Injury Severity Score (ISS) Calculator
Injury Severity Score (ISS) Calculator
Sum the squares of the three highest body-region AIS severities you already hold. Any AIS 6 forces 75 by convention, and the resulting scale is ordinal and full of holes.
Injury Severity Score
Six AIS regions to 0–75Head or neck AIS 4, chest AIS 3, extremity AIS 3; face, abdomen and external all 0
Formula
where A, B and C are the highest AIS severities in the three most severely injured of the six ISS body regions · any AIS 6 sets ISS = 75
- three regions, not three injuries
- each region contributes only its single highest code, and only three regions contribute at all. Two critical chest injuries are one chest code. This is the one rule that separates the ISS from the NISS, and it is the rule that makes the ISS under-read multiple severe injury to one part of the body
- the six ISS body regions
- head or neck, face, chest, abdominal or pelvic contents, extremities or pelvic girdle, and external and other. NSW Health’s Institute of Trauma and Injury Management states it plainly: there are “only six ISS body regions to which injuries can be assigned”, and these are a grouping of the nine AIS regions rather than the AIS regions themselves
- AIS 6 forces 75
- “if an injury has an AIS of 6, the ISS score is automatically assigned 75”. A convention, not arithmetic: the squares would give 36. The published range is quoted as 1 to 75
- not continuous, and not linear
- only 45 distinct values are attainable on 0 to 75, and 31 integers between 1 and 75 cannot occur at all — 7, 15, 23, 28, 31, 37, 39, 40, 44, 46, 47, 49, 52, 53, 55, 56, 58 and every integer from 60 to 74 except 66. Nor is 6 twice 3. The original authors judged the severity relationship “almost certainly nonlinear” and chose squares as “the simplest nonlinear function”; a later analysis concludes that “the value of the ISS is only ordinal”, which is an argument against taking means of it
- where the codes come from
- The Abbreviated Injury Scale codes themselves come from a copyrighted dictionary licensed by the Association for the Advancement of Automotive Medicine, which licenses it for software only through an API and forbids its reproduction. This page therefore does not and cannot assign an AIS code to an injury: it takes the severities the reader already holds from a licensed coder or a trauma registry and applies the published formula over them.
- derivation
- published in 1974 from hospital and medical examiner data for “more than two thousand persons”, in which “substantial correlation was demonstrated” between AIS severity and survival, and in which injuries not normally life-threatening were found to have a marked effect on mortality when combined with others. That second finding is the reason for summing three regions rather than taking the worst
Worked example
Head or neck AIS 4, chest AIS 3, extremity AIS 3; face, abdomen and external all 0
Three regions are injured, so all three contribute: 4, 3 and 3
4² + 3² + 3² = 16 + 9 + 9 = 34 ISS points
34 sits in the top reported stratum, 25 or more
Now suppose the same patient's two chest injuries were both AIS 4 and the extremity injury AIS 3, with nothing in the head. The ISS takes one chest code: 4² = 16 plus 3² = 9, total 25. The NISS on the same three injuries is 4² + 4² + 3² = 41
Push it further: three AIS 5 injuries confined to the chest give an ISS of 5² = 25 and a NISS of 75. A 50-point gap, and it opens widest in exactly the patients it matters for
Change the head code to 6 and the total becomes 75 whatever else is entered — the convention overrides the arithmetic
The six ISS body regions
| ISS region | What it covers | The trap |
|---|---|---|
| Head or neck | Brain, skull, cervical spine and cord | Separate from face, so a basal skull fracture and a mandibular fracture are two regions |
| Face | Facial bones, eye, ear, mouth, nose | Almost never reaches the top three, so a devastating facial injury can leave the ISS untouched |
| Chest | Thorax, diaphragm, thoracic spine and cord | Flail chest with bilateral contusions is ONE code |
| Abdomen or pelvic contents | Abdominal and pelvic viscera, lumbar spine and cord | The pelvic girdle is not here — it is in extremities |
| Extremities or pelvic girdle | Limbs and the bony pelvis | Four limb fractures are one code; the ISS cannot see the other three |
| External and other | Burns, lacerations, abrasions, contusions, hypothermia, drowning, asphyxia | Where a burn is scored, so burn size feeds the ISS through this region |
Reported strata, and the holes in them
| ISS | Texas registry label | What to watch |
|---|---|---|
| 0 | none — the report has a row for it and no band | No injury coded in any region. The published range is usually quoted as 1 to 75 |
| 1 – 8 | Mild | Attainable values here are 1, 2, 3, 4, 5, 6 and 8. There is no ISS of 7 |
| 9 – 15 | Major | Collides with the commoner convention in which major trauma is an ISS above 15. Name the scheme |
| 16 – 24 | Severe | 16 is one severe injury alone; 24 needs injury in three regions |
| 25 + | Critical, written as “ISS > 25” | An ISS of exactly 25 is attainable and falls in no published band of that report |
Three regions, squared — and what that throws away
The Injury Severity Score takes the highest Abbreviated Injury Scale severity in each of six body regions, picks the three worst regions, squares each and adds them. It was published in 1974 from hospital and medical examiner records for more than two thousand injured people, and its central finding was not that the worst injury predicts death but that injuries which are individually survivable have a marked effect on mortality when they occur together. Summing three regions rather than taking the single worst is a direct consequence of that observation, and it is why the score caught on.
The squaring is the part that gets misread. The original authors judged the relationship between severity and outcome to be almost certainly nonlinear and chose squares as the simplest nonlinear function available to them; the exponent carries no further meaning. An ISS of 6 is not twice an ISS of 3, differences between ISS values are not comparable across the range, and a later formal analysis concludes that the value of the score is only ordinal — which is an argument against reporting mean ISS as though it were a measurement. The scale is also full of holes. Only 45 of the 76 integers from 0 to 75 can occur; thirty-one, including 7, 15, 23 and every value from 60 to 74 except 66, are unreachable.
The one-code-per-region rule is the score’s defining limitation. A patient with three critical chest injuries and nothing else scores 25, because the chest contributes one 5. The same three injuries on the New Injury Severity Score give 75. The gap is widest in the patients the score exists to identify, and that is what Osler’s modification was written to fix. Separately, an AIS of 6 — an injury described as currently untreatable — sets the ISS to 75 by convention rather than by arithmetic, so the top of the scale is reached by two quite different routes.
This page applies the published formula to severities the reader supplies and does not assign them, because the AIS dictionary is copyrighted and licensed by the Association for the Advancement of Automotive Medicine. Enter the codes your registry or coder produced — and note that registries on different AIS dictionary versions will not score the same patient identically.
Frequently asked questions
How is the Injury Severity Score calculated?
Take the highest AIS severity in each of the six ISS body regions, select the three most severely injured regions, square each of those three severities and add them. The result runs 1 to 75. Any single injury with an AIS of 6 sets the score to 75 regardless of everything else.
Why does an AIS 6 give an ISS of 75?
By convention. The arithmetic would give 36. An AIS 6 denotes an injury described as currently untreatable, and assigning 75 stops such a patient being ranked below someone with three critical but survivable injuries. NSW Health’s trauma institute states it directly: if an injury has an AIS of 6, the ISS is automatically assigned 75.
Is the ISS a continuous scale from 0 to 75?
No, and a page or a statistical model that treats it as one is wrong. Only 45 distinct values are attainable. Thirty-one integers between 1 and 75 cannot occur — 7, 15, 23, 28, 31, 37, 39, 40, 44, 46, 47, 49, 52, 53, 55, 56, 58, and everything from 60 to 74 apart from 66.
Can this calculator assign the AIS codes for me?
No. The AIS is a copyrighted dictionary licensed by the Association for the Advancement of Automotive Medicine, whose licence terms permit software use only through an API connection and forbid reproducing or amending the content. This page takes the severities you already hold from a licensed coder or a trauma registry and applies the published ISS formula to them.
What is the difference between ISS and NISS?
One rule. The ISS takes the highest severity in each of three different body regions; the NISS takes the three highest severities anywhere, so two severe injuries to the same region both count. The NISS is therefore never lower than the ISS and is often much higher — three critical chest injuries give an ISS of 25 and a NISS of 75.
Related calculators
References
- Agency for Clinical Innovation, Institute of Trauma and Injury Management. Injury scoring. NSW Health. aci.health.nsw.gov.au/networks/trauma/data/injury-scoring
- Transport Research International Documentation record 133403: The Injury Severity Score: A Method for Describing Patients with Multiple Injuries and Evaluating Emergency Care (1974). Abstract read at trid.trb.org/View/133403; the full paper was not reachable.
- On Some Statistical and Axiomatic Properties of the Injury Severity Score. arXiv:2102.04233 — the attainable value set and the argument that the ISS is ordinal rather than interval.
- Association for the Advancement of Automotive Medicine. AIS License — commercial (software) and healthcare system licensing terms for the Abbreviated Injury Scale. aaam.org/ais/ais-products/ais-license
- Texas Department of State Health Services, Injury Prevention Unit. Texas EMS and Trauma Registries — 2021 trauma data, 18 August 2023.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/