SIL and PFD Calculator
Safety Integrity Level is set by the average probability of failure on demand of a safety instrumented function. PFDavg depends on the dangerous undetected failure rate, the architecture, and above all the proof test interval — testing more often is frequently the cheapest route to a higher SIL.
Formula
Worked example
A single transmitter in 1oo1 with λ_DU = 2 × 10⁻⁶ per hour, proof tested annually (TI = 8760 h).
- PFDavg = 2 × 10⁻⁶ × 8760 / 2
- = 2 × 10⁻⁶ × 4380 = 8.76 × 10⁻³
- This falls in the range 10⁻³ to 10⁻², so SIL 2.
- Halving the proof test interval to 6 months: PFDavg = 4.38 × 10⁻³ — still SIL 2, but with more margin.
PFDavg is 8.76 × 10⁻³, which is SIL 2 but sits close to the SIL 1 boundary. Note that in the 1oo2 formula, once β (common cause) is non-zero, that term dominates at long test intervals — beyond a point, adding redundancy stops helping and only shorter proof tests or better diagnostics move the number.
How to interpret the result
| SIL 1 | PFDavg 10⁻² to 10⁻¹ (RRF 10 to 100) |
| SIL 2 | PFDavg 10⁻³ to 10⁻² (RRF 100 to 1,000) |
| SIL 3 | PFDavg 10⁻⁴ to 10⁻³ (RRF 1,000 to 10,000) |
| SIL 4 | PFDavg 10⁻⁵ to 10⁻⁴ (RRF 10,000 to 100,000) |
Common mistakes
- Quoting a SIL for a device in isolation. SIL applies to the complete safety instrumented function — sensor, logic solver and final element together, with the final element usually dominating.
- Omitting the common-cause beta factor in redundant architectures, which produces optimistic results by orders of magnitude.
- Assuming proof tests are perfect. Realistic proof test coverage below 100% leaves a residual that accumulates over the lifetime.
- Confusing SIL capability (what the hardware can support) with achieved SIL (what your installed function actually delivers).
Related tools
About Reliability Pro
SIL / PFD is part of a suite of 126 reliability, maintenance and quality engineering tools covering life data analysis, accelerated testing, system reliability, FMEA and root cause, SPC, and design for reliability. It runs in the browser and as native iOS and Android apps, so the same calculation is available at a desk or in front of the asset.