ASIL Determination Calculator (ISO 26262)
Pick a severity, an exposure and a controllability for the hazardous event. The calculator returns the ASIL, shows how it was reached, and lists the quantitative hardware targets that then apply.
| What this ASIL then requires | Target |
|---|---|
| Single-point fault metric (SPFM) | – |
| Latent-fault metric (LFM) | – |
| PMHF | – |
Take these into the FIT and PMHF budget calculator to see what failure rate and diagnostic coverage they imply.
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How the classification works
ISO 26262 does not assign an integrity level to a component. It assigns one to a hazardous event — a malfunction, in a specific operational situation, producing a specific harm. The same electronic failure can be QM in one situation and ASIL D in another, and that is the point of the method: it forces the conversation about where the part is used before anyone argues about how reliable it is.
Three questions decide it. Severity asks how badly people are hurt if the event happens. Exposure asks how much of the vehicle’s operating life is spent in the situation where it could happen. Controllability asks whether a normal driver, or another road user, can act to avoid the harm once it starts.
Each answer carries an index, and the three indices add. A total of six or below is QM — outside the scope of ISO 26262’s safety requirements, though not outside the scope of engineering judgement. Seven, eight, nine and ten give ASIL A through ASIL D. Because it is a sum, no single axis dominates: a life-threatening outcome that is almost never encountered and is easy to control lands lower than a moderate injury that is common and unavoidable.
The three arguments people lose
- Exposure is about the situation, not the fault. It measures how often the vehicle is in the operational situation, not how often the electronics break. Conflating the two is the most common error, and it deflates the ASIL by a grade or more.
- Controllability is not “a good driver could handle it”. It asks whether a typical driver, already surprised, can act in time. Assessors are unsympathetic to claims that rest on expert reaction.
- Severity counts everyone. Harm to other road users and pedestrians counts, not only to the vehicle’s occupants.
What follows from the answer
The ASIL sets the quantitative hardware targets shown above, and much else besides: the rigour of the requirements process, the independence of the people reviewing it, whether decomposition is available, and how much of the verification has to be independent. ASIL A carries no quantitative hardware-architectural-metric or PMHF target, which is often misread as “no analysis needed” — the qualitative requirements still apply in full.
If the answer is B or above, the next question is whether your architecture can actually reach those metrics. That is what the FIT and PMHF budget calculator is for, and it is the point at which most projects discover their diagnostic coverage assumptions were optimistic.
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