Sizing Migrations

As mentioned in the article, How long does a migration take, it can be difficult to accurately predict how long a migration will take, due to several factors, such as system load, bandwidth, average item size, and Source or Destination bottlenecks.​

However, here is a rough breakdown of what speeds you can expect. Note: this is by no means an SLA, but just an example based on a very large sample of migration speeds experienced by our Partners.

  • Low-end: 250MB per hr/per user
  • Mid-end: 750MB per hr/per user
  • High-end: 1.25GB per hr/per user

If you want a predictable timeline of how long your migration will take, we recommend sizing your migration before it happens. This is particularly important for large migration projects.

Sizing your migration consists of performing a proof of concept (POC) migration ahead of time, and using the statistics generated to extrapolate duration and speed. This POC migration should be performed against a few larger mailboxes.

  • Once the POC is completed, you should delete the items from the Destination, and reset statistics. Here is our KB on how to reset statistics Reset Item Statistics.
  • For large migration projects, you should reach out to your BitTitan sales representative. They can provide you with some free licenses for use in your POC.

To size your migration, perform the following steps:

  1. Perform a POC. This should include the migration of three (3) larger-sized mailboxes. To perform this POC, follow the steps in the migration guide that matches your scenario. Migration guides can be found here. Note: We recommend that you perform a single Full migration pass, rather than following a Pre-Stage migration strategy. Important: The migration speeds seen during your POC may vary from your actual migration speeds, because many factors, such as available bandwidth and server resources at the time of migration, may be different between migrations. More factors that determine migration speed are discussed in How long does a migration take.
  2. After the migration completes, view the statistics for the mailbox and make note of the lower data speed (the lower data speed of the Source or Destination). The lower of the two speeds is the rate in which the migration is moving.  In the example in the table below, the lowest migration speeds are at the Source. Therefore, the average mailbox speed will be the average value of the Source speeds. In the example below, this average speed = 600 MB/hr.


​Mailbox Name ​Source speed MB/hr ​Destination speed MB/hr
​mailbox 1 ​500 ​700
​mailbox 2 ​600 ​800
​mailbox 3 ​700 ​900

Once you have completed the steps above, you will have some accurate data to help you determine how long your migration project will take to complete.

To help you understand how to predict this, here is an example project:

Project specifics:

Source: On-Premises Exchange 2010

Destination: Office 365

Average mailbox size: 3GB

Number of mailboxes: 500

Preparation steps: Create an Admin account on Source, and throttling disabled against this account

Migration strategy: pre-stage strategy 




  • Say 90% of their data is in mail items older than 90 days. Then 90% of 3GB = 2.7GB.
  • Let's say that on average they get migration speeds of 500MB per user per hour. BitTitan can only migrate as fast as the speeds that the Source and Destinations allow. For this example, a lower-than-average speed has been used.
  • Also, they are leaving the Advanced Option of maximum concurrent mailboxes (under the Performance section) at 100. You should lower or increase this number based on Source server resources and upload bandwidth speeds.

Pre-Stage time for completion:

Each mailbox should take 5.4 hours to complete. 100 get completed in the first 5.4 hours. Then another 100, and so on. Remember that when the first one completes, then the next is started so they don't all wait until all others in the 100 batch have completed. However, to try to estimate:

500/100= 5.

Therefore 5.4 x 5 = 27

27 hours / 24 hours = 1.125 days for Pre-Stage to complete, if leaving migrations going on all 24 hours (some Partners like to run migrations only at night so as not to overload the server).

Full/Delta pass:


0.3GB left per mailbox.

Full/Delta Migration calculation:

Therefore at 500mb per hour per user, each mailbox will complete in 36 minutes. First 100 in 36 minutes. Multiply by 5 to get 500 mailboxes migrated = 180 mins. 180/60 = 3 hours for Delta/Full pass to complete.


We highly recommend that you allow extra time for your Pre-Stage migration pass to complete. Therefore, in this scenario, we recommend that you start the Pre-Stage at least one week prior to MX record cutover.

We also recommend that you create your MigrationWiz project, set the Advanced Options and import users into the project early (at least two weeks prior to MX record) and then run Verify Credentials and Trial Migrations (these do not require licenses). If these succeed, you can be very confident that your Pre-Stage and subsequent Full migration (post-MX record cutover) will work.

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