Is "regardless of whether or not" proper grammar?

I have a sentence like this:

I will go to the store regardless of whether or not it's raining

Meaning that the weather has no bearing on my intent to go to the store. The "regardless of whether or not" is awkward, and it feels like bad grammar, but I can't think of a better way to say it.

What would the proper grammar be in this case?

Solution 1:

The grammar is fine, but there are certainly less laboured ways to say it!

I will go to the store, whether it rains or not.

Raining or not, I'm going.

Solution 2:

"Regardless of whether or not..." is correct, but it can be shortened to "Regardless of the weather",..." but can also be improved with "Regardless of whether it rains or not..." It does seem long, but the awkwardness is negligible, in my opinion. The idiom is like "in spite of"- the "of" is necessary; unlike "despite" which does not need the 'of'. 'Regardless' can be replaced by 'heedless', which has the same meaning and is also followed by "of".

Solution 3:

There is nothing wrong with its grammar.

Its style is another matter, and you are right that it is more wordy than it strictly needs to be. If ever “omit needless words” applied, it would be here.