Capacity planning involves aligning infrastructure requirements with business goals e.g. as of today your current system gracefully handles 100 orders per hour, business estimates 20% increase in orders per hour going forward, can your existing infrastructure handle the 20% increase in order transactions?
There are two main methods of determining current capacity of the system:
1) Benchmarking – Conducting load tests with different volumes to identify current capacity. This also involves Scalability testing to find out the impact on capacity with horizontal and vertical scaling.
2) Forecasting – Historical data is used to determine the current capacity and plan for future requirements. Data is looked upon at from various forecasting algorithms.
Tools:
Popular performance testing tools like Performance Center, JMeter can be used for capacity testing and planning using the method of Benchmarking.
The only tool I have known that uses Forecasting for Capacity planning is BMC TCO (Truesight Capacity Optimization). But if you have historical data for your environment and are aware of Forecasting algorithms, this can be done using an excel sheet as well.