π Injury Severity Score (ISS) Calculator
The trauma score behind triage and research.
For informational purposes only, not medical advice. Consult a healthcare professional.
The Injury Severity Score compresses a multi-injury trauma patient into one number from 0 to 75. Each of six body regions gets an AIS grade from 1 (minor) to 6 (unsurvivable); the three highest are squared and summed. Squaring is the point: it weights severe injuries far more than a pile of minor ones.
ISS above 15 defines major trauma in research and triage worldwide, and mortality climbs steeply past 25. Enter the AIS per region and the calculator applies the squaring rule, including the automatic 75 for any AIS 6.
How to Use This Calculator
Grade each body region with the Abbreviated Injury Scale: 1 minor, 2 moderate, 3 serious, 4 severe, 5 critical, 6 unsurvivable, 0 uninjured. Use the worst injury in each region. The calculator takes the three highest, squares each, and sums them. Any region graded 6 sets the ISS to 75 automatically.
Grade the worst injury in each of the six body regions on the 1 to 6 AIS scale.
Enter 0 for uninjured regions.
Read the ISS: the squared sum of the three highest region scores.
Compare against 15 (major trauma) and note the severity band.
Why the Squares
Any AIS 6 -> ISS = 75
Baker and colleagues introduced ISS in 1974 after finding that simple AIS sums underweighted severe injuries: three AIS-2 injuries (sum 6) looked as bad as one AIS-4 plus two AIS-1s (also sum 6), but the patients were not equally sick. Squaring fixes the ranking: 4+1+1 becomes 16+1+1 = 18, while 2+2+2 becomes 4+4+4 = 12. The automatic 75 for AIS 6 reflects injuries considered unsurvivable by definition. The six regions are head/neck, face, chest, abdomen, extremities, and external, and only the single worst injury per region counts.
Worked Example
Head AIS 4, chest 3, abdomen 2: the top three are 4, 3, 2, and 4^2 + 3^2 + 2^2 = 29 gives an ISS of 29, which clears the ISS > 15 cutoff of 15. A single AIS 5 head injury alone scores 25 (5 squared), showing how the squaring punishes severe single-region injuries.
Reading the Score
ISS 1 to 8 is minor trauma, 9 to 15 moderate, 16 to 24 major, and above 25 severe to critical. The widely used research cutoff is 15: above it, the patient counts as major trauma in registries and studies. Mortality is low through the teens, then rises sharply: published series show mortality roughly doubling with each band above 25. ISS also correlates with length of stay, ICU need, and cost, which is why administrators care about it too.
| ISS | Band | Typical meaning |
|---|---|---|
| 1-8 | Minor | Ward care, low mortality |
| 9-15 | Moderate | Possible admission |
| 16-24 | Major | Trauma team, likely ICU |
| 25-75 | Severe/critical | High mortality risk |
A Retrospective Research Tool
ISS is scored after the full injury picture is known, often at discharge, and it does not guide real-time resuscitation: trauma teams treat physiology (blood pressure, consciousness, bleeding), not the score. Accurate AIS grading needs training and the official AIS dictionary. Students and researchers can use this calculator to learn the math; clinical registry scoring belongs to trained coders. For informational purposes only, not medical advice. Consult a healthcare professional.
Related tools: Glasgow-adjacent NIH Stroke Scale and Morse fall scale.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a bad Injury Severity Score?
Above 15 is major trauma; above 25 is severe with steeply rising mortality. An ISS of 75 means an injury graded unsurvivable.
What is the difference between AIS and ISS?
AIS grades one injury from 1 to 6. ISS combines the worst AIS from the three most injured body regions by squaring and summing them.
Why square the AIS scores?
So severe injuries dominate the total. Without squaring, three moderate injuries would outscore one critical injury, which misranks how sick patients are.
What does ISS 75 mean?
At least one body region had an AIS 6 injury, graded unsurvivable. By definition the ISS becomes 75 automatically.
Is ISS calculated in the emergency department?
Usually not for care decisions; it is scored retrospectively once all injuries are known. ED teams resuscitate based on vital signs and exam.
What is the NISS?
The New ISS uses the three highest AIS scores regardless of body region, fixing the flaw where two severe injuries in one region only count once. Many registries now report both.
Does ISS predict death accurately?
It correlates strongly with mortality at the group level but is weaker for individuals, and it ignores age and physiology, which is why scores like TRISS add those in.
Who assigns AIS grades?
Trained trauma registrars using the official Abbreviated Injury Scale dictionary. Consistent grading needs training, not just the 1-to-6 labels.