“Whether” is a useful word to distinguish between two different scenarios, categories and more. But using “or not” with it can be redundant and a waste of space. 

For example: 

  • I wonder whether it will rain much this monsoon season. 
  • I wonder whether or not it will rain much this monsoon season.

In the above examples, we don’t need the “or not.” That’s because the clause is the object of the verb “wonder.” 

You also don’t need “or not” when: 

  • The clause is the object of a preposition: The judge will base her decision on whether aggravating circumstances outweigh mitigating circumstances.
  • The clause is the subject of the sentence: Whether Vanessa Nygaard will stay head coach of the Phoenix Mercury will depend on a decision by upper management.

You do need “or not” when the clause modifies a verb: They will perform Rent tomorrow whether or not the director is over her bout of food poisoning.