The correct answer is: 1024 (but the error is so small if you use 1023, it actually doesn't matter since the reference volt will vary with a much larger margin of error than your calculation will introduce). But in case you are still wondering why not 1023, here it is. After reading many responses…

Dividing by 1023 or 1024? The final verdict on analogRead

robtillaart March 25, 2018, 3:30pm 6

... unless you are using float

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