Load Testing measures the application under test under maximum load at which the system would crash. The main purpose of load testing is to identify the peak point after which the users would not be able to use the system. Both Load and Scalability come under the Performance Testing methodology.

Scalability differs from Load Testing in the fact that scalability test measures the system at the minimum and maximum loads at all levels, including the software, hardware, and database levels. Once the maximum load is found out, developers need to respond appropriately to ensure that the system is scalable after a particular load.

Example: If scalability testing determines the maximum load to be 10,000 users, then for the system to be scalable, developers need to take measures on factors such as decreasing response time after the 10,000 user limit is reached or increasing the RAM size to accommodate the growing user data.

Load Testing involves placing a maximum load on the developed applications at one go, while scalability testing involves gradually increasing the load over a period of time progressively. Load testing determines the point at which the application crashes, while scalability tries to identify the reason for the application crash and take steps to resolve the issue. In short, Load Testing helps to identify the performance problems while scalability testing helps to identify if the system can scale up to the growing number of users.