What the frequency and severity rates are
The accident frequency rate shows how many lost-time injuries occur for every million hours worked. The severity rate shows how many workdays are lost, also per million hours. The first says how often people get hurt; the second, how badly.
Several bases are in use: 1,000,000 hours (LTIR/LTIFR in many multinationals and in Brazil's NBR 14280), 200,000 hours (OSHA's TRIR and LTIR in the US) and 1,000 full-time workers (Germany). Knowing the base is essential before comparing numbers. These rates support the performance monitoring and measurement required by ISO 45001 (clause 9.1).
Visual map of frequency and severity rates
The map shows the two formulas, the 250-employee example, the conversion to the OSHA base and how to read the results.
Frequency rate: formula and example
Lost-time injury frequency rate (LTIFR) = lost-time injuries × 1,000,000 ÷ hours worked
Hours worked are the actual hours of all employees in the period, overtime included, vacation, absence and leave excluded. Decide beforehand whether contractors and commuting accidents are in scope, and keep the rule.
Example (fictional): a plant with 250 employees logged 480,000 hours worked in the year (about 1,920 h per person), 6 lost-time injuries and 4 injuries needing medical treatment without lost time.
LTIFR = 6 × 1,000,000 ÷ 480,000 = 12.5
That is 12.5 lost-time injuries per million hours worked. The 4 cases without lost time are not part of the lost-time rate; many companies track a separate rate for them.
Severity rate: formula and example
Severity rate = (days lost + days charged) × 1,000,000 ÷ hours worked
Days lost are the days away from work. Days charged are fixed values that Brazil's NBR 14280 assigns to fatalities and permanent disabilities so that a serious accident weighs more than its days of absence (6,000 days for death and permanent total disability). Other systems count only days away.
| Lost-time injury | Days lost |
|---|---|
| 1 | 5 |
| 2 | 8 |
| 3 | 10 |
| 4 | 12 |
| 5 | 21 |
| 6 | 28 |
| Total | 84 |
With no fatality or permanent disability, there are no charged days:
Severity rate = 84 × 1,000,000 ÷ 480,000 = 175
Average severity per injury: 84 ÷ 6 = 14 days. If one fatality had occurred in the year, 6,000 charged days would add 6,000 × 1,000,000 ÷ 480,000 = 12,500 to the rate, which shows how a single severe case dominates the figure.
TRIR and LTIR (OSHA base)
Companies with US parents or global customers ask for rates on the OSHA base of 200,000 hours (equal to 100 employees × 40 h × 50 weeks):
TRIR = recordable cases × 200,000 ÷ hours worked
LTIR = days-away cases × 200,000 ÷ hours worked
| Metric | Calculation | Result |
|---|---|---|
| TRIR (6 days-away + 4 medical treatment) | 10 × 200,000 ÷ 480,000 | 4.17 |
| LTIR (days away only) | 6 × 200,000 ÷ 480,000 | 2.50 |
| Conversion | LTIFR ÷ 5 = 12.5 ÷ 5 | 2.50 |
The conversion is simple (200,000 is one fifth of 1,000,000), but recording criteria differ: OSHA defines what a recordable case is (medical treatment beyond first aid, restricted work, days away), and NBR 14280 has its own classification. To compare, align the definitions before aligning the base. OSHA also uses the DART rate (days away, restricted or transferred), on the same 200,000-hour base.
How to read the results
Take the following year in the same example: 500,000 hours, 4 lost-time injuries and 40 days lost.
| Metric | Year 1 | Year 2 | Change |
|---|---|---|---|
| Hours worked | 480,000 | 500,000 | +4.2% |
| Lost-time injuries | 6 | 4 | −2 |
| Frequency rate | 12.5 | 8.0 | −36% |
| Days lost | 84 | 40 | −44 |
| Severity rate | 175 | 80 | −54.3% |
| Days per injury | 14 | 10 | −4 |
- Read frequency and severity together. A falling frequency with a rising severity means fewer but worse injuries: real risk has not gone down.
- Few cases, large swings. With 4 to 6 injuries a year, one more case moves the rate by tens of percent. Watch the 12-month trend, not just the month.
- It is a lagging indicator. The rates count what already happened. Add leading indicators: near misses, hazards corrected, inspections and training.
- Analyze causes. Use a Pareto chart by type, area and body part, and the 5 Whys on each injury, with the action logged as a corrective action.
- Prevent. Anticipate risks with the risk matrix and preliminary risk analysis, guard machines as in machine safety (NR-12), apply LOTO in maintenance and cut strain with ergonomics.
- Visual management. Safety is the "S" of the SQDCM board; show days without injury and the frequency rate on the visual management board.
Common mistakes
- Using headcount instead of hours. The base is hours worked; headcount distorts with shifts, overtime and leave.
- Comparing rates on different bases (1,000,000, 200,000 or 1,000 workers) as if they were equal.
- Underreporting to "improve" the metric. Hiding injuries destroys the chance to learn and creates legal risk.
- Forgetting contractors, temporary workers and overtime in the numerator or the denominator, with no single rule.
- Counting days differently from year to year (calendar or working days, including the day of the accident or not).
- Treating a low rate as guaranteed safety. Few injuries can be luck; risk only falls with controls.
Frequently asked questions
What is the accident frequency rate formula?
Frequency rate = lost-time injuries × 1,000,000 ÷ hours worked. In the example: 6 × 1,000,000 ÷ 480,000 = 12.5. OSHA uses 200,000 hours instead.
How is the severity rate calculated?
Severity rate = (days lost + days charged) × 1,000,000 ÷ hours worked. In the example: 84 × 1,000,000 ÷ 480,000 = 175.
What is the difference between TRIR and LTIR?
TRIR counts all recordable cases (including medical treatment and restricted work); LTIR counts only cases with days away. Both use the 200,000-hour base.
Why does OSHA use 200,000 hours?
It equals 100 full-time employees working 40 hours a week for 50 weeks, so the rate reads as injuries per 100 full-time workers per year.
How do I convert a 1,000,000-hour rate to the OSHA base?
Divide by 5. In the example, 12.5 ÷ 5 = 2.5. First align which cases are counted.
What is a good accident frequency rate?
There is no universal number; it depends on the sector and the recording criteria. Compare with your own history and with peers using the same base and definitions.
Sources
- OSHA (USA). 29 CFR Part 1904: Recording and Reporting Occupational Injuries and Illnesses.
- ISO 45001:2018. Occupational health and safety management systems: requirements with guidance for use.
- ABNT. NBR 14280: Cadastro de acidente do trabalho: procedimento e classificação.
Keep studying: trusted sources and links
- ISO: ISO 45001, Occupational health and safety
- Ministério do Trabalho e Emprego: Normas Regulamentadoras (NRs) (in Portuguese)
Save this address: Manufatura Enxuta is your school for lean manufacturing. Come back whenever you need to study, with a complete article, a visual map and trusted sources. Follow one of the learning paths or check the A–Z glossary.
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.
