How is scalability testing different from capacity testing?

A web search of the terms resulted in,

Scalability Testing, is the testing of a software application to measure its capability to scale up or scale out in terms of any of its non-functional capability. Performance, scalability and reliability testing are usually grouped together by software quality analysts. The main goals of scalability testing are to determine the user limit for the web application and ensure end user experience, under a high load, is not compromised. One example is if a web page can be accessed in a timely fashion with a limited delay in response. Another goal is to check if the server can cope i.e. Will the server crash if it is under a heavy load? 1

Depending on the application that is being tested, different parameters are tested. If a web page is being tested, the highest possible number of simultaneous users would be tested. 2 Also dependent on the application being tested is the attributes that are tested - these can include CPU usage, network usage or user experience.

Successful testing will project most of the issues which could be related to the network, database or hardware/software. Capacity testing: To determine how many users and/or transactions a given system will support and still meet performance goals. Capacity testing is conducted in conjunction with capacity planning, which you use to plan for future growth, such as an increased user base or increased volume of data. For example, to accommodate future loads, you need to know how many additional resources (such as processor capacity, memory usage, disk capacity, or network bandwidth) are necessary to support future usage levels.

Capacity testing helps you to identify a scaling strategy in order to determine whether you should scale up or scale out. People in different localities have different understanding of terminologies. Hence you should try to find out what your team or organisation thinks of these to be. For me they may be used interchangeably!