² + 2² = 29 (if face is the next highest at AIS 2), while NISS = 4² + 4² + 3² = 41. Studies have shown NISS is a better predictor of mortality than ISS for patients with multiple injuries in the same body region, particularly thoracic injuries.
• How is ISS used in clinical practice and research? ▼
  ISS serves several key functions. In clinical practice, an ISS of 15 or above guides triage to major trauma centres and activates trauma team responses. In trauma registries such as TARN (Trauma Audit and Research Network) in the UK and NTDB (National Trauma Data Bank) in the USA, ISS is a standard data field used for case-mix adjustment and outcome benchmarking. In research, ISS is used for stratifying patients in clinical trials and defining study populations. ISS also contributes to the TRISS methodology (combined with RTS and age) for calculating predicted probability of survival. It is worth noting that ISS calculation requires full diagnostic workup — CT scans and operative findings — so final ISS is typically available after admission rather than at the point of initial triage.
• What are the limitations of the ISS? ▼
  ISS has several recognised limitations. First, as discussed, only one injury per body region contributes to the score, underestimating severity in patients with multiple injuries in the same region — the main driver of NISS development. Second, AIS coding requires training and access to the official AIS dictionary, which is a proprietary AAAM document, making coding inconsistent between centres without standardised training. Third, ISS does not account for physiological response — a patient with the same ISS may have very different survival depending on their haemodynamic reserve, comorbidities, and access to care. Fourth, ISS correlates with blunt trauma better than penetrating trauma; some authorities suggest penetrating trauma is better scored with AIS-based systems that account for the unique injury patterns. Fifth, ISS is retrospective — it can only be calculated after full diagnostic evaluation, limiting its use for immediate triage decisions.

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