PFDavg Calculator (SIL Verification, 1oo1)
By Santaji GadepatilinUpdated October 28, 2026
In a safety instrumented system, each safety function must reach a target probability of failure on demand (PFDavg) that sets its SIL. For a single channel, PFDavg depends on the dangerous undetected failure rate and how often it is proof tested. This PFDavg calculator does the 1oo1 calculation — see the PFDavg calculation formula, what SIL is and SIL verification.
How to use the PFDavg calculator
Enter the dangerous undetected failure rate λDU for the whole safety function (sensor + logic solver + final element) in FIT, and the proof test interval in years. You get PFDavg, the risk reduction factor and the SIL band that PFDavg alone supports.
PFDavg formula (1oo1, low demand)
PFDavg ≈ λDU × TI ÷ 2 · RRF = 1 ÷ PFDavg
This simplified formula assumes perfect proof testing and ignores repair time and common-cause failures.
SIL bands (low-demand mode)
| SIL | PFDavg | Risk reduction factor |
|---|---|---|
| SIL 1 | ≥ 10⁻² to < 10⁻¹ | 10 – 100 |
| SIL 2 | ≥ 10⁻³ to < 10⁻² | 100 – 1,000 |
| SIL 3 | ≥ 10⁻⁴ to < 10⁻³ | 1,000 – 10,000 |
| SIL 4 | ≥ 10⁻⁵ to < 10⁻⁴ | 10,000 – 100,000 |
PFDavg vs proof test interval (λDU = 500 FIT)
| Test interval | PFDavg | RRF |
|---|---|---|
| 3 months | 5.48e-04 | 1,826 |
| 6 months | 1.10e-03 | 913 |
| 1 year | 2.19e-03 | 457 |
| 2 years | 4.38e-03 | 228 |
| 5 years | 1.09e-02 | 91 |
Redundant architectures
| Architecture | Simplified PFDavg |
|---|---|
| 1oo1 | λDU TI / 2 |
| 1oo2 | (λDU TI)² / 3 + β λDU TI / 2 |
| 2oo3 | (λDU TI)² + β λDU TI / 2 |
β is the common-cause factor (typically 2–10%). For a full assessment, follow IEC 61508/61511 — see SIL determination methods.
Frequently asked questions
What is the PFDavg formula for 1oo1?
PFDavg ≈ λDU × TI ÷ 2, where λDU is the dangerous undetected failure rate per hour and TI the proof test interval in hours.
What PFDavg does each SIL need?
SIL 1 — 10⁻² to 10⁻¹; SIL 2 — 10⁻³ to 10⁻²; SIL 3 — 10⁻⁴ to 10⁻³; SIL 4 — 10⁻⁵ to 10⁻⁴ (low-demand mode, IEC 61511).
Is PFDavg enough to claim a SIL?
No. You must also meet architectural constraints (hardware fault tolerance), systematic capability and proof-test coverage requirements.
How do I lower PFDavg?
Test more often, choose devices with lower λDU, add redundancy (1oo2, 2oo3) or improve diagnostic coverage.