How to Calculate Severity Rate
TRIR tells you how often incidents happen. DART tells you which ones caused lost time. But neither tells you how bad those incidents actually were. That's what Severity Rate measures — and most safety programs ignore it entirely.
What Is Severity Rate?
Severity Rate — also called the Lost Workday Rate — measures the total number of lost workdays per 100 full-time employees per year. It quantifies the impact of workplace injuries, not just their frequency.
Two companies can have identical TRIR and DART rates but completely different Severity Rates. One company's incidents might result in 2-day restrictions while another's result in 90-day surgeries. Without Severity Rate, you can't tell the difference from the numbers alone.
This is why OSHA, the BLS, and safety professionals treat TRIR, DART, and Severity Rate as a three-metric system — each one tells you something the others can't.
The Severity Rate Formula
The calculation uses the same 200,000-hour baseline as TRIR and DART, normalizing results per 100 full-time employees so you can compare across organizations of different sizes:
The 200,000 figure represents 100 employees × 40 hours/week × 50 weeks/year. This standardization is what makes your number comparable to BLS industry benchmarks.
A Worked Example
A distribution operation with 300 employees works 624,000 hours in a year. They have 8 recordable incidents resulting in 42 total lost workdays:
Their Severity Rate is 13.46 lost workdays per 100 FTEs — below the BLS all-industry average of ~16, meaning their incidents are causing less disruption than typical. But paired with their incident count, you now have a complete picture of both frequency and impact.
Average Severity Per Incident
The second number worth calculating is simple but powerful:
In the example above: 42 days ÷ 8 incidents = 5.25 days per incident on average.
This number tells you whether your injuries are mostly minor (1–3 days) or frequently severe (30+ days). An operation with a low incident count but high average severity likely has a specific hazard — a piece of equipment, a task, a department — generating disproportionately serious injuries. That's where your investigation effort belongs.
Industry Benchmarks
The Bureau of Labor Statistics publishes Days Away from Work rates by industry annually. General reference ranges:
| Severity Rate | Assessment | What It Suggests |
|---|---|---|
| < 8 | Excellent | Well-controlled incidents, effective return-to-work program |
| 8 – 16 | Good to Average | Near or below BLS all-industry average (~16) |
| 16 – 30 | Above Average | Incidents trending more serious, investigate injury types |
| 30 – 50 | High | Systemic issue — specific hazard or task driving severity |
| > 50 | Critical | Immediate root cause analysis required |
Always compare your rate to your specific NAICS industry code — a construction company's benchmark is very different from a food distribution operation's.
How to Use Severity Rate in the Real World
Justify Return-to-Work Programs
Every day an injured worker is out drives your Severity Rate up. Modified duty and return-to-work programs directly reduce lost workdays — and Severity Rate gives you the before/after data to prove ROI to leadership. A program that cuts average lost days from 12 to 6 per incident cuts your Severity Rate in half.
Identify Your Worst Hazards
Break Severity Rate down by department, task type, or injury category. The department with the highest average days per incident is telling you something specific — usually a single hazard that generates rare but severe injuries that wouldn't show up clearly in your TRIR alone.
Make the Budget Conversation
Leadership understands "we lost 42 days of productive work this year to injuries" better than they understand a TRIR of 2.56. Severity Rate translates safety performance into operational impact — hours lost, productivity lost, overtime cost to backfill. It's the metric that turns safety data into a business case.
Track Improvement Over Time
Use Severity Rate year-over-year to measure whether your safety program is not just preventing incidents (TRIR) but also preventing serious ones (Severity). You can have a flat TRIR while your Severity Rate drops significantly — which means your program is shifting injuries from severe to minor. That's real progress worth recognizing.
Severity Rate vs DART Rate — What's the Difference?
DART Rate counts the number of incidents that resulted in days away, restriction, or transfer. Severity Rate measures the total days lost from those incidents. They answer different questions:
- DART Rate: "How often are our incidents serious enough to affect work?"
- Severity Rate: "When incidents do happen, how much work time do we actually lose?"
A low DART Rate with a high Severity Rate means you have few serious incidents, but the ones you do have are very bad. A high DART Rate with a low Severity Rate means incidents are frequent but mostly minor. Each combination points to a different intervention strategy.
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