Originally Posted by JuiceMonkey

The 95th percentile is a widely used mathematical calculation to evaluate the regular and sustained utilization of a network connection. It is commonly used among all major internet transit and peering networks, as well as datacenters and ISPs for both capacity planning and/or calculating usage. It roughly means ?for most of the time this was the throughput on the line?.

The 95th percentile is a good number to use for billing as it can allow the customer throughput bursts without additional financial compensation. Basically the 95th percentile says that 95% of the time, the usage is below this amount. Conversely, 5% of the time, usage is above that amount.

There are important factors to percentile calculation:

Sampling interval, or how often samples are taken (called also "data points").

A percentile is calculated on some set of data points.
Every data point represents the average bandwidth used through the sampling interval, calculated as the number of bytes (or KB/MB/GB etc.) transferred divided by the sampling interval length in seconds (effectively representing the average utilization for single sampling interval). The number is expressed in a data transfer rate as bits per second (kbit/s/Mbit/s/Gbit/s).