Injury Severity Score (ISS) Calculator
Enter the Abbreviated Injury Scale (AIS) severity rating for each of the six body regions. The calculator selects the three highest scores, squares each one, and sums them to give the ISS. A score above 15 meets the clinical threshold for major trauma. Results update instantly as you adjust any region.
What is the Injury Severity Score?
The Injury Severity Score (ISS) is an anatomical scoring system developed by Baker and colleagues in 1974 to provide a single number that describes the overall severity of multiple trauma. It is calculated by assigning an Abbreviated Injury Scale (AIS) severity code to the single worst injury in each of six body regions, then summing the squares of the three highest scores. The resulting number ranges from 0 to 75, where higher values reflect more severe and life-threatening combinations of injuries. ISS is the most widely used anatomical severity score in trauma research and triage, and it is the cornerstone of trauma registry data collection worldwide.
The AIS and six body regions
Each body region is scored on the Abbreviated Injury Scale: 0 means no injury, 1 is minor, 2 is moderate, 3 is serious, 4 is severe, 5 is critical, and 6 denotes an unsurvivable injury. The six regions used for ISS are: Head and Neck (brain, skull, cervical spine and cord), Face (skeleton, nose, mouth, eyes, ears), Chest or Thorax (rib cage, sternum, thoracic spine, diaphragm), Abdomen (abdominal organs, lumbar spine), Extremities and Pelvic Girdle (arms, legs, pelvic ring), and External (lacerations, burns, hypothermia). Only the worst injury per region is counted. If any region receives an AIS of 6, the ISS is immediately and automatically set to 75 regardless of the other scores.
Major trauma, mortality, and clinical use
An ISS above 15 is universally accepted as the threshold for major trauma or polytrauma, and most trauma systems use this cut-off to trigger a full trauma team response or to define which patients belong in a Level I trauma centre. Research consistently shows that ISS correlates with probability of mortality, time in an intensive care unit, and overall hospital length of stay across large populations. An ISS of 75 carries an extremely high mortality rate, while patients with scores below 9 are generally managed without specialized trauma activation. The score is also used for benchmarking hospital performance, quality improvement, and trauma research. Clinicians and registrars should note that AIS coding requires training and standardized dictionaries; the same physical injury can receive different AIS codes depending on the edition of the AIS dictionary used.
Limitations and the New Injury Severity Score (NISS)
ISS has two well-known limitations. First, it counts only the single worst injury per body region, so two serious injuries in the same region (for example, two AIS-4 injuries both in the chest) contribute no more to the score than one of them would alone. Second, patients with multiple serious injuries concentrated in fewer regions may score lower than patients with the same total tissue damage spread across different regions. The New Injury Severity Score (NISS), proposed by Osler in 1997, addresses the first limitation by using the three worst injuries anywhere in the body, regardless of region. NISS tends to out-perform ISS for predicting mortality in patients with multiple injuries in the same region, but ISS remains the standard for most trauma registries and international comparisons because of its long history and broad adoption.
ISS severity classification
| ISS range | Severity | Clinical significance |
|---|---|---|
| 0 | No injury | No recorded injury in any region |
| 1-8 | Minor | Generally does not require hospitalization |
| 9-15 | Moderate | Hospitalization typically warranted |
| 16-24 | Severe | Major trauma threshold exceeded (ISS > 15) |
| 25-74 | Very Severe | High mortality risk; ICU care usually required |
| 75 | Unsurvivable | AIS 6 in at least one region; injury incompatible with survival |
Standard ISS bands with corresponding clinical severity and the major trauma threshold used in most trauma systems.
Frequently asked questions
What ISS score is considered major trauma?
An ISS greater than 15 is the universally accepted threshold for major trauma (polytrauma). Most trauma systems use this cut-off to activate a full trauma team, route patients to a Level I trauma centre, and flag cases for specialized care. An ISS of 16 or above means the patient has at least two AIS-3 injuries in different body regions, or one AIS-4 injury plus at least one other injury of any severity.
Why does an AIS 6 automatically make ISS equal 75?
An AIS score of 6 represents an injury that is considered unsurvivable or incompatible with life, regardless of what is done for the patient. Rather than have such a score mixed into a mathematical formula that could theoretically produce a result below 75, the system sets ISS to 75 automatically. This preserves the logical consistency of the scale: 75 is the theoretical maximum (6 squared times three regions), and any AIS-6 injury alone is treated as the worst possible scenario.
What is the difference between ISS and NISS?
Both scores square AIS values and sum three of them, but they differ in which injuries they select. ISS takes the highest AIS score from each of the three most severely injured body regions, so multiple injuries within one region only count once. NISS (New Injury Severity Score) takes the three worst AIS scores anywhere in the body, regardless of region. NISS can be higher than ISS for patients with several serious injuries in the same body region, and some studies find it better predicts mortality in that group.
Can ISS be used to predict survival?
ISS correlates with mortality at a population level and is a standard variable in survival-prediction models such as TRISS (Trauma and Injury Severity Score), which combines ISS with physiological parameters like revised trauma score, age, and mechanism of injury. However, ISS alone is not sufficient to predict outcomes for an individual patient, because it captures anatomy only and ignores physiological response to injury, comorbidities, time to treatment, and quality of care.
How is ISS used in trauma registries?
Trauma registries collect ISS as a standard data point for every admitted patient. It is used to measure casemix severity across hospitals, to risk-adjust mortality comparisons (so a hospital treating more severely injured patients is not unfairly penalized), to monitor trends in injury patterns over time, and to stratify patients for research studies. Most registries use the AIS 2005 or AIS 2008 update dictionary for consistent coding.
Does a higher ISS always mean the patient will die?
No. ISS reflects statistical risk at a population level, not an individual guarantee. Some patients with very high ISS values survive, particularly with rapid access to high-level trauma care. Factors like age, physiological reserve, time to surgery, blood product availability, and surgical expertise all influence outcome independently of the ISS. An ISS of 75 does not mean every such patient will die, though it carries an extremely high mortality rate in published trauma series.
Sources
- Baker SP, O'Neill B, Haddon W, Long WB. The Injury Severity Score: a method for describing patients with multiple injuries and evaluating emergency care. J Trauma. 1974;14(3):187-196.
- Osler T, Baker SP, Long W. A modification of the Injury Severity Score that both improves accuracy and simplifies scoring. J Trauma. 1997;43(6):922-926.