The classic Risk Priority Number multiplies three ratings:
RPN = Severity (S) × Occurrence (O) × Detection (D)
This tool also supports optional weighting to emphasize a factor:
Weighted RPN = (S × WS) × (O × WO) × (D × WD)
- Describe the failure mode and its potential effect on people, assets, or schedule.
- Pick Severity: worst credible outcome if the failure happens.
- Pick Occurrence: how often the failure could reasonably happen.
- Pick Detection: likelihood your controls detect the failure before harm.
- Adjust weights if your project needs stricter emphasis on one factor.
- Set threshold limits to match your site risk matrix and governance.
- Calculate, assign an owner, and verify risk reduction after actions.
| Failure mode | Effect | S | O | D | Base RPN | Category |
|---|---|---|---|---|---|---|
| Missing toe-board at scaffold platform | Falling objects could injure workers below | 7 | 5 | 6 | 210 | High |
| Unmarked trench edge near pedestrian route | Trips and falls causing minor to moderate injury | 5 | 4 | 4 | 80 | Low |
| Poor housekeeping at material laydown | Delayed access, minor injuries, and rework | 4 | 6 | 3 | 72 | Low |
Professional Guidance: Using RPN on Construction Projects
Why RPN Matters on Construction Sites
Risk Priority Number (RPN) helps teams rank failure modes in activities like lifting, excavation, and temporary works. By combining Severity, Occurrence, and Detection on a 1–10 scale, supervisors can compare hazards consistently across crews, shifts, and subcontractors. For construction planning, it supports daily pre-task briefs and weekly risk reviews, keeping resources aimed at the biggest exposures first.
Scoring Severity with Real Consequences
Severity reflects the credible worst outcome, not the most likely one. A score of 10 fits fatality or permanent disability, while 1 fits negligible first-aid. Use task-specific references: fall exposure, struck-by energy, confined space atmosphere, and electrical contact voltage. To stay objective, document assumptions, include weather, crew experience, and access constraints, and review scores with the foreman and safety lead before work begins daily.
Estimating Occurrence from Field Data
Occurrence estimates how often the failure could happen during the work period. Use evidence: near-miss logs, inspection findings, equipment hours, and similar-job history. For example, repeated missing toe boards may justify 6–7, while a rare design anomaly may be 2–3.
Rating Detection and Controls
Detection rates how likely current controls will catch the failure before harm. Strong detection includes pre-lift checklists, permit-to-work verifications, engineered interlocks, and competent-person inspections. Weak detection occurs when issues rely on informal observation or end-of-day reporting.
Calculating RPN and Weighted RPN
Basic RPN equals S×O×D, producing a range from 1 to 1000. This calculator also supports weighting, so critical dimensions can be emphasized, such as increasing Severity weight for high-energy work. Weighted RPN uses (S×wS)×(O×wO)×(D×wD).
Setting Practical Risk Bands
Risk bands convert numbers into decisions. Many sites treat ≤80 as low, 81–200 as medium, and >200 as high, then adjust to match local tolerance and regulatory expectations. The thresholds in the tool are editable, enabling alignment with your project risk matrix.
Turning Scores into Corrective Actions
Once ranked, focus on the top items first. Apply the hierarchy of controls: eliminate the step, substitute materials, engineer barriers, add administrative controls, then PPE. Set a target RPN reduction, assign an owner, and verify effectiveness with follow-up scoring.
Documenting Reviews and Continuous Improvement
RPN is most valuable when tracked over time. Store results per activity, date, location, and assessor, then trend reductions after audits or method changes. Use the CSV and PDF exports for toolbox talks, client reporting, and closeout packages that prove due diligence.
FAQs
1. What is a typical scoring scale for RPN inputs?
Most teams use 1–10 for Severity, Occurrence, and Detection. Define anchors for each value (for example, 10 = fatality, 1 = negligible) so different assessors score consistently across similar tasks.
2. How do I choose Occurrence without guesswork?
Base it on evidence: recent near-misses, inspection defects, equipment hours, and similar-project experience. If data is limited, start conservative, then refine the score after the first week of observations and audits.
3. What does a high Detection number mean?
High Detection means poor detectability. If controls are unlikely to catch the failure before it reaches workers, Detection should be higher. Add checkpoints, permits, or engineered safeguards to improve detection and reduce the score.
4. When should I use weighting?
Use weights when one dimension should dominate decisions. For high-energy work, increase Severity weight; for repetitive tasks with frequent exposure, increase Occurrence weight. Keep weights documented so stakeholders understand the rationale.
5. How should we set low, medium, and high thresholds?
Start with your site risk matrix and regulatory expectations. Many projects begin with 80 and 200 as breakpoints, then adjust after reviewing a month of results to match your actual incident and near-miss patterns.
6. Can RPN replace a full risk assessment?
No. RPN ranks issues; it does not capture every legal requirement, exposure duration, or cumulative risk. Use it alongside method statements, permits, and engineering reviews, especially for critical lifts, excavations, and confined spaces.
7. What should we do after controls are implemented?
Re-score the same failure mode using the updated controls and record before/after values. Confirm changes in the field, then export the updated log for reporting. Trend reductions to show improvement and prioritize the next actions.
Use these scores to prevent incidents and rework today.