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How Defect Rate is Calculated

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A defect rate is the percentage of output that fails to meet a quality target. Defect rates can be used to evaluate and control programs, projects, production, services and processes.


A defect rate is calculated by testing output for non-compliances to a quality target. Quality is typically specified by functional and non-functional requirements. The following formula can be used to calculate defect rate.
defect rate = (defects / output tested) x 100
Defects is the number of items that failed quality tests. Output tested is the total number of tests conducted.


A bicycle production line tests every unit for defects. In a week, 4000 bicycles are produced and 3 fail quality tests.
defect rate = (3 / 4000) × 100 = 0.075%
Product lines may calculate multiple defect rates based on different levels of testing. For example, all bicycles may undergo standard quality testing with a small percentage randomly selected for more detailed testing. This results in a standard defect rate and detailed defect rate.


Software testing tests 500,000 lines of code and discovers 33 defects.
defect rate = (33/500000) × 100 = 0.0066%
Code defects are also commonly measured as defects per thousand lines of code. This can be calculated with the formula:
defects per thousand lines of code = defects / (lines of code /1000)
In the example above this results in a defects per thousand lines of code of:
defects per thousand lines of code = 33 / (500000 / 1000) = 0.066

Use Cases

Software testing tests 33 use cases and discovers 4 defects.
defect rate = (4/33) × 100 = 12.12%
It is common to calculate defect rate according to the number of user stories, use cases, requirements or function points that are tested.


Services may be tested for quality control with sampling of service interactions. For example, a call center evaluates 400 calls in a week for customer service quality and finds that 4 calls fail to meet quality standards.
defect rate = (4/400) × 100 = 1%
Automated services can also be tested for quality control purposes. For example, a website is tested for speed and errors 1000 times a day with 3 defects detected.
defect rate = (3/1000) × 100 = 0.3%
Overview: Defect Rate
The percentage of output that fails to meet a quality target.
Related Concepts


This is the complete list of articles we have written about quality.
Conformance Quality
Cost Of Poor Quality
Credence Quality
Critical To Customer
Critical To Quality
Customer Satisfaction
Data Quality
Defect Density
Defect Rate
Defensive Design
Design Quality
Document Quality
Ease Of Use
Experience Quality
Figure Of Merit
Fit For Purpose
Information Quality
Non-Functional Testing
Operational Testing
Perceived Value
Planned Obsolescence
Product Quality
Quality Assurance
Quality Control
Quality Culture
Quality Examples
Quality Improvement
Quality Management
Quality Metrics
Quality Objectives
Quality Of Life
Quality Policy
Quality Requirements
Quality Testing
Recovery Paradox
Regression Testing
Reliability Engineering
Service Quality
Types of Quality
Acceptance Testing
Work Quality
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