Glossary · p99
p99 (99th percentile)
The value below which 99% of measurements fall — the worst-case excluding outliers.
If your network latency p99 is 80 ms, then 99 out of every 100 requests complete in 80 ms or less, but 1 in 100 takes longer. p99 catches the spikes that average and median hide. Standard for capacity planning, anti-cheat, and any system where worst-case matters more than typical-case. p99.9 adds another decimal place of conservatism.