Quantitative Risk Assessment (QRA) — Individual/Societal Risk Calculator

IR = Individual Risk (risk of fatality to a specific person per year) | SR = Societal Risk (risk to groups, shown via F-N relationships). Aggregates hazard scenario data into overall risk figures and compares them against published UK HSE (Health and Safety Executive) tolerability criteria.

Scope note: This tool aggregates hazard scenario frequencies and consequence probabilities that you supply (from HAZOP, fault tree analysis, consequence modeling, or historical data) into overall Individual and Societal Risk figures. It does not perform consequence modeling itself (dispersion, fire, explosion modeling — e.g., PHAST/ALOHA-type analysis) or spatial population integration for a full F-N curve. A full QRA requires specialist software and a competent risk assessor; this tool supports the aggregation and criteria-comparison step only.

Individual Risk (IR)

Societal Risk (F-N)

Receptor Type UK HSE applies different tolerability bands to workers vs. members of the public.

Hazard Scenarios

Total Individual Risk (per year)

—

Tolerability Category

Hazard Scenarios

Formula Reference

Individual Risk (per scenario) = Frequency (per year) × Probability of Fatality × Occupancy/Exposure Fraction
Total Individual Risk = Σ (Individual Risk per scenario)
Societal Risk: Cumulative Frequency F(N or more fatalities) = Σ Frequency of all scenarios causing ≥ N fatalities

UK HSE Individual Risk Criteria (R2P2)

Workers — Broadly Acceptable	< 1×10⁻⁶ /year
Workers — Tolerable (ALARP)	1×10⁻⁶ to 1×10⁻³ /year
Workers — Intolerable	> 1×10⁻³ /year
Public — Broadly Acceptable	< 1×10⁻⁶ /year
Public — Tolerable (ALARP)	1×10⁻⁶ to 1×10⁻⁴ /year
Public — Intolerable	> 1×10⁻⁴ /year

Societal Risk Reference Point (UK HSE)

HSE has proposed that a societal risk of 50 or more fatalities in a single event should be regarded as intolerable if its frequency exceeds 1 in 5,000 per year (2×10⁻⁴/yr). A full societal risk assessment plots the entire F-N curve against sloped criterion lines (slope −1) spanning many values of N — this single reference point is a simplified screening check only.

Criteria based on UK HSE's "Reducing Risks, Protecting People" (R2P2, 2001) framework, widely used in industry though not themselves prescribed in law. These are risk tolerability screening criteria, not a certification of adequate safety — ALARP (As Low As Reasonably Practicable) demonstration and full QRA remain necessary regardless of which band a result falls into. Only a competent risk assessor should rely on this tool's output for safety case or regulatory submissions. For customized calculators tailored to your organization, please contact Impact Tech Solutions.