ALARP / Cost-Benefit Risk Calculator

ALARP = As Low As Reasonably Practicable. Calculates the Cost of Preventing a Fatality (CPF) and Gross Disproportion Factor (GDF) for a proposed risk reduction measure, following the UK HSE cost-benefit framework used to test whether further risk reduction is "reasonably practicable."

Important context: The Gross Disproportion Factor is a decision-support input, not an automatic pass/fail rule. UK HSE case law and guidance require a collective, professional judgement — considering risk severity, proximity to the intolerable boundary, vulnerable groups, and societal concern — not a mechanical cut-off. This calculator performs the arithmetic; the ALARP judgement itself must be made by a competent person, typically the Assessment/Inspection team collectively, per HSE guidance.

Risk Reduction Measure

Total Cost of Measure (£) Capital cost plus present value of ongoing/operating costs over the measure's lifetime.

Risk Reduction, ΔPLL (fatalities/year) Reduction in expected fatalities per year (Potential Loss of Life) this measure achieves — from your QRA.

Measure Lifetime (years) Period over which the risk reduction benefit applies (e.g., remaining plant life).

Valuation & Proportionality

Value of Preventing a Fatality, VPF (£) UK HSE has cited figures in the ~£1–2 million range historically (updated periodically for inflation) — confirm the current official figure before formal use.

Proportion Factor Threshold Judgement-based. Historical reference range: ~2 for low risk to public, ~3 for workers, up to ~10 for high risk/severe consequence.

Fatalities Prevented (lifetime)

£0

Cost of Preventing a Fatality

0.0×

Gross Disproportion Factor

Formula Reference

Fatalities Prevented (lifetime) = Risk Reduction (ΔPLL) × Measure Lifetime
Cost of Preventing a Fatality (CPF) = Total Cost of Measure ÷ Fatalities Prevented (lifetime)
Gross Disproportion Factor (GDF) = CPF ÷ VPF

Historical Proportion Factor Reference Points

Minimum (baseline)	1 — values below imply bias against safety
Low risk to public	~2 (Sizewell B inquiry reference)
Risk to workers	~3 (Sizewell B inquiry reference)
High risk / severe consequence	up to ~10

HSE guidance is explicit that there is no fixed, prescribed disproportion factor — "the greater the risk, the more that should be spent in reducing it." These figures are historical reference points from UK case law and inquiries, not a current statutory table.

Methodology, interpretation & sources

Purpose. ALARP = As Low As Reasonably Practicable. Calculates the Cost of Preventing a Fatality (CPF) and Gross Disproportion Factor (GDF) for a proposed risk reduction measure, following the UK HSE cost-benefit framework used to test whether further risk reduction is "reasonably practicable."

Primary references. Use the current edition/version that applies to your jurisdiction and organization:

Editorial note. Prepared and maintained by Impact Tech Solutions. Editorially updated 28 September 2026. No individual professional credential is claimed on this page; high-consequence, statutory, tax, engineering or health decisions should be independently verified against the current primary source and, where required, a qualified professional.

ALARP / Cost-Benefit Risk Calculator

Formula Reference

Historical Proportion Factor Reference Points

Methodology, interpretation & sources

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