ISS Calculator - Free Online Tool

Calculate ISS, NISS, and TRISS probability of survival from AIS body region severity scores

The Injury Severity Score (ISS) Calculator is a validated clinical tool used worldwide to quantify the overall severity of traumatic injuries in patients with multiple injuries. Developed by Baker, O'Neill, Haddon, and Long in 1974, ISS has become the standard anatomic injury severity measure in trauma research, quality improvement programs, and comparative outcome studies across trauma centers globally. ISS is calculated by scoring six body regions using the Abbreviated Injury Scale (AIS), which assigns a severity code from 0 (no injury) to 6 (unsurvivable) to each injured body region. The ISS formula then takes the three highest AIS scores from different body regions, squares each one, and sums the squares. This produces a final score ranging from 0 to 75, where higher scores indicate greater injury burden and correlate with higher mortality, longer hospital stays, and more intensive resource utilization. The six body regions assessed in the ISS system are: Head and Neck (including the brain and cervical spine), Face (including facial bones, nose, mouth, eyes, and ears), Thorax (including the chest wall, rib cage, thoracic spine, lungs, and heart), Abdomen (including abdominal organs, pelvic contents, and lumbar spine), Extremities including the pelvic girdle (arms, legs, and pelvis), and External or Other (including skin injuries such as lacerations, burns, hypothermia, and near-drowning). A critical rule in ISS scoring is that if any single body region receives an AIS score of 6, the ISS is automatically set to 75, regardless of injuries in other regions. An AIS-6 injury is defined as unsurvivable, and this automatic maximum score reflects the clinical reality that such injuries are incompatible with life. ISS is widely used in trauma registries, trauma quality improvement programs, research publications, and resource allocation decisions. A score above 15 is the internationally accepted threshold for major trauma or polytrauma — a level associated with approximately 10% mortality. Patients with ISS greater than 25 have very severe trauma with high mortality risk and typically require intensive care unit admission. Our calculator also computes the New Injury Severity Score (NISS), a refinement introduced by Osler, Baker, and Long in 1997. While ISS restricts each body region to a single score, NISS uses the three highest AIS scores from any body regions, allowing multiple serious injuries within the same region to be counted. This makes NISS more sensitive for penetrating trauma and concentrated single-system injuries, where ISS may systematically underestimate severity. For a complete survival probability estimate, our calculator includes the Revised Trauma Score (RTS) sub-calculator and TRISS (Trauma Injury Severity Score) computation. RTS quantifies physiologic status using the Glasgow Coma Scale, systolic blood pressure, and respiratory rate. TRISS combines anatomic severity (ISS), physiologic status (RTS), age, and injury type (blunt vs. penetrating) to generate a probability of survival, making it the most comprehensive bedside trauma outcome prediction tool available. Important limitations: ISS is an anatomic scoring system requiring complete injury assessment. It cannot be used as a real-time triage tool in the acute resuscitation phase, as many injuries are not diagnosed until full evaluation is complete. Accurate ISS coding requires knowledge of the AIS system and clinical expertise. Two patients can have the same ISS with very different injury patterns and outcomes. For pediatric patients, the Modified Injury Severity Score (MISS) uses five body areas and may be more appropriate. This calculator is intended as an educational and research tool. All clinical decisions must be made by qualified healthcare professionals based on the complete clinical picture. Results should not be used in isolation as the basis for treatment decisions.

Understanding the ISS System

The Injury Severity Score (ISS) is the global standard for measuring anatomic injury burden in trauma patients. It translates AIS body region codes into a composite severity measure that predicts mortality, hospital resource use, and patient outcomes.

The AIS Scale and Body Regions

The Abbreviated Injury Scale (AIS) rates each body region from 0 (no injury) to 6 (unsurvivable). The six ISS regions are: Head/Neck, Face, Thorax, Abdomen, Extremities (incl. pelvic girdle), and External/Other. Only the worst (highest) AIS score per region is used in ISS calculation. Injuries within the same region that do not receive the highest AIS score are not counted — this is the key limitation that NISS addresses. AIS coding requires clinical expertise and complete injury diagnosis.

ISS Formula and Special Rules

ISS equals the sum of the squares of the three highest AIS scores from three different body regions (A² + B² + C²). Squaring the scores amplifies the penalty for severe injuries — an AIS-5 injury contributes 25 points while an AIS-3 adds only 9. If any region receives AIS-6 (unsurvivable), ISS is automatically set to 75, the maximum possible score, regardless of other scores. This means ISS ranges from 0 (no injury) to 75 (unsurvivable or three AIS-5 injuries).

ISS vs. NISS: When They Differ

The New Injury Severity Score (NISS) uses the three highest AIS scores across all regions, regardless of which region they come from. When a patient has multiple serious injuries in the same body region — for example, two chest injuries both scoring AIS-4 — ISS only counts the highest (one AIS-4), while NISS counts both. NISS is always greater than or equal to ISS. Research shows NISS outperforms ISS for predicting outcomes in penetrating trauma and concentrated same-region injuries, while ISS performs comparably for classic multisystem blunt trauma.

TRISS and Survival Probability

TRISS (Trauma Injury Severity Score) is a logistic regression model that predicts probability of survival (Ps) using ISS, the Revised Trauma Score (RTS), age group (over or under 54 years), and injury mechanism (blunt vs. penetrating). RTS scores physiologic parameters: Glasgow Coma Scale, systolic blood pressure, and respiratory rate. TRISS was developed from the Major Trauma Outcome Study (MTOS) database and is used for trauma audit, benchmarking trauma system performance, and identifying unexpected survivors and non-survivors for quality review.

ISS Formulas

Injury Severity Score (ISS)

ISS = A² + B² + C²

Sum of the squares of the three highest AIS scores from three different body regions, where A ≥ B ≥ C. If any region has AIS = 6, ISS is automatically set to 75.

New Injury Severity Score (NISS)

NISS = A² + B² + C²

Sum of the squares of the three highest AIS scores from any body regions, regardless of which region they belong to. NISS is always ≥ ISS.

Revised Trauma Score (RTS)

RTS = 0.9368 × GCS_c + 0.7326 × SBP_c + 0.2908 × RR_c

Weighted sum of coded Glasgow Coma Scale, systolic blood pressure, and respiratory rate values (each coded 0–4). Range: 0 (worst) to 7.84 (normal).

TRISS Probability of Survival

Ps = 1 / (1 + e^(−b))

Where b = b₀ + b₁(RTS) + b₂(ISS) + b₃(Age). Coefficients differ for blunt vs. penetrating trauma. Based on the MTOS database (Boyd et al. 1987).

ISS Reference Tables

AIS Severity Scale

The Abbreviated Injury Scale codes each body region injury from 0 to 6 based on threat to life.

AIS Code	Severity	Description
0	No Injury	Region uninjured
1	Minor	Minor injuries not requiring hospital admission
2	Moderate	Injuries that may require hospitalization but are not life-threatening
3	Serious	Injuries that are life-threatening but survival is probable
4	Severe	Injuries that are life-threatening with uncertain survival
5	Critical	Injuries with critical survival probability
6	Unsurvivable	Injuries incompatible with life — ISS automatically set to 75

ISS Severity Classification

ISS score ranges and their clinical severity classification with approximate mortality.

ISS Range	Classification	Clinical Significance
0	No Injury	No trauma documented
1–8	Minor Trauma	Low priority; outpatient management typically sufficient
9–15	Moderate Trauma	Hospital admission may be required; ISS > 15 = major trauma threshold
16–24	Severe Trauma	Significant morbidity; trauma team activation warranted (~10% mortality)
25–74	Very Severe Trauma	High mortality risk; ICU care usually required
75	Maximal / Unsurvivable	Incompatible with life (AIS-6 present or three AIS-5 injuries)

Worked Examples

Multi-system blunt trauma patient

A 42-year-old male involved in a motor vehicle collision presents with a subdural hematoma (Head AIS = 3), bilateral rib fractures with hemothorax (Thorax AIS = 4), and an open femur fracture (Extremities AIS = 2). Face, Abdomen, and External regions are uninjured (AIS = 0).

Identify the three highest AIS scores from different regions: Thorax = 4, Head = 3, Extremities = 2

Square each score: 4² = 16, 3² = 9, 2² = 4

Sum the squares: ISS = 16 + 9 + 4 = 29

Classify severity: ISS 29 falls in the 25–74 range → Very Severe Trauma

ISS > 15 → meets the international major trauma threshold

ISS = 29 (Very Severe Trauma). This patient meets polytrauma criteria and requires ICU-level care with trauma team activation.

Penetrating trauma with same-region injuries (ISS vs NISS)

A 28-year-old male with multiple gunshot wounds to the abdomen presents with a liver laceration (Abdomen AIS = 4), bowel perforation (Abdomen AIS = 3), and a minor chest wall contusion (Thorax AIS = 1). All other regions uninjured.

ISS calculation: Top 3 from different regions → Abdomen = 4, Thorax = 1 (only 2 injured regions)

ISS = 4² + 1² = 16 + 1 = 17

NISS calculation: Top 3 AIS scores regardless of region → 4, 3, 1

NISS = 4² + 3² + 1² = 16 + 9 + 1 = 26

NISS (26) > ISS (17) because multiple serious abdominal injuries are underweighted by ISS

ISS = 17 (Severe), NISS = 26 (Very Severe). NISS better captures the concentrated abdominal injury burden in this penetrating trauma case.

TRISS survival probability calculation

A 60-year-old female with blunt trauma: ISS = 22, GCS = 12 (coded value 3), SBP = 95 mmHg (coded value 4), RR = 22 breaths/min (coded value 4).

Calculate RTS: 0.9368 × 3 + 0.7326 × 4 + 0.2908 × 4 = 2.8104 + 2.9304 + 1.1632 = 6.904

Apply TRISS blunt trauma coefficients: b₀ = −1.2470, b₁ = 0.9544, b₂ = −0.0768, b₃ = −1.9052

Age > 54, so age coefficient applies: b = −1.2470 + 0.9544(6.904) + (−0.0768)(22) + (−1.9052)(1)

b = −1.2470 + 6.5910 − 1.6896 − 1.9052 = 1.7492

Ps = 1 / (1 + e^(−1.7492)) = 1 / (1 + 0.1738) ≈ 0.852

TRISS Probability of Survival ≈ 85.2%. This is a moderate-risk blunt trauma patient with a favorable but not certain prognosis.

How to Use the ISS Calculator

Assess and Score Each Body Region

For each of the six ISS body regions (Head/Neck, Face, Thorax, Abdomen, Extremities, External), select the AIS severity code that best describes the worst injury in that region. Use 0 (None) for uninjured regions. AIS codes range from 1 (Minor) through 5 (Critical) to 6 (Unsurvivable). Score based on complete injury diagnosis, not initial presentation alone.

Review Your ISS and Severity Classification

The calculator automatically computes ISS as the sum of the squares of the top three AIS scores from different regions. If any region scores AIS-6, ISS is automatically set to 75. Review the severity classification (Minor: 1-8, Moderate: 9-15, Severe: 16-24, Very Severe: 25-74, Maximal: 75) and note the major trauma flag if ISS exceeds 15.

Compare ISS with NISS

Review the NISS (New Injury Severity Score), which counts the three highest AIS scores regardless of body region. If NISS equals ISS, the three most severe injuries are in different regions. If NISS is greater than ISS, the patient has multiple serious injuries in the same body region, which ISS underweights. NISS is typically preferred for penetrating trauma patients.

Calculate TRISS Survival Probability (Optional)

Expand the TRISS Calculator section and enter the patient's Glasgow Coma Scale score (3-15), systolic blood pressure (mmHg), respiratory rate (breaths/min), age, and injury mechanism (blunt or penetrating). The calculator will compute the Revised Trauma Score (RTS) and TRISS probability of survival — a population-level estimate useful for trauma audit and quality improvement.

Frequently Asked Questions

What is the Injury Severity Score (ISS) and who uses it?

The Injury Severity Score (ISS) is a validated anatomic scoring system that quantifies overall trauma severity in patients with multiple injuries. It was developed by Baker et al. in 1974, initially validated on motor vehicle collision patients in Baltimore. ISS is used worldwide by trauma surgeons, emergency physicians, trauma registrars, and researchers. It is required for designation by the American College of Surgeons Committee on Trauma and is used in trauma registries such as NTDB, TQIP, and ACS TQIP. Primary applications include trauma quality improvement, benchmarking between trauma centers, research publications, resource allocation, and regulatory reporting. ISS is not appropriate for acute bedside triage, as complete injury diagnosis is required before scoring.

Why does ISS = 75 automatically when any region scores AIS-6?

An AIS-6 injury is by definition unsurvivable — it represents injuries that are incompatible with life based on the AIS coding system. Examples include decapitation, complete crush injury to the head, traumatic hemipelvectomy, or massive burns exceeding 90% body surface area. When any body region is assigned AIS-6, no mathematical calculation is needed because the injury itself is defined as non-survivable. Setting ISS automatically to 75 prevents misleading scores — without this rule, a patient with AIS-6 head and AIS-1 facial injuries would score only 36 + 1 = 37, which fails to reflect the gravity of the situation. The automatic maximum of 75 is also mathematically the highest achievable score (5² + 5² + 5² = 75).

What is the difference between ISS and NISS, and when does it matter?

ISS (Injury Severity Score) uses only the highest AIS score from each of the six body regions, then squares and sums the top three regional scores. This means if a patient has two chest injuries rated AIS-4 and AIS-3, ISS only counts the AIS-4 for the thorax region. NISS (New Injury Severity Score) removes this restriction and uses the three highest AIS scores from any body regions. In our chest injury example, NISS would count both AIS-4 and AIS-3 thorax injuries, giving a higher and arguably more accurate score. Research has shown NISS outperforms ISS for predicting mortality and complications in penetrating trauma patients and those with concentrated single-region injuries. For classic multisystem blunt trauma affecting different regions, ISS and NISS often agree. In the absence of multiple same-region injuries, NISS always equals ISS.

What does TRISS probability of survival mean clinically?

TRISS (Trauma Injury Severity Score) is a logistic regression model that generates a probability of survival (Ps) from 0 to 100 percent for a given patient profile. The Ps is calculated using ISS (anatomic severity), RTS (physiologic status: GCS + SBP + RR), age group, and injury mechanism. A Ps of 0.85 means that based on historical MTOS database outcomes, 85% of patients with this profile would be expected to survive. TRISS is not a prognosis for the individual patient — it is a population-level benchmark. Its primary use is in trauma quality improvement: if a patient with a Ps of 0.90 dies, clinicians investigate whether care factors contributed. If a patient with Ps of 0.10 survives, this is documented as an unexpected survivor, highlighting potentially exceptional care or underestimated resilience.

What is the Revised Trauma Score (RTS) and how is it calculated?

The Revised Trauma Score (RTS) is a physiologic scoring system that quantifies hemodynamic and neurological status using three parameters: Glasgow Coma Scale (GCS), systolic blood pressure (SBP), and respiratory rate (RR). Each parameter is coded to a value of 0-4 based on specific ranges. GCS 13-15 = 4, GCS 9-12 = 3, GCS 6-8 = 2, GCS 4-5 = 1, GCS 3 = 0. SBP greater than 89 = 4, SBP 76-89 = 3, SBP 50-75 = 2, SBP 1-49 = 1, SBP 0 = 0. RR 10-29 = 4, RR over 29 = 3, RR 6-9 = 2, RR 1-5 = 1, RR 0 = 0. These coded values are weighted and summed: RTS = (0.9368 × GCS_coded) + (0.7326 × SBP_coded) + (0.2908 × RR_coded), giving a range of 0 to 7.84. An RTS below 4 suggests the patient should be transferred to a designated trauma center.

How accurate is ISS, and what are its main limitations?

ISS has been validated across hundreds of thousands of trauma patients and reliably predicts mortality, hospital length of stay, ICU admission rates, and resource utilization at the population level. Its primary limitations are well established. First, only the highest AIS per body region is counted, causing ISS to underestimate severity when multiple serious injuries occur in the same region — NISS addresses this. Second, ISS requires complete injury assessment including imaging, operative findings, and final diagnoses, so it cannot be calculated during the acute resuscitation phase. Third, the scoring is entirely anatomic with no physiologic data — a patient with identical ISS scores can have vastly different outcomes based on physiologic reserve, age, comorbidities, and time to care. Fourth, accurate AIS coding is complex and requires dedicated training, meaning inter-rater reliability can vary. Fifth, ISS was developed and validated in adult trauma populations and is less well-calibrated for pediatric injuries.