It is critically important that you truly understand any indicator that is part of your trading decision process. This includes being able to calculate it by hand. The reality is you simply won't understand the nuances and potentially downfalls of an indicator until you can get your hands dirty.
Here is a free tool that will teach you how to calculate RSI
In addition here is my video explaining RSI, how people trade it, and walking you through on how to calculate it.
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