Enter Hazard Details
Use the matrix to score initial risk, then apply exposure and control effectiveness to estimate residual risk.
Example Data Table
These sample entries show how likelihood and severity map to risk levels.
| Activity | Hazard | L | S | Exposure | Controls % | Initial | Residual | Residual Level |
|---|---|---|---|---|---|---|---|---|
| Scaffold access | Fall from height | 4 | 5 | 1.25 | 35 | 20 | 16.25 | High |
| Cutting rebar | Eye injury | 3 | 2 | 1.00 | 60 | 6 | 2.40 | Low |
| Excavation | Trench collapse | 3 | 5 | 1.50 | 20 | 15 | 18.00 | Extreme |
Formula Used
- Initial Risk Score: Initial = Likelihood × Severity
- Adjusted Score (Exposure): Adjusted = Initial × ExposureMultiplier
- Residual Score (Controls): Residual = Adjusted × (1 − ControlsEffectiveness/100)
- Risk Levels (default bands): Low ≤ 4, Medium ≤ 9, High ≤ 16, Extreme > 16
Exposure is clamped to 0.5–2.0 and controls to 0–100 for stability.
How to Use This Calculator
- Enter the project, activity, and one primary hazard.
- Select a likelihood score from 1 to 5.
- Select a severity score from 1 to 5.
- Set exposure multiplier based on time and frequency.
- Estimate how effective existing controls will be.
- Click Calculate Risk to view initial, adjusted, residual results.
- Download CSV or PDF to attach to your records.
Professional Guide: Using a Risk Assessment Matrix on Site
1) Purpose of a matrix in construction
A risk assessment matrix turns judgement into a repeatable score so teams can compare hazards consistently. A 5×5 approach uses likelihood (1–5) and severity (1–5) to produce an initial score from 1 to 25, helping prioritize actions, permits, and supervision.
2) Likelihood scoring with clear triggers
Likelihood should reflect real conditions: task frequency, workforce competence, housekeeping, weather, and proximity to moving plant. Use 1 for rare scenarios, 3 for events seen on similar jobs, and 5 when exposure is continuous or controls are missing.
3) Severity anchored to credible outcomes
Severity is tied to the worst credible consequence, not the most common injury. Work at height or confined spaces may justify 4–5 due to permanent harm or fatality potential, while low-energy tasks may sit at 1–2. Stable definitions improve reporting.
4) Exposure multiplier adds realism
This calculator applies an exposure multiplier (0.5–2.0) to represent time-at-risk. Short, controlled tasks might use 0.75–1.0, while repeated lifts, congested access routes, or long shifts can justify 1.25–2.0. The adjusted score separates brief peaks from constant exposure.
5) Control effectiveness models residual risk
Control effectiveness (0–100%) estimates how strongly current measures reduce the adjusted risk. Engineering barriers, verified permits, competent supervision, and inspected fall protection typically outperform admin-only controls. Residual risk is calculated as Adjusted × (1 − effectiveness/100).
6) Interpreting Low, Medium, High, Extreme
Default bands are Low (≤4), Medium (≤9), High (≤16), and Extreme (>16). Link bands to decisions: Low needs routine checks, Medium needs task briefing and monitoring, High needs stronger engineered controls and permits, and Extreme should pause until risk drops.
7) Documentation that stands up to reviews
Strong records include the activity, hazard, location, and a short control summary. Capturing both initial and residual scores shows improvement and supports learning. CSV exports help dashboards and trends; the PDF summary supports daily briefings, permits, and client reporting.
8) Practical example for planning
If likelihood is 4 and severity is 5, initial risk is 20. With exposure 1.25, adjusted risk becomes 25. With controls at 35% effective, residual risk is 16.25, still High. That outcome signals stronger controls, revised sequencing, or an alternate method.
FAQs
1) What is the difference between initial and residual risk?
Initial risk scores the hazard before considering exposure weighting and control effectiveness. Residual risk reflects the adjusted score after applying current controls, helping you judge whether work can proceed safely.
2) How do I pick a fair likelihood score?
Base likelihood on task frequency, site congestion, competence, and recent near-misses. Use consistent triggers across teams so similar activities receive comparable scores, improving decision quality and trend reporting.
3) Why use an exposure multiplier?
Exposure accounts for time-at-risk. Two hazards with the same L×S can require different priorities if one happens once per month and the other occurs all day, every day.
4) What does control effectiveness mean in practice?
It is an estimate of how much your controls reduce the adjusted risk. Verified engineered controls and strong supervision usually score higher than signage or toolbox talks alone.
5) Should severity always be the worst possible outcome?
Use the worst credible consequence based on realistic failure modes, not unlikely extremes. This keeps scoring defensible and prevents overrating low-impact tasks.
6) Can I customize the risk level bands?
Yes. Many organizations adjust bands to match internal standards. Update the riskLevel() thresholds to align with your company matrix, then keep the definitions stable for reporting.
7) Is this calculator a replacement for a full risk assessment?
No. It supports consistent scoring and documentation. You still need a competent assessment of hazards, legal duties, method statements, and verification that controls are implemented and working.