One of the most common questions when you start to use SQL Azure is related to the choice of the level of service needed for your needs. On the cloud every wasted resource is a tangible additional cost, so it is good to chose the best service level the fits your needs, no more and no less. You can always scale it up later if needed.
The "problem" is that the level is measured in DTU - Database Transaction Units - which a value that represents a mix of CPU, memory and I / O. The problem is that it is very difficult, if not impossible, to calculate this value for an existing on-premises server, so that you can have a compare it with the performance of your well-known on-premises server.
Well, it *was* impossible. Now you can, thanks to this tool:
Azure SQL Database DTU Calculator
developed by Justin Henriksen, a Solution Architect specializing on Azure, that simplifies a lot the estimation effort. After running a PowerShell script to detect some metrics on the On-Premises Server, you have to upload the collected values n that site to get an idea of what level of DTU is optimal in case you want to move that database or server to the cloud.
Of course the more your workload is representative of a real-world scenario, the better estimates you will have: keep this in mind before taking any decision. In addition to this website, there are also two links very useful to better understand what level of service is best suited to your situation: