What are the ASIL levels?

ISO 26262 defines four ASIL levels — A, B, C and D — where ASIL D demands the most rigour and ASIL A the least. A fifth outcome, QM (Quality Management), means the risk is low enough that normal quality processes suffice. The level is set per hazardous event and inherited by its safety goal and every requirement derived from it.

How is ASIL determined?

ASIL is determined during Hazard Analysis and Risk Assessment (HARA) by rating each hazardous event on three parameters: Severity (S0–S3), Exposure (E0–E4) and Controllability (C0–C3). The combination is looked up in the ISO 26262-3 ASIL table; the worst case, S3 with E4 and C3, gives ASIL D, while lower ratings drop to ASIL C, B, A or QM.

What is ASIL decomposition?

ASIL decomposition is an ISO 26262-9 technique that splits a safety requirement across two sufficiently independent elements, each developed to a lower ASIL. For example, an ASIL D requirement can be decomposed into ASIL B(D) plus ASIL B(D), or ASIL C(D) plus ASIL A(D). It holds only if independence is demonstrated, typically through a dependent-failure analysis.

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Glossary · Functional Safety

ASIL

Also known as: Automotive Safety Integrity Level

Definition

ASIL (Automotive Safety Integrity Level) is the risk classification ISO 26262 assigns to each safety goal of a vehicle function, from ASIL A (lowest) to ASIL D (highest). It is derived in the HARA from the severity, exposure and controllability of a hazardous event, and it sets how rigorous hardware and software development and verification must be.

Where this matters: Functional Safety Engineering (ISO 26262)

ASIL is determined per safety goal during HARA via Severity, Exposure, and Controllability ratings. It scales the rigour of every later activity — from quantitative hardware-fault metrics, to coverage targets in software (Statement at A, Branch at B/C, MC/DC at D).

Why it matters

ASIL sets the rigour of nearly every downstream safety activity, so getting HARA and the ASIL assignment right early prevents both under-engineering (a compliance gap) and over-engineering (wasted cost). It also drives co-engineering with cybersecurity, since a safety goal and a security control often touch the same function.

FAQ

Common questions about ASIL

Related terms

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