Failure Rate To PFD Calculator

Work out failure rate to pfd instantly with clear inputs, formula shown and shareable results.

About Failure Rate To PFD Calculator

For a simplex channel tested periodically, average probability of failure on demand is the undetected dangerous failure rate times the test interval, divided by two, because on average a fault exists for half the interval. Diagnostic coverage removes the detected portion, so both better diagnostics and shorter test intervals improve PFD proportionally.

Formulas

Average PFD

PFDavg = lambda undetected x test interval hours / 2

Undetected rate

lambda undetected = dangerous failure rate x (1 - diagnostic coverage)

Worked example

With the default values below, Failure Rate To PFD Calculator returns Average probability of failure on demand of 0.00175200. Change any input above and every figure here is recalculated in your browser.

Inputs used

Dangerous failure rateminimum 0 per hour: 0.000001 per hour
Proof test intervalminimum 0.5 months: 12 months
Diagnostic coverageaccepts 0–99 %: 60 %

Results

Average probability of failure on demand: 0.00175200
Safety integrity level achieved: SIL 2
Risk reduction factor: 570.8
Mean time between dangerous failures: 114.2 years

Step by step

1Values used
2Average PFD
3Undetected rate
4Average probability of failure on demand= 0.00175200
5Safety integrity level achieved= SIL 2
6Risk reduction factor= 570.8
7Mean time between dangerous failures= 114.2 years

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 8 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Functional safety verification must follow the applicable standard using validated failure rate data and full architectural analysis.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Functional safety verification must follow the applicable standard using validated failure rate data and full architectural analysis.

Frequently Asked Questions

How is the Failure Rate To PFD figure worked out?

PFDavg = lambda undetected x test interval hours / 2. For a simplex channel tested periodically, average probability of failure on demand is the undetected dangerous failure rate times the test interval, divided by two, because on average a fault exists for half the interval.

Can I reach SIL 3 by testing more often?

Only up to a point. Systematic capability, architectural constraints on redundancy and proof test effectiveness all limit the achievable SIL regardless of test frequency.

What values do I need to enter?

This calculator takes 3 inputs: Dangerous failure rate, Proof test interval, Diagnostic coverage. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.