Injury Severity Score Calculator (ISS from AIS by body region)

Understanding the Injury Severity Score calculator — concept and principle

The Injury Severity Score (ISS) summarises how badly a person with several injuries is hurt, on a scale of 0 to 75. Trauma teams, registries and researchers use it to define major trauma, compare patients and audit trauma care.

Square the three worst regions: with the default inputs the result is ISS 29 (above 15 → major trauma).

How it is calculated

ISS = A² + B² + C²

Each injury is graded with the Abbreviated Injury Scale (AIS) from 1 (minor) to 6 (maximal).
The body is divided into six regions: head or neck, face, chest, abdomen or pelvic contents, extremities or pelvic girdle, and external (skin).
Take the highest AIS in each region and square the three highest regional values; A, B and C are those three values.
If any injury is AIS 6, the ISS is set to 75.

The principle

Baker and colleagues found in 1974 that death rates rose with the square of the AIS rather than in a straight line, and that adding the squares of the worst injuries in different body regions tracked mortality well. Using three regions captures the effect of multiple serious injuries spread over the body. Only one injury per region counts, which keeps the score simple but means several injuries in one region raise it less.

Worked example

Default inputs: head 3, face 1, chest 4, abdomen 2, extremities 2, external 1.

The three highest values are chest 4, head 3 and one of the 2s.
ISS = 4² + 3² + 2² = 16 + 9 + 4 = 29.
29 is above 15 → major trauma Yes.
Regions with an injury = 6.

How to read the result

Where does your result fall?

An ISS above 15 is the usual definition of major trauma and often triggers care in a major trauma centre or inclusion in registries. Higher scores mean higher expected mortality, but age, physiology on arrival and the exact injuries matter a lot. ISS is calculated after injuries are fully known, so it is mainly a research and audit tool.

Common mistakes

Adding three injuries from the same region.
Using AIS codes from different versions of the dictionary without noting it.
Forgetting the rule that any AIS 6 sets ISS to 75.

Keep in mind: This is a scoring tool for trained staff, not a diagnosis or triage instruction. AIS coding requires the official AAAM dictionary and training.

Frequently asked questions

What ISS counts as major trauma?

An ISS above 15 is the most widely used threshold.

Why are the AIS values squared?

Because mortality rises much faster than linearly with injury severity.

What is the New Injury Severity Score?

NISS uses the three highest AIS values regardless of region, which handles several injuries in one region better.

Sources: Baker SP, O'Neill B, Haddon W Jr, 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 · Association for the Advancement of Automotive Medicine, Abbreviated Injury Scale

Checked 9 Nov 2026. Baker SP, O’Neill B, Haddon W Jr, 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; regions, AIS-6 rule and major-trauma threshold as summarised in Wikipedia, “Injury Severity Score”. AIS codes themselves come from the AAAM Abbreviated Injury Scale dictionary.

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