λ to PFD Calculator
Convert a component's dangerous failure rate (λ_DU) into a Probability of Failure on Demand (PFD_avg, low-demand mode) or per-hour failure frequency (PFH, high-demand mode), and read off the corresponding SIL band per IEC 61508 / IEC 61511. Runs entirely in your browser — nothing leaves your device.
Dangerous undetected failure rate. Typical: 1e-7 to 1e-5.
e.g. 8,760 = annual; 4,380 = 6-monthly.
Mean time to repair after detection.
PFD_avg
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The formulas
For a 1-out-of-1 channel in low-demand mode (the typical SIS protection layer):
PFD_avg = (λ_DU · T_PT) / 2 + λ_DU · MTTR
The first term dominates when proof test interval is large; the MTTR term matters when repair time is comparable to test interval. For high-demand or continuous mode (e.g. an automotive ASIL-D function operating continuously):
PFH ≈ λ_DU (per hour)
SIL bands (IEC 61508 Part 1, Table 2 & 3)
| SIL | Low-demand PFD_avg | High-demand PFH |
|---|---|---|
| SIL 1 | 10⁻² to < 10⁻¹ | 10⁻⁶ to < 10⁻⁵ |
| SIL 2 | 10⁻³ to < 10⁻² | 10⁻⁷ to < 10⁻⁶ |
| SIL 3 | 10⁻⁴ to < 10⁻³ | 10⁻⁸ to < 10⁻⁷ |
| SIL 4 | 10⁻⁵ to < 10⁻⁴ | 10⁻⁹ to < 10⁻⁸ |
Caveats
This is the simplified 1-out-of-1 calculation. Multi-channel architectures (1oo2, 2oo3 voting), diagnostic coverage, common-cause failure (Beta-factor), and partial proof testing all change the result. For full SIL verification of a redundant SIS with CCF, model the architecture as a fault tree in FTA Studio and quantify the top event directly.