If there is any one question that I am asked the most it is this: how much bandwidth do customers really need? Of course, what most carriers are really asking me is how fast they should make their data products. It’s a very good question.

The glib answer is that every family is different. But it is possible to talk about the kind of bandwidth that various common activities require and to make some generalizations about the average home. You might recall this is what the FCC did back when they reset the definition of broadband at 25 Mbps download. They looked at expected broadband usage for homes of two, three and four people and used that as a way to justify increasing the definition of broadband.

Since video is now the largest use of data for most home that’s the natural place to start any calculation of needed bandwidth. The problem is that there is no standard size of a transmitted video and the size of a video stream is going to depend upon the compression techniques used by the company sending the signal. Rather than a standard size of a video stream there is instead a pretty wide range.

The speed at which an ISP might see video just got a lot more complicated to predict by Netflix. They have always tried to send an HD video at 5.6 Mbps. If the customer’s broadband connection was not good enough to support that speed, then Netflix cut the quality of the video stream and sent it at a much lower rate. Netflix announced that they are now going to be more dynamic in the way they size and send video. They will send high action HD video, for example, at a higher data rate than a low-action movie. They will also have more than one option for downsizing the video so that it doesn’t have to drop all the way to SD. This means a whole array of different speeds of video from Netflix.

While video is the primary way that households currently use bandwidth, you can’t ignore all of the other uses, most of which are growing quickly. For example, there is now a significant amount of data used automatically in the background without direction from the user. Programs update automatically or constantly communicate in both directions with the cloud. One big and growing use of household data is cellphone data offloading. People tend to forget that their cellphone on WiFi is busily using their home bandwidth. There is a lot of talk in the industry of migrating away from cellphone apps and running more cellphone programs directly in the cloud, and that will mean a significant increase in cellphone data usage.

It’s also important to recognize that the Internet is not perfect and that every bit that is sent to us doesn’t arrive perfectly the first time. Depending upon the quality of the connection with the ISPs at both end of a transmission, there can be anywhere from a few to a relatively large number of bits that must be sent multiple times to complete a file download. For example, it might really require 1.2 gigabits of bandwidth to download a gigabit file. One measure of this is latency, and while not a perfect predictor of the amount of re-sent bit packets, we know that the higher the latency the more packets that must be sent multiple times.

Another thing to consider is that you can’t use every bit of your Internet connection at one time. For example, if you have a 10 Mbps connection you can’t view two 5 Mbps video streams at the same time. This is due to what the industry calls overhead, which is the background processes that enable your device to communicate with the Internet. The amounts of overhead can vary, but it’s not usual to see 10% to 20% overheads in a home network – bandwidth that is used by your router and the ISP to communicate in the background or to provide buffers between different data streams. The more things you do at the same time, the greater the overhead becomes, which to engineers is called contention.

I’ve used my own home as an example before. We are two adults and a teen who don’t have traditional cable TV. We all have cellphones and we work and play using bandwidth a lot. There have been a few times when our Internet connection from Comcast slows down. We see that anytime that it hits about 25 Mbps that we start having trouble doing things if we are all trying to use the Internet. So for our household, for right now, 25 Mbps  seems to be our magic number. But that number constantly grows and I would expect our threshold to get higher month after month as more and more parts of our lives use the web.