Burstable Billing allows you to use more than your agreed bandwidth without the financial penalty of purchasing a higher Committed Information Rate (CIR) from an ISP.

Usage charges are generally based upon the 95th percentile which effectively filters the peaks from your usage profile. Using this approach means that you could burst to full capacity continuously for up to 65 min per day with no financial penalty.

Most ISPs use 5 minute sampling and 95% utilization when calculating the burstable rate.

95th Percentile

The 95th percentile is a widely used mathematical calculation to evaluate the regular and sustained utilisation of a network pipe. It is commonly used among bandwidth providers for both capacity planning and/or calculating metered use and 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 any financial penalty. 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 two 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.) transfered divided by the sampling interval length in seconds(effectively representing the average utilisation for single sampling interval). In most cases, the throughput is measured every 5 minutes(i.e. the sampling interval is 5 min) and these samples are used as the data set for the daily percentile calculation.

How is the burstable rate actually calculated?

The ISP records your throughput every five minutes. Over a 24 hour period this represents 288 readings. Once sorted into ascending order the 274th reading is the 95th percentile value.

Provided 95th Percentile value is less than your purchased rate there is no additional charge.

What this means is that you could run at the full burst value for 65 min a day with no penalty.