How to Calculate Severity Rate: A Comprehensive Guide
In occupational health and safety, the Severity Rate is a crucial metric used to quantify the impact of workplace injuries. Unlike the incident rate, which simply counts how many accidents happened, the severity rate measures how "severe" those accidents were by looking at the amount of time lost from work.
The Severity Rate Formula
The standard formula used by OSHA (Occupational Safety and Health Administration) to calculate the severity rate is:
Severity Rate = (Total Lost Work Days / Total Employee Hours Worked) × 200,000
Understanding the Components
Total Lost Work Days: This includes the number of days employees were away from work or on restricted duty due to work-related injuries or illnesses.
Total Employee Hours Worked: This is the total number of hours actually worked by all employees. It should not include vacation, sick leave, or holidays.
200,000 Multiplier: This number represents the equivalent of 100 employees working 40 hours per week, 50 weeks per year. It provides a standard base for comparison regardless of company size.
Practical Example
Imagine a manufacturing plant with the following data for the year:
✅ Lost Work Days: 45 days
✅ Total Hours Worked: 150,000 hours
Calculation: (45 / 150,000) × 200,000 = 60.0
This means for every 100 employees, 60 work days were lost due to injury over the course of the year.
Why Track Severity Rate?
Tracking the severity rate allows safety managers to identify trends. A low incident rate with a high severity rate suggests that while accidents are rare, they are extremely dangerous when they do occur. This data is essential for insurance premium negotiations and for prioritizing safety equipment investments.
function calculateSeverityRate() {
var lostDays = document.getElementById("lostDays").value;
var totalHours = document.getElementById("totalHours").value;
var multiplier = document.getElementById("multiplier").value;
var resultDiv = document.getElementById("severityResult");
var scoreDisplay = document.getElementById("severityScore");
var interpretation = document.getElementById("severityInterpretation");
// Validation
if (lostDays === "" || totalHours === "" || totalHours <= 0) {
alert("Please enter valid numbers for both Lost Days and Total Hours Worked.");
return;
}
var days = parseFloat(lostDays);
var hours = parseFloat(totalHours);
var mult = parseFloat(multiplier);
// Severity Rate Calculation
var severityRate = (days / hours) * mult;
var roundedRate = severityRate.toFixed(2);
// Display Logic
resultDiv.style.display = "block";
scoreDisplay.innerText = roundedRate;
var status = "";
if (severityRate === 0) {
status = "Excellent! You have zero lost time days for this period.";
} else if (severityRate < 20) {
status = "This is a relatively low severity rate, suggesting injuries are minor or well-managed.";
} else if (severityRate < 70) {
status = "This is a moderate severity rate. Consider reviewing safety protocols and return-to-work programs.";
} else {
status = "This is a high severity rate. This indicates significant time lost per injury, which may require immediate safety intervention and root cause analysis.";
}
interpretation.innerHTML = "Interpretation: " + status + "This means that for every " + (mult === 200000 ? "100" : "500") + " full-time workers, your company lost approximately " + roundedRate + " days to injury or illness.";
// Smooth scroll to result
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}