What is the 95th percentile?

The 95th percentile is a method for metering bandwidth that allows a customer to burst over their Committed Information Rate (CIR). Unlike a fixed network link, a customer is able subscribe to a CIR at a fraction (usually 10-20%) of the interface speed, but when necessary burst above and even consume the entire interface. The 95th percentile is an alternative to either fixed or GB transfered billing methods and is ideal for datacenter applications.

Every five minutes the network interface is sampled for the total amount of bytes transfered. This is averaged over 300 seconds to estimate the average transfer rate per second. These averages are collected every five minutes and stored in a database. When the month is over, the samples are arranged from highest to lowest and the top 5% of samples are removed. The next highest sample is the 95th percentile.

A 30 day month has 36 hours of free peak traffic.

30 days * 24 hours * 5% = 36 hours

The 95th percentile is not a mean or an average, its represents the value that your bandwidth is at or under 95% of the time. To avoid bursting charges you can think of this one of two ways:

  1. 95% of the time the usage should be at or below the CIR, or
  2. 5% of the time the usage can be over the CIR.

If you are looking for more information, I'd recommend you start with Wikipedia's article on Burstable Billing.