Glossary · Monitoring

Apdex (Application Performance Index)

Apdex is a 0 to 1 score that summarizes user satisfaction with response times by classing each request as satisfied, tolerating or frustrated.

Updated October 6, 2026

Apdex (Application Performance Index) is an open standard for turning response times into a single satisfaction score between 0 and 1. You choose a target threshold T. Requests at or under T are satisfied, those between T and 4T are tolerating, and slower ones are frustrated.

Why it matters

A score is easy to put on a dashboard and to compare across services, which helps non-specialists see whether performance is acceptable. It reduces a mass of timing data to one number that can be tracked over time or alerted on.

Example

The formula is:

Apdex = (satisfied + tolerating / 2) / total samples

With T = 0.5 s and 100 requests: 80 under 0.5 s, 15 between 0.5 and 2 s, 5 slower.

Apdex = (80 + 15/2) / 100 = 0.875

A common reading is 0.94 to 1.0 excellent, 0.85 to 0.93 good, 0.70 to 0.84 fair, and lower is poor.

Common mistakes

  • Choosing T carelessly. A loose T makes the score look great while users wait.
  • Using the same T for very different operations, such as search and report export.
  • Treating the number as precise. It hides the shape of the distribution, so check percentiles as well.

Get notified when this term updates

Plus occasional practical tips on uptime and monitoring, nothing else. Unsubscribe anytime.

Keep reading

Glossary · MonitoringAPM (Application Performance Monitoring)APM tools instrument application code to measure response times, errors and dependencies so teams can find slow transactions and failing code paths.Glossary · MonitoringBaseline (Monitoring)A monitoring baseline is the normal range of a metric over time, such as typical response time or traffic, used to judge whether current values are abnormal.Glossary · MonitoringBrownoutA brownout is a partial service degradation in which a system still works but with reduced capacity, slower responses or disabled features.Guide · MonitoringDDoS Basics for Website Owners: Symptoms, Triage and ProtectionWhat a DDoS attack is, how to tell it from a legitimate traffic spike, and the basic defenses (CDN, WAF, rate limiting) plus what to tell your hosting provider.8 min readGuide · MonitoringSynthetic vs Real User Monitoring: Differences and When to Use EachCompare synthetic monitoring and real user monitoring (RUM), what each can and cannot see, when to use which, and how to get both with free options.8 min readGuide · E-commerceBlack Friday Website Readiness Checklist: 8 Weeks to Launch DayA dated checklist for getting an online store ready for Black Friday: capacity, caching, DNS, SSL, payments, monitoring, change freeze and rollback.7 min read

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.