Technological Innovation

How do you calculate SIL level?

Calculating Safety Integrity Level (SIL) is a crucial aspect in determining the reliability and dependability of safety-related systems. SIL is a measure of the probability of failure of a system to perform its intended function correctly when called upon. This article aims to provide an easy-to-understand explanation of how SIL level is calculated.

Safety Integrity Level (SIL)

Safety Integrity Level, commonly referred to as SIL, is a measurement of the overall risk reduction provided by a safety function. It is defined in international standard IEC 61508 and is widely used in industries that involve systems with safety-critical functions, such as automotive, aerospace, chemical, and nuclear.

SIL level ranges from SIL 1 to SIL 4, with SIL 4 being the highest level of safety and SIL 1 being the lowest. Each level corresponds to a specific achievable target for the probability of failure on demand (PFD), which represents the likelihood of a system failing to carry out its safety function when required.

Calculating SIL Level

The calculation of SIL level involves several steps and considerations:

Hazard Analysis: A thorough hazard analysis is conducted to identify potential risks associated with the system. This includes identifying hazards, assessing their severity, and determining the necessary risk reduction measures.

Mitigation Measures: Based on the hazard analysis, appropriate mitigation measures are implemented to reduce the risks to an acceptable level. These measures can include redundancy, design changes, monitoring systems, and safety instrumented systems (SIS).

Probabilistic Risk Assessment: A probabilistic risk assessment is performed to quantify the likelihood of failures and the effectiveness of the mitigation measures. This involves analyzing historical data, failure rates of components, and system architecture to calculate the probability of failure on demand (PFD).

SIL Determination: Using the PFD obtained from the risk assessment, the SIL level is determined by comparing it with the target limits specified in the IEC 61508 standard. The lower the PFD, the higher the SIL level.


Calculating SIL level is an essential process in ensuring the safety and reliability of safety-related systems. By conducting a hazard analysis, implementing appropriate mitigation measures, and quantifying the risks through probabilistic risk assessment, organizations can determine the appropriate SIL level for their systems. It is imperative to follow the guidelines set by international standards, such as IEC 61508, to achieve reliable and safe operations in various industries.


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