What Is the 95th Percentile and How It Can Apply to You

In the early days of the Internet, traffic was usually measured in megabytes (MB) or gigabytes (GB).

Most web analysis software would look at the logs from a webserver and add up all the "bytes transferred" numbers and the dates and use that information to arrive at a total amount of traffic sent by that server in a given time period.
Some colocation providers continue to measure traffic this way, but most now use the 95th percentile metric. What is 95th percentile? What does it mean and how does it work? In this article, we'll try to briefly explain 95th percentile and the way it measures traffic in today's networks.

95th Percentile is a way to measure traffic that better fits with the way communications companies build their networks.

For example, if a customer will have peaks of 900Mb/s occasionally, but an average of only 50Mb/s, the telecommunications company still has to maintain network equipment capable of handling the 900Mb/s bursts even though the average usage is much less.
95th percentile is essentially a weighted average that is a fair way to balance the costs of handling the peak load while still giving the customer some accommodation for their lower average utilization.

Notice that the numbers are followed by "Mb" (lower case "b")? That's because 95th percentile is measuring megabits per second, not megabytes.

This is what is known as a rate-based metric since it measures rate and not volume. The normal way this is implemented is to take a sample of the data rate every 5 minutes, every day, all month long.

This will result in 8,640 samples per typical 30-day month.

At the end of the month we'll take the rate at which 95% of those samples fall at or below and we'll essentially "throw out" the highest 432 samples.

Another way to think of this is that you're effectively getting about 1.

5 days of "free" time and you're billed for the rate you're using the rest of the time.
Just remember that that "1.

5 days" is comprised of 432 individual 5-minute samples that can be scattered around any given month.
Most commercial facilities such as datacenters and telecommunications companies will bill for Internet usage using the 95th percentile metric.
There will usually be two components to the pricing, the first being the "committed" rate (what you buy by contract regardless of what you actually use), and the second being "overage" (what you use above the committed rate). For example, if you have a 10Mb/s commit on a burstable 100Mb/s circuit, you have the ability to burst up to 100Mb/s but overage above the 10Mb/s committed rate will be billed on 95th percentile at a per-megabit rate. Many times the burst rate is the same as the per-megabit committed rate but sometimes it will be less or even more. Be sure to check your contract when planning for bandwidth costs!