By:  Mike Colburn (StreamMediaGuy)

You will want to have some idea as to how much bandwidth you will use during a live webcast, webinar, or video event.  This will help you estimate costs and plan capacity. There is a fairly simple formula to calculate this.  Keep in mind that this will be an estimate and depending on your encoder, codecs, and compression methods, this number could vary.  You can use the formula to figure out how large a file will be once you encode and save your video as well.

There are 3 things we must know.
1. Video Bit Rate – this is the bit rate that you encode you video at.  If you are encoding above 1000Kbps then convert the number to Kbps don’t use Mbps.  In other words.  1Mbps=1000Kbps   2.5Mbps=2500Kbps  etc.

2. Length of the video – This can be tricky.  You don’t want to calculate the actual length, you want to estimate the amount of time each person is likely to watch.  If you are calculating for On-Demand viewing, then YES, use the whole length of the video.  But for live streams, figure about 10-20% of the actual event per viewer.  Don’t kid yourself, no one is going to watch your 4 hour antique auction in it’s entirety.  They are going to watch 30 minutes tops.

In my experience, the only types of live broadcasts which get watched in their entirety are big name concerts and church services.  Otherwise, plan on 10-20% of the length of the stream per user.

3.  Lastly, how many people will watch the event?  You may have 2 Million visitors to your site each year, or a mailing list of 500,000 but consider this when planning.   What is the event?  When is it?  Is it a concert on a Saturday night (do you think your audience is home in front of their computer Saturday night or out doing what they do then?  Oprah Winfrey got 500,000 people to watch her book club online webinar.  She has a viewing audience of 50,000,000 dedicated viewers.  This content was ONLY going to be available on the web, not TV as well.  She has about as much pull in the media as anyone anywhere.  So if you think you’re going to get 1Million people to watch your stream, think again.  (unless you’re Oprah or maybe Barack Obama).

Here’s the formula.

Bit Rate X Time (in seconds)

———————————  = Y

………………..8

Take Y and divide that number by 1000.  This will give your number for 1 Viewer watching for that amount of time represented in MegaBytes (MB)

Now multiply that number by the number of viewers you’re expecting and that will be how much bandwidth you will use.

Remember:

  • 1000MB=1GB
  • 1000GB=1TB
  • 1000TB=1PB
  • Approximately 1TB=10Mbps
  • I know 1MB is really 1024KB.  Most CDNs and ISPs will round 1 MB to 1000KB, it makes for better billing

Example: 750Kbps for 1 Hour watched by 1500 people

750 X 3600 =2700000 / 8 = 337500/1000 = 337.50 X 1500= 506250MB or 506GB

You can use this same formula to estimate how large a file will be once you encode it.  Just don’t multiply it at the end by the number of viewers.  Of course if want to estimate your Content Delivery Network (CDN) usage, then just take the file size and multiply it by the number of monthly views.  This will estimate how much you should commit to your provider.