Injury Severity Score Calculator

What Is the Injury Severity Score (ISS)?

The Injury Severity Score (ISS) is an anatomical scoring system used to quantify the overall severity of traumatic injuries. It provides a standardized method for assessing patients with multiple injuries across different body regions. The ISS correlates with mortality, morbidity, and hospital length of stay, making it a critical tool in trauma triage and outcomes research.

Developed as an extension of the Abbreviated Injury Scale (AIS), the ISS takes the three most severely injured body regions and sums the squares of their AIS scores. The result is a single numerical value ranging from 0 to 75 that represents the cumulative severity of all injuries.

How the ISS Is Calculated

The calculation follows a specific methodology based on the Abbreviated Injury Scale (AIS). The body is divided into six regions:

• Head and neck
• Face
• Chest
• Abdomen and pelvic contents
• Extremities and pelvic girdle
• External (skin and soft tissue)

Each injury is assigned an AIS severity score from 1 (minor) to 6 (unsurvivable). The ISS calculation then follows these steps:

1. Identify the three most severely injured body regions — only the highest AIS score from each region is considered.
2. Square each of those three AIS scores.
3. Sum the three squared values.

The formula is: ISS = A² + B² + C², where A, B, and C are the highest AIS scores from three different body regions.

If any injury has an AIS score of 6, the ISS is automatically set to 75, as this indicates an unsurvivable injury.

How to Use This Calculator

To calculate the ISS, you need the AIS severity scores for each injured body region. Enter the AIS score for each region that has sustained an injury. Leave regions with no injury blank or set to 0. The calculator will automatically identify the three highest scores from different regions and compute the total.

Note that only one score per body region is used — if a region has multiple injuries, use the highest AIS score for that region.

Interpreting the Results

The ISS score provides a general indication of trauma severity:

• ISS less than 9 — Minor to moderate trauma. Low mortality risk.
• ISS 9–15 — Moderate to severe trauma. Increasing risk of complications.
• ISS 16–24 — Severe trauma. Significant mortality risk. Often used as a threshold for major trauma designation.
• ISS 25 or higher — Critical trauma. High mortality risk.
• ISS 75 — Maximum score, indicating at least one unsurvivable injury.

An ISS of 16 or higher is commonly used in trauma research and clinical practice to define major trauma. However, the ISS is a research and triage tool — it does not replace clinical judgment. Individual patient factors such as age, pre-existing conditions, and physiological response also significantly affect outcomes.

Limitations of the ISS

The ISS has several well-documented limitations that users should understand:

• Single score per region — The ISS only uses the highest AIS score from each body region, potentially underestimating severity when a region has multiple serious injuries.
• No physiological data — The ISS is purely anatomical and does not account for vital signs, age, or comorbidities.
• Non-linear scoring — The squared scoring system means that a single very severe injury can dominate the total score.
• Region boundaries — Some injuries may span multiple regions, making classification ambiguous.
• Not for individual prognosis — The ISS is designed for population-level analysis and trauma system evaluation, not for predicting individual patient outcomes.

Common Mistakes When Calculating ISS

• Using more than one score per region — Only the highest AIS score from each region should be used. Multiple injuries in the same region do not increase the region's contribution.
• Including more than three regions — Only the three most severely injured regions are used. Scores from additional regions are ignored.
• Confusing AIS and ISS — The AIS score is the input; the ISS is the calculated output. They are not interchangeable.
• Misclassifying body regions — Ensure each injury is assigned to the correct body region according to the AIS classification.

Practical Use Cases

The ISS is widely used in:

• Trauma research — To stratify patients by injury severity for clinical studies and outcome analysis.
• Trauma center benchmarking — To compare expected versus observed mortality rates across institutions.
• Prehospital triage — As part of field triage protocols to identify patients who may benefit from transport to a Level I trauma center.
• Quality improvement — To evaluate trauma care processes and identify areas for improvement.
• Injury epidemiology — To describe injury patterns and severity in populations.