Calculator
Example data table
| Task | Hazard | Frequency | Duration | People | Probability | Controls (%) | Adjusted likelihood (typical) |
|---|---|---|---|---|---|---|---|
| Excavation near services | Utility strike | 4 | 3 | 2 | 3 | 40 | Possible |
| Working at height | Fall from edge | 4 | 4 | 3 | 4 | 30 | Likely |
| Concrete cutting | Flying fragments | 3 | 2 | 2 | 3 | 60 | Unlikely |
| Traffic management | Vehicle intrusion | 5 | 4 | 4 | 4 | 35 | Almost Certain |
Formula used
MultipliedScore = BaseScore × EnvironmentFactor × HumanFactor × HistoryFactor
AdjustedScore = MultipliedScore × (1 − ControlEffectiveness/100)
How to use this calculator
- Describe the task and hazard in plain, specific terms.
- Select frequency, duration, people exposed, and event probability.
- Apply environment, human, and incident history factors as needed.
- Estimate current control effectiveness as a realistic percentage.
- Click Calculate likelihood to view the rating above the form.
- Use the recommended actions to refine controls and planning.
- Download CSV or PDF to keep a simple audit trail.
Notes and limitations
- Ratings support decision-making; they do not replace permits or competent review.
- Use consistent scoring definitions across projects for comparability.
- Update the assessment when scope, crew, or conditions change.
Hazard likelihood rating guide
1) Why likelihood matters on active sites
Likelihood drives daily planning because the same hazard can behave differently across crews, shifts, and environments. This calculator standardizes judgement using a 1–5 rating, helping supervisors compare tasks and allocate controls before work starts.
2) The 1–5 exposure scales used
Four inputs are scored from 1 to 5: exposure frequency, exposure duration, people exposed, and event probability. With these scales, the base score ranges from 1 to 625 (5×5×5×5). This provides enough spread to separate low, medium, and high likelihood work.
3) Adjustment factors add site realism
Multipliers account for conditions that commonly raise likelihood. Environment ranges from 1.00 to 1.30 for factors like low lighting, congestion, traffic interface, or severe weather. Human factors range from 1.00 to 1.25 for fatigue, complexity, and experience mix. History ranges from 1.00 to 1.20 when near misses or incidents are trending.
4) Control effectiveness reduces the score
Controls are entered as a percentage reduction from 0% to 95%. For example, a multiplied score of 120 with 35% effectiveness becomes 78 (120×0.65). This encourages realistic scoring: strong engineered controls and verified supervision should materially reduce likelihood, while paperwork-only controls should not.
5) Practical thresholds for the 1–5 rating
The adjusted score is mapped to bands: Rare (<15), Unlikely (15–40), Possible (41–90), Likely (91–180), and Almost Certain (>180). These thresholds are designed so common daily tasks sit in the middle band unless conditions or history push the score upward.
6) Using the rating to pick actions
Ratings 1–2 usually mean maintain controls, brief the crew, and monitor triggers. Rating 3 signals additional supervision, a safer sequence, or improved barriers. Ratings 4–5 typically require engineered controls, permits, isolation, or a method redesign before work proceeds.
7) Typical data points for consistent scoring
Use consistent definitions: “Daily” exposure (4) means most shifts; “Multiple times per day” (5) means repeated exposure blocks. Duration “3–6 hours” (4) should reflect cumulative exposure, not a single short step. Keep probability tied to credible failure paths, not worst-case consequences.
8) Reporting, audits, and continuous improvement
Exporting a CSV creates a simple register for audits, pre-task briefings, and trend review. Track repeated high ratings to trigger design changes, supplier substitutions, or training. Recalculate when crews change, weather shifts, or temporary works are modified.
FAQs
1) What is the difference between base score and adjusted score?
Base score uses the four 1–5 exposure inputs only. Adjusted score applies environment, human, and history factors, then reduces the result by control effectiveness to better reflect real site conditions.
2) How do I choose a realistic control effectiveness percentage?
Use evidence: installed barriers, tested isolation, verified permits, and active supervision justify higher values. If controls rely mostly on reminders or signage, keep the percentage modest and improve the physical controls.
3) Can I use this rating for permits and method statements?
Yes, as a supporting input. The rating helps prioritize controls and sign-offs, but it does not replace your permit system, temporary works approvals, or competent-person reviews where required.
4) What should I do if the rating is 4 or 5?
Pause and strengthen controls. Introduce engineered measures, isolation, additional supervision, and staged sequencing. If the task cannot be made safe with practical controls, redesign the method before starting.
5) How often should I recalculate likelihood?
Recalculate when conditions change: weather, lighting, traffic interface, crew composition, equipment, or sequence. A quick recheck is also useful after any near miss or when controls are removed or altered.
6) Does a low likelihood mean the hazard is low risk?
Not necessarily. Likelihood measures chance of occurrence; severity is separate. A rare but catastrophic hazard still needs strong controls. Pair this likelihood rating with your consequence or severity assessment process.
7) Why include incident history as a multiplier?
Near misses and incidents are leading indicators that controls may be failing. The history factor increases the score to prompt action, reinforcing learning and preventing repetition of the same exposure pattern.