Injury Severity Score Calculator

Q: How is ISS used in trauma registries?

Trauma registries use ISS to standardize injury severity for benchmarking outcomes, triaging trauma center resources, risk-adjusting mortality comparisons, and supporting research such as the TRISS survival model.

Body Region	Includes
Head/Neck	Brain, skull, cervical spine, cervical spinal cord
Face	Facial skeleton, eyes, ears, nose, mouth
Chest	Thoracic spine, ribs, sternum, thoracic organs, diaphragm
Abdomen	Lumbar spine, abdominal and pelvic organs
Extremity	Limbs, bony pelvis, pelvic girdle
External	Skin, burns, soft-tissue injuries not captured elsewhere

Tier	ISS Range
Minor trauma	0–8
Moderate trauma	9–15
Severe / major trauma	16–75

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Walk-through

How to Use This Calculator

6 steps

1

Review the 6 AIS body regions

The Injury Severity Score covers six standardized body regions: Head/Neck, Face, Chest, Abdomen, Extremity, and External. Each region gets one Abbreviated Injury Scale (AIS) value, not a value per injury.

2

Select the AIS severity for each injured region

For every region with a coded injury, choose its AIS severity from 0 (no injury) to 6 (unsurvivable). Leave a region blank if it was not injured or not yet coded — you need at least 3 regions filled in to see a score.

3

Let the tool find the 3 highest regions

The calculator automatically identifies the three body regions with the highest AIS values among the ones you entered. It never combines two injuries from the same region — one AIS value per region, by design.

4

Read the sum-of-squares result

The Injury Severity Score is the sum of the squares of those three AIS values, ranging from 0 to 75. The result card also lists which three regions were counted.

5

Check for the AIS-6 override

If any single region is coded AIS 6 (unsurvivable), the score is automatically set to the maximum, 75 — regardless of what the other regions show. This is a fixed rule from the original ISS definition, not a calculation quirk.

6

Read the severity tier

The Severity Tier tab classifies the score as minor (under 9), moderate (9 to 15), or severe/major (16 and above) — the threshold many trauma centers use to define major trauma.

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Reference

Formula & Methodology

2 formulas

Injury Severity Score

ISS = AIS₁² + AIS₂² + AIS₃²

ISS is the sum of the squares of the AIS severity scores (0-6) of the three most severely injured of the six AIS body regions (Head/Neck, Face, Chest, Abdomen, Extremity, External), with at most one AIS value counted per region. The theoretical range is 0 to 75. Source: 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.

AIS-6 unsurvivable override

If any AIS = 6, then ISS = 75

AIS 6 denotes an injury considered unsurvivable even with maximal treatment. By the published convention, a single AIS-6 injury automatically sets ISS to its maximum value of 75, overriding the sum-of-squares calculation.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Editorial Team · Updated 2026-07-22 · 3 sources

Editorial accountability

Author: Calculover Editorial Team - Clinical Content

Owner: Calculover Editorial Team - Clinical Content

Reviewer: Calculover Editorial Team - Clinical Review

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Implements the Injury Severity Score as published by Baker SP, O'Neill B, Haddon W Jr, and Long WB (J Trauma, 1974): ISS is the sum of the squares of the AIS severity values (0-6) of the three most severely injured of six AIS body regions, with a fixed rule that any AIS-6 injury sets ISS to 75.

Assumption: AIS severity values entered into this tool are assumed to have already been correctly coded by a trained AIS coder using the current AAAM coding dictionary — this calculator does not verify or assign AIS codes.

Assumption: Only one AIS value is considered per body region, per the original ISS methodology, even if multiple injuries exist within the same region.

Assumption: At least 3 of the 6 body regions must be entered before a score is shown, since ISS is undefined with fewer than 3 coded regions.

Limitations & guidance

ISS cannot fully capture severity when a patient has multiple severe injuries within the same body region, since only the single highest AIS in that region counts.

ISS does not incorporate age, comorbidities, or physiologic status; combined tools such as TRISS add these factors for a fuller mortality-risk picture.

Incorrect AIS coding upstream of this tool will produce a misleading ISS — always have AIS codes assigned or verified by a trained trauma coder.

Professional guidance: This calculator is for clinical decision support and educational purposes only. It is not a substitute for professional medical judgment, trauma team assessment, or a formally coded trauma registry entry. AIS coding requires formal training; this tool only sums values that have already been coded.

Primary 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 (AIS)

MDCalc — Injury Severity Score (ISS)

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Glossary

Key Terms Explained

8 terms

Injury Severity Score (ISS) An anatomical trauma severity score from 0 to 75, calculated as the sum of the squares of the AIS values of the three most severely injured body regions.

Abbreviated Injury Scale (AIS) A standardized, anatomically based injury severity scale (0 = no injury to 6 = unsurvivable) maintained by the Association for the Advancement of Automotive Medicine (AAAM), used to code the severity of individual injuries.

Body region One of the six anatomical regions ISS groups injuries into: Head/Neck, Face, Chest, Abdomen, Extremity, and External. Only the single highest AIS value within a region is used.

Sum of squares The ISS calculation method: the AIS value of each of the three highest-scoring body regions is squared, then the three squares are added together. Squaring weights more severe injuries disproportionately more than a simple sum would.

Unsurvivable injury (AIS 6) The maximum AIS severity code, reserved for injuries considered unsurvivable regardless of treatment. Any single AIS-6 injury automatically sets ISS to 75.

Major trauma A classification commonly applied when ISS is 16 or higher, used by many trauma centers and trauma systems to trigger resource allocation and outcome tracking.

Trauma severity A general term for how physiologically and anatomically serious a patient's combined injuries are, of which ISS is one standardized anatomical measure.

Trauma registry A structured database that trauma centers maintain to track patient injuries, care, and outcomes; ISS is one of the core fields recorded for every registry patient to allow risk-adjusted comparisons.

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Scenarios

Real-World Examples

3 worked examples

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Minor trauma case

A patient has a moderate extremity fracture, minor external abrasions, and a minor facial laceration.

Extremity AIS 2 (Moderate) External AIS 1 (Minor) Face AIS 1 (Minor)

ISS = 2² + 1² + 1² = 4 + 1 + 1 = 6, which falls in the minor trauma range (0-8). The extremity injury drives most of the score — a jump to AIS 3 there would move the total to 11 (moderate).

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Moderate trauma case

A patient has a serious chest injury, a moderate head/neck injury, and a minor facial injury.

Chest AIS 3 (Serious) Head/Neck AIS 2 (Moderate) Face AIS 1 (Minor)

ISS = 3² + 2² + 1² = 9 + 4 + 1 = 14, which falls in the moderate trauma range (9-15). Because ISS squares each value, the chest injury alone contributes nearly two-thirds of the total score.

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Severe / major trauma case

A patient has a severe head/neck injury, a serious chest injury, and a serious abdominal injury from a high-speed collision.

Head/Neck AIS 4 (Severe) Chest AIS 3 (Serious) Abdomen AIS 3 (Serious)

ISS = 4² + 3² + 3² = 16 + 9 + 9 = 34, well above the ISS ≥ 16 major-trauma threshold many trauma centers use. Note that a single AIS-6 region anywhere would instead force ISS straight to 75, regardless of these other values.

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Deep Dive

Understanding the Injury Severity Score

4 chapters · 5 min read

The Injury Severity Score (ISS) is one of the most widely used anatomical trauma severity scores in emergency medicine and trauma research. Developed by Baker and colleagues in 1974, it condenses injuries across six body regions into a single 0-75 number that correlates with mortality, complication rates, and length of stay. This calculator sums pre-coded AIS values — it does not assign AIS codes itself.

How the Injury Severity Score works

ISS starts from the Abbreviated Injury Scale (AIS), a system maintained by the Association for the Advancement of Automotive Medicine that assigns each injury a severity code from 0 (no injury) to 6 (unsurvivable) based on anatomical criteria. Injuries are first grouped into six AIS body regions — Head/Neck, Face, Chest, Abdomen, Extremity, and External — and only the single highest AIS value within each region carries forward.

ISS then takes the three body regions with the highest AIS values and sums the square of each: ISS = AIS₁² + AIS₂² + AIS₃². Squaring rather than simply adding weights the most severe injuries disproportionately, which Baker et al. (1974) found tracked mortality more closely than an unweighted sum. Because the maximum AIS in any one region is 5 for the purposes of this sum (a value of 6 triggers the override below), the highest ordinary ISS achievable is 5² + 5² + 5² = 75.

The AIS-6 unsurvivable override

AIS 6 is reserved for injuries considered unsurvivable even with maximal treatment — for example, a near-complete decapitation or a massive uncontained aortic injury. By definition, a single AIS-6 injury in any body region automatically sets ISS to its maximum score of 75, bypassing the sum-of-squares calculation entirely. This reflects that no combination of lesser injuries elsewhere changes the fact that one injury alone is expected to be fatal.

Reading the severity tiers

There is no single universal ISS cutoff, but a common convention splits scores into minor (ISS under 9), moderate (ISS 9-15), and severe or major trauma (ISS 16 and above). Many trauma centers, state trauma systems, and the American College of Surgeons' triage guidance use ISS ≥ 16 as a working definition of major trauma for resource allocation and outcome benchmarking, though exact thresholds vary by system.

Limits and what this calculator does not do

This tool performs the ISS arithmetic only — it does not assign AIS codes to injuries. Correct AIS coding requires formal training and access to the current AIS coding dictionary; miscoding an injury's AIS severity will produce a misleading ISS regardless of the arithmetic. ISS also has known limitations: it can only count one injury per body region (so a patient with two severe injuries in the same region is underscored), and it does not account for a patient's age, comorbidities, or physiologic status the way combined scores like TRISS do. Use ISS as one input into a broader clinical or trauma-registry assessment, never as a stand-alone diagnostic or triage tool.

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Questions

Frequently Asked Questions

6 questions

What are the 6 body regions used in the Injury Severity Score?+

The ISS uses six AIS body regions: Head/Neck, Face, Chest, Abdomen (including pelvic contents), Extremity (including the bony pelvis), and External (skin). Only the single highest AIS value within each region counts toward the score.

What is the Abbreviated Injury Scale (AIS)?+

AIS is a standardized anatomical injury severity scale from 0 (no injury) to 6 (unsurvivable), maintained by the Association for the Advancement of Automotive Medicine (AAAM) and assigned by trained coders using published coding dictionaries. This calculator sums pre-coded AIS values — it does not assign them.

Why does ISS use sum of squares instead of a simple sum?+

Squaring the three highest AIS values weights more severe injuries disproportionately. Baker et al. (1974) found this sum-of-squares approach correlated more strongly with mortality, complication rates, and hospital length of stay than a simple additive score.

What happens if a body region is coded AIS 6?+

AIS 6 denotes an unsurvivable injury. By definition, any single AIS-6 injury automatically sets the Injury Severity Score to its maximum value of 75, regardless of the severity of any other body region.

What ISS is considered major trauma?+

Many trauma centers and registries define an ISS of 16 or higher as major trauma. Below that, ISS is generally categorized as minor (under 9) or moderate (9 to 15). Exact cutoffs can vary between trauma systems.

How is ISS used in trauma registries?+

Trauma registries record ISS to standardize injury severity for benchmarking outcomes, triaging trauma center resources, risk-adjusting mortality comparisons across hospitals, and supporting research such as the TRISS survival-probability model, which combines ISS with physiologic data.

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AIS Severity by Body Region