When planning for Exchange Online, you need to estimate the Exchange Online network bandwidth that your organization is going to need. To do this, you'll need to make some assumptions about your user base. For example, you'll need to make a determination of what the average email size is that your users send and receive. A common estimate for this metric is 50 kilobytes. You'll also need to make some assumptions regarding the number of messages that are actually read. For this assumption, organizations will typically assume every message is read. At the same time, you should think about how much mail is deleted on a regular basis. Using a ratio of 50% isn't uncommon. You'll also want to estimate how many OWA users there are, and how often those users log on and log off of OWA every day. 

As you can see, there is actually quite a bit that goes into bandwidth estimation for Exchange Online. The table that you see on your screen shows what typical estimates would look like when planning for Exchange Online bandwidth. It breaks things out by light, medium, heavy, and very heavy email users. It is this type of information that you need to gather when planning bandwidth. 

This table here shows an estimated amount of network traffic that each type of email user generates. To use this information in your bandwidth planning, consider the following example: Assume that your organization employs 50 heavy Outlook users. To calculate the average network traffic that they generate, use the formula that you see on your screen. Essentially, this formula is network bytes per second equals the number of users times the bandwidth usage divided by 28.8k. 28.8k is the number of seconds in an eight hour day. Bandwidth usage refers to the bandwidth used by the type of user you're estimating for. Number of users is the number of users that you're planning for. Using this formula, we can determine that 50 heavy users will require nine kilobytes per second because 50 heavy users times 5,200 kilobytes per user, per day times 28,800 seconds per day equals 9.027 kilobytes per second. 

Now, if you assume daily peak activity of twice that, you should really be budgeting for 18 kilobytes per second for 50 heavy Outlook users. 

Phew, that's quite a bit of math, but, it's necessary when trying to determine the necessary bandwidth before deploying Exchange Online.