What the calculator is for
A LOPA or risk graph gives a required risk reduction factor; a SIL verification gives an achieved PFDavg. The two meet in the SIL tables of IEC 61508-1 and IEC 61511-1. This converter moves between them instantly and shows which band a number falls in, how much margin it has to a target, and the exact boundary values.
- RRF = 1 / PFDavg in low-demand mode.
- The bands are open at the lower edge: PFDavg = 10⁻² exactly (RRF 100) is SIL 1, not SIL 2. SILProof’s engine uses the same rule.
- High demand or continuous mode uses PFH, the average frequency of dangerous failure per hour.
Worked example
A LOPA finds that the scenario needs a risk reduction factor of 250. The SIF therefore needs PFDavg below 1 / 250 = 4 × 10⁻³, which sits in the SIL 2 band (RRF above 100 and up to 1,000).
The design is verified at PFDavg = 1.6 × 10⁻³, an RRF of 625. It meets the requirement with a factor of 2.5 to spare, and it is in the SIL 2 band. Note the order: compare against the required RRF, not just the band. A SIL 2 PFDavg of 8 × 10⁻³ would not meet an RRF of 250.
SIL tables
| SIL | PFDavg | RRF |
|---|---|---|
| SIL 4 | 10⁻⁵ ≤ PFDavg < 10⁻⁴ | 10,000 < RRF ≤ 100,000 |
| SIL 3 | 10⁻⁴ ≤ PFDavg < 10⁻³ | 1,000 < RRF ≤ 10,000 |
| SIL 2 | 10⁻³ ≤ PFDavg < 10⁻² | 100 < RRF ≤ 1,000 |
| SIL 1 | 10⁻² ≤ PFDavg < 10⁻¹ | 10 < RRF ≤ 100 |
| SIL | PFH (per hour) |
|---|---|
| SIL 4 | 10⁻⁹ ≤ PFH < 10⁻⁸ |
| SIL 3 | 10⁻⁸ ≤ PFH < 10⁻⁷ |
| SIL 2 | 10⁻⁷ ≤ PFH < 10⁻⁶ |
| SIL 1 | 10⁻⁶ ≤ PFH < 10⁻⁵ |
Common mistakes
- Claiming SIL from PFDavg alone. The claimed SIL is the lowest of the quantitative band, the architectural constraints (HFT and SFF, see the SFF / HFT calculator) and systematic capability.
- Using PFDavg in high demand. If the demand rate is more than once a year, IEC 61511 treats the function as high demand and PFH applies.
- Rounding into a band. 1.0 × 10⁻² is SIL 1; 9.9 × 10⁻³ is SIL 2. Keep the unrounded value.
- SIL 4 in the process sector. Values below 10⁻⁵ fall outside the table, and IEC 61511 discourages SIL 4 SIFs; revisit the risk analysis instead.
Basis
Bands follow IEC 61508-1:2010 Tables 2 and 3 and IEC 61511-1:2016 Tables 4 and 5. The calculator uses exactly the band rule of the SILProof engine, which is checked against IEC 61508-6 Annex B. It gives the quantitative band only.