Capacity
Capacity is the maximum workload or number of users, transactions, or resources that a system can handle while meeting specified performance requirements.
“The degree to which the maximum limits of a component or system parameter meet requirements.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Capacity?
Capacity defines the operational limits of a system before performance becomes unacceptable. Understanding capacity helps organizations plan infrastructure and future growth.
Workload limits: Capacity may be measured in users, requests, transactions, or data volume.
Performance relationship: Capacity focuses on maximum supported load while maintaining acceptable response times.
Planning value: Capacity estimates support infrastructure and scalability decisions.
Real World Example
A cloud storage service is contracted to support up to 50,000 simultaneous file uploads during a planned product launch, per the signed SLA.
The team has stress-tested the service well past that number and confirmed it "doesn't crash," but nobody has actually confirmed whether the documented limit written into the contract — 50,000 concurrent uploads — is genuinely met with acceptable performance, as opposed to just eventually breaking somewhere higher.
The tester evaluates Capacity directly against the requirement: confirming the system sustains exactly the contracted 50,000 concurrent uploads within the agreed latency and error-rate limits, rather than simply finding wherever the system happens to break.
Testing reveals the system technically survives 50,000 uploads but with latency far outside the SLA's acceptable range — the documented parameter limit is not actually met by the required standard, a finding distinct from "does it crash," and one a stress test alone, without checking it against the specific contracted requirement, would have missed.
Practice Questions
Question 1
Which metric BEST indicates capacity?
Question 2
A service saturates CPU at 75% of expected traffic. What is BEST next action?
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