Comcast: How fast is 10 Mbps?

My Current Comcast Connection Speed

Measured Every 30 Minutes

Table of Contents


When we moved to our new apartment in San Francisco, I found that DSL wasn't an option. Desperate for a net connection, I held my nose and purchased 10 Mbps cable internet from Comcast. I'd heard that Cable companies oversold their capacity, but how bad could it be?

At 7pm on a Thursday, running a quick test demonstrated a download speed of 700 Kilobits / second -- that's only 7 percent of the 10 Mbps that Comcast advertised.

So, I decided to apply some scientific rigor to my conundrum: How fast is 10 megabits/second in Comcast's universe?


The speed of my internet connection is inversely proportional to the probability of my using it.

In other words, when everyone is either sleeping or at work, my internet service will function as as advertised.


To move my evidence beyond the anecdotal, I need to regularly record the current download capacity of my internet connection, even when I am not at home. To ensure the accuracy of the results, the software must take into account any competing downloads by measuring total traffic received, rather than the speed of an individual download.

With this in mind, I wrote a small piece of software to run on my home FreeBSD router. It fetches a 5 megabyte file every 30 minutes, and determines the current download speed by measuring the actual bytes received on the external interface. Thus, simultaneously downloading Trading Places from the iTunes Music Store will not grossly affect the test results. Additionally, the "control" file is hosted on a network I control, with guaranteed adequate bandwidth for this test.

In addition to measuring my own bandwidth, Jonathan is measuring the bandwidth of his 6 Mbps AT&T DSL line. For comparison with Comcast, I'm automatically fetching and grinding his connection statistics.

Live Data

Past 24 Hours

Past 7 Days

Past 30 Days

The light blue line signifies the ISP's advertised bandwidth. The orange line shows actual measured bandwidth.

Bandwidth measurements are taken every 30 minutes.


There's currently insufficient data to draw any final conclusions. However, bandwidth does appear to be significantly reduced during the peak evening hours. The experiment is ongoing -- check back soon!