Injury frequency and severity rates: calculation and reading

How the calculation is carried out

1. 1

   Set the period

   Month, quarter or year. All three indicators must cover the same period as the hours worked, otherwise the ratio means nothing.

2. 2

   Count the hours actually worked

   These are hours of effective presence, not contractual hours: leave, absence and time off are excluded. It is the denominator of both rates, and therefore the costliest mistake.

3. 3

   Retain lost time injuries

   An injury with no time away from work does not enter the French frequency rate. Commuting accidents follow their own rule: state in your procedure whether you include them, and keep the same convention from one period to the next.

4. 4

   Count the days lost

   Days of absence counted within the period, including those arising from an injury that occurred in an earlier period and is still ongoing.

5. 5

   Read the result over time

   A single figure says nothing. It is the twelve-month rolling trend, and the gap between frequency and severity, that guide action.

A fully worked example

Take the values already suggested in the fields above: 3 lost time accidents, 45 lost working days, 180,000 hours worked and an average headcount of 120 over the period.

A frequency rate of 16.67 reads as follows: roughly one lost time accident every 60,000 hours worked. It is the same figure said differently, and it is often the wording that lands best in a management review.

The same figures against 200,000 hours give 3.33 instead of 16.67. This is not a conversion: the French frequency rate counts lost time accidents, the North American TRIR counts recordable injuries, which are more numerous. Two different case definitions do not compare, even brought to the same base.

The most common calculation mistakes

• ▪Using contractual hours instead of hours actually worked, which mechanically understates both rates.
• ▪Changing convention mid-year, for instance by including commuting accidents from one quarter onwards: the series is no longer comparable with itself.
• ▪Comparing a frequency rate on a 1,000,000 base with a TRIR on a 200,000 base, even after conversion, when the events counted differ.
• ▪Leaving out agency staff and contractors present on site, whose hours and injuries often fall within the reporting perimeter.
• ▪Tracking only the frequency rate: an organisation can improve its frequency while the severity of its injuries worsens.
• ▪Stopping the count of days lost at the end of the period while the absence continues, which artificially shifts severity to the next period.

Frequently asked questions

Should commuting accidents be counted?

Conventions vary between organisations and between countries. What matters is to set the rule in your procedure, write it down, and stop changing it: a series of indicators is only worth something if it remains comparable with itself over time.

What is the difference between the frequency rate and the frequency index?

The frequency rate relates injuries to hours worked, the frequency index relates them to headcount. The first neutralises variations in working time, the second compares more easily between organisations of similar size.

Does ISO 45001 require these indicators?

No. The standard requires occupational health and safety performance to be monitored, measured, analysed and evaluated, and the chosen indicators to be relevant to the organisation. It requires neither the frequency rate, nor the severity rate, nor any particular formula: these are widely used professional conventions, not normative requirements.

Are these figures sent to HEMC?

No. The calculation runs entirely in your browser. None of the data entered is transmitted, stored or linked to your organisation.