Is Chess.com's elo equivalent to FIDE's elo? In other words, would a player with an elo of 1800 on Chess.com have a FIDE elo of 1800 if he was FIDE?
No.
First off, chess.com doesn't have one rating. There are separate ratings for many time controls.
FIDE ratings are usually for over the board slow chess.
Secondly, chess.com doesn't use Elo. It uses Glicko. Read about it. https://www.chess.com/article/view/chess-ratings---how-they-work
Is Chess.com's elo equivalent to FIDE's elo? In other words, would a player with an elo of 1800 on Chess.com have a FIDE elo of 1800 if he was FIDE?