How to calculate TRIR, DART, LTIFR and severity rate
All of these rates divide a count by the hours worked and scale it to a fixed base, so a 50 person shop and a 5,000 person plant can be compared:
TRIR = recordable cases × 200,000 ÷ hours worked
DART rate = DART cases × 200,000 ÷ hours worked
LTIR = lost time injuries × 200,000 ÷ hours worked
LTIFR = lost time injuries × 1,000,000 ÷ hours worked
Severity rate = lost workdays × 200,000 ÷ hours worked
| Term | What it means |
|---|---|
| Hours worked | hours actually worked by all employees in the period, overtime included, vacation, sick leave and holidays left out |
| Recordable case | a work related injury or illness that meets the OSHA recording criteria (29 CFR 1904): beyond first aid, days away, restricted work, loss of consciousness and so on |
| DART case | a recordable case with days away from work, restricted work or a job transfer |
| Lost time injury | an injury that keeps the person off work for at least one full shift after the day it happened |
| Lost workdays | calendar days away from work because of those cases |
The 200,000 base is 100 full time employees × 40 hours × 50 weeks. That turns every rate into “cases per 100 full time workers per year”, which is how OSHA and the Bureau of Labor Statistics (BLS) report them. The UK, Australia and the ILO usually express lost time per 1,000,000 hours instead, so the LTIFR is five times the LTIR for the same data.
If you do not have total hours handy, the calculator works them out as employees × hours per week × weeks. Payroll or time records are always more accurate.
Worked example
The calculator opens with a plant of 120 employees working 40 hours a week for 50 weeks:
- Hours worked = 120 × 40 × 50 = 240,000 h
- Recordable cases: 6, of which 3 DART cases
- Lost time injuries: 2, with 34 lost workdays
The math:
- TRIR = 6 × 200,000 ÷ 240,000 = 5.00
- DART rate = 3 × 200,000 ÷ 240,000 = 2.50
- LTIR = 2 × 200,000 ÷ 240,000 = 1.67
- LTIFR = 2 × 1,000,000 ÷ 240,000 = 8.33
- Severity rate = 34 × 200,000 ÷ 240,000 = 28.33, or 34 ÷ 2 = 17 days per lost time case
How to read the results
- TRIR is the broadest measure: every recordable case counts the same, from a few stitches to an amputation.
- DART narrows it to cases serious enough to keep someone off their normal job. A DART rate close to the TRIR means most of your recordables are serious.
- LTIR and LTIFR count only lost time, the figure most global companies and contractors report.
- Severity rate weighs cases by days lost. Few cases with a high severity rate means a small number of serious injuries.
With few hours worked, a single case swings the rate a lot. For small sites, look at a rolling 12 months instead of a single month. To see where injuries cluster, build a Pareto chart by body part, area or injury type. To get ahead of injuries on machinery, score each hazard with the HRN risk assessment calculator.
Lagging rates tell you how you did, not how to improve. Compare against the BLS average for your industry (its NAICS code) and, above all, against your own trend.
Common mistakes
- Counting hours nobody worked. Vacation, sick leave and holidays are not exposure. Using scheduled or paid hours inflates the denominator and makes the rate look better than it is.
- Mixing recordable and lost time. A TRIR built from lost time cases, or an LTIFR built from all recordables, cannot be compared with anyone else’s.
- Comparing rates on different bases. An LTIFR of 8 per million hours is the same as an LTIR of 1.6 per 200,000. Brazilian NBR 14280 rates also use 1,000,000 hours. Always check the base before comparing.
- Leaving out temporary or contract workers when you count their injuries, or the other way round. Cases and hours must cover the same people.
- Counting days away without the OSHA cap. OSHA recordkeeping caps days away or restricted at 180 per case, and counts calendar days, not just scheduled shifts.
This calculator checks the arithmetic. Deciding what is recordable and keeping the OSHA 300 log are the employer’s responsibility under 29 CFR 1904.
Sources
- OSHA, 29 CFR Part 1904, Recording and Reporting Occupational Injuries and Illnesses.
- US Bureau of Labor Statistics, Survey of Occupational Injuries and Illnesses (SOII), incidence rates by industry.
- ILO, resolution on statistics of occupational injuries (16th ICLS, 1998), frequency rates per 1,000,000 hours.