HSE Risk Matrix Explained: Understanding Risk Ratings (UK Guide)
RiskGen Team
Health & Safety ExpertsThe HSE risk matrix (officially called the "Initial Decision Matrix") is the Health and Safety Executive's own tool for deciding how serious a risk is. Unlike generic numerical matrices, it uses three factors — severity of harm, number of people affected, and likelihood — to produce a clear Red, Amber, or Green rating that tells you exactly what action to take.
What Is the HSE Risk Matrix?
The HSE's Initial Decision Matrix is the official risk assessment tool published by the Health and Safety Executive. It considers three things:
- • Severity of harm: How serious would the injury be?
- • Number of people: Could it affect one person or many?
- • Likelihood: How likely is it to actually happen?
The result is a simple colour code: Red (priority action), Amber (follow up needed), or Green (no further action). There are no numerical scores to calculate — just clear categories and a straightforward outcome.
How the HSE Risk Matrix Works
You assess each hazard by working through three steps:
- 1. Choose the severity band that matches the worst realistic outcome.
- 2. Decide whether multiple people or a single/low number could be affected.
- 3. Select the likelihood level that best fits the situation.
Where these three factors intersect on the matrix gives you the risk rating.
Severity Bands
The HSE uses three severity bands, each with two casualty levels:
- Serious Personal Injury: Death, major fractures, amputations, crush injuries, permanent disability. The most severe outcomes.
- Significant Injury (RIDDOR reportable): Injuries that must be reported under RIDDOR — over-7-day injuries, broken bones, burns requiring hospital treatment, occupational diseases.
- Minor Injury (non-RIDDOR): First aid injuries that don't require formal reporting — bruises, minor cuts, small scrapes.
For each severity band, you also consider whether the hazard could affect multiple casualties or just a single or low number of people. This gives six possible severity/casualty combinations.
Likelihood Levels
The HSE uses four likelihood levels:
- Probable: Could happen at any time. Controls are clearly inadequate or absent.
- Possible: Could happen sometime. Controls exist but may not be fully effective.
- Remote: Unlikely, though conceivable. Good controls are in place.
- Nil/Negligible: So unlikely that it's not worth considering further.
The HSE Initial Decision Matrix
Here is the complete HSE matrix showing how severity, casualty count, and likelihood combine to give a risk rating:
| Severity / Casualties | Probable | Possible | Remote | Nil/Negligible | |
|---|---|---|---|---|---|
| Serious Personal Injury | Multiple | RED | RED | RED | AMBER |
| Single or low | RED | RED | AMBER | AMBER | |
| Significant Injury (RIDDOR) | Multiple | RED | AMBER | AMBER | GREEN |
| Single or low | AMBER | AMBER | GREEN | GREEN | |
| Minor Injury (non-RIDDOR) | Multiple | AMBER | GREEN | GREEN | GREEN |
| Single or low | GREEN | GREEN | GREEN | GREEN | |
Understanding the Risk Ratings
Each colour tells you what to do next:
- Red — Priority Action: The risk is unacceptable. Stop work if necessary and implement controls immediately. These hazards need urgent attention and must not be left unaddressed.
- Amber — Follow Up: The risk needs attention. Plan and implement additional control measures. These are not emergencies, but you must take action to reduce the risk.
- Green — No Further Action: Current controls are adequate. The risk is acceptably low. Continue to monitor but no additional measures needed.
Example: Assessing a Construction Hazard
Let's say you're assessing the risk of a worker falling from scaffolding:
- • Severity: Serious Personal Injury (falls from height can cause death or major fractures)
- • Casualties: Single or low (typically one worker at a time)
- • Likelihood: Possible (could happen if guardrails or harnesses aren't used correctly)
Result: RED
This is a priority risk. You must ensure scaffolding is properly erected, guardrails are in place, harnesses are worn, and workers are trained before any work at height begins.
Another Example: Manual Handling
Assessing the risk of back strain from lifting heavy materials:
- • Severity: Significant Injury / RIDDOR (back injuries can result in over-7-day absence)
- • Casualties: Single or low
- • Likelihood: Possible (with proper training and equipment, but still a risk)
Result: AMBER
Follow-up action needed. Implement mechanical aids, provide manual handling training, and review lifting procedures.
Why the HSE Model Is Different from Generic Matrices
You may have seen generic "5x5 risk matrices" that use numerical scores (1-25). The HSE model is fundamentally different:
- • No numerical scores: The HSE model uses categorical judgements, not multiplication. This avoids the false precision of arbitrary numbers.
- • Considers casualty count: A hazard that could injure 20 people is treated differently from one that could injure 1. Generic matrices don't account for this.
- • Three clear outcomes: Red/Amber/Green is simpler and more actionable than four or five colour bands.
- • Official source: This is the model published by the UK's Health and Safety Executive, the regulator itself.
When to Use the HSE Risk Matrix
The HSE risk matrix is used across UK workplaces for:
- • Risk assessments (RAMS documents)
- • COSHH assessments (chemical risks)
- • Construction site safety reviews
- • Fire risk assessments
- • Pre-task safety planning and toolbox talks
It helps you comply with the Management of Health and Safety at Work Regulations 1999 and the Health and Safety at Work Act 1974.
Common Mistakes to Avoid
- ✗ Underestimating severity: Always consider the worst realistic outcome, not the most common one. A fall from height doesn't usually result in death, but it can.
- ✗ Ignoring multiple casualties: Consider whether a hazard (like a gas leak or structural collapse) could affect several people at once.
- ✗ Not reassessing after controls: Rate the risk before AND after control measures. The "after" rating should show your controls are working.
- ✗ Using the wrong model: Generic numerical matrices (1-25 scores) are not from the HSE. If you want to reference HSE methodology, use this categorical model.
Frequently Asked Questions
Is this risk matrix required by law?
No specific risk matrix is required by law. However, you are legally required to assess risks under the Management of Health and Safety at Work Regulations 1999. The HSE's own model is the most authoritative tool for doing this in the UK.
What does RIDDOR mean?
RIDDOR stands for Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013. It's the law that requires employers to report certain workplace injuries to the HSE. The "Significant Injury" band in the matrix aligns with RIDDOR-reportable injuries.
Can I still use a numerical risk matrix?
You can use any reasonable method to assess risk. However, numerical matrices (like the 5x5 grid with scores 1-25) are not from the HSE and can give a false sense of precision. The HSE's categorical model is simpler, more defensible, and directly aligned with the regulator's approach.
Do I need to include a risk matrix in my RAMS?
Many principal contractors and clients expect to see risk ratings in your RAMS. Including the HSE model shows you've assessed risk using the regulator's own methodology, which carries more weight than a generic scoring system.
What's the difference between "before controls" and "after controls"?
You should assess risk twice: once for the inherent risk (before any control measures) and once for the residual risk (after controls are in place). This shows your controls are actually reducing risk. For example, a scaffold fall might be Red before controls but Amber or Green after guardrails, harnesses, and training are implemented.
Final Thoughts
The HSE's Initial Decision Matrix is the most authoritative risk assessment tool available to UK businesses. It's simple, clear, and directly aligned with how the regulator thinks about risk.
By using severity bands, casualty count, and likelihood categories instead of arbitrary numbers, you get risk ratings that genuinely help you decide what to do — which is the whole point of risk assessment.
RiskGen implements the HSE's Initial Decision Matrix directly in our platform, so every risk assessment you create uses the official model. Create professional RAMS documents with correct risk ratings in minutes.
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