Skip to content
ENExplore Calculators

Safety Integrity Level Calculator

Work out safety integrity level instantly with clear inputs, formula shown and shareable results.

About Safety Integrity Level Calculator

The required safety integrity level follows from the gap between the unmitigated hazard frequency and the tolerable frequency, divided by the risk reduction the existing layers already provide. The required probability of failure on demand is the reciprocal of that factor, and the SIL bands are defined in decades: SIL 1 covers PFD from 0.1 to 0.01, SIL 2 to 0.001, SIL 3 to 0.0001.

Formulas

Risk reduction required

RRF = unmitigated frequency / tolerable frequency / existing risk reduction

SIL bands

PFD 0.1 to 0.01 is SIL 1, 0.01 to 0.001 SIL 2, 0.001 to 0.0001 SIL 3

Worked example

With the default values below, Safety Integrity Level Calculator returns Probability of failure on demand required of 0.01000000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Unmitigated hazard frequencyminimum 0 per year
0.1 per year
Tolerable hazard frequencyminimum 0 per year
0.0001 per year
Risk reduction from existing layersminimum 1
10

Results

Probability of failure on demand required
0.01000000
Risk reduction factor required
100.00
Safety integrity level
SIL 2
Total risk reduction with existing layers
1,000.00

Step by step

  1. 1Values used
  2. 2Risk reduction required
  3. 3SIL bands
  4. 4Probability of failure on demand required= 0.01000000
  5. 5Risk reduction factor required= 100.00
  6. 6Safety integrity level= SIL 2
  7. 7Total risk reduction with existing layers= 1,000.00

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 or fees or charges 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. SIL determination and verification must be carried out to the applicable functional safety standard by competent practitioners.

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

Preliminary estimate only. SIL determination and verification must be carried out to the applicable functional safety standard by competent practitioners.

Frequently Asked Questions

How is the Safety Integrity Level figure worked out?

RRF = unmitigated frequency / tolerable frequency / existing risk reduction. The required safety integrity level follows from the gap between the unmitigated hazard frequency and the tolerable frequency, divided by the risk reduction the existing layers already provide.

Is a higher SIL always better?

No. Each SIL step demands an order of magnitude more testing, redundancy and management effort, and SIL 4 is generally regarded as a sign the process design itself should be changed instead.

What values do I need to enter?

This calculator takes 3 inputs: Unmitigated hazard frequency, Tolerable hazard frequency, Risk reduction from existing layers. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Safety Integrity Level Calculator | Calcrivo