You wouldn't have a fide rating. They start at 1400
How do you calculate your fide rating based on your chess.com rating
There is surely some formula.
I want to know approximately. I'm looking for an answer like this
400 + 20x years you played + 0.5 times hours you study
You have to create a list of questions, to invite many players with both ratings, to hold a poll, to collect the data, to conduct some multidimensional nonlinear regression analysis, to fix the list of questions, to hold the poll again... Until you look like this:

So a fide rating is “normally” anywhere within 150-300 points lower than your chesscom rapid. Unfortunately, this calculation doesn’t really factor in that players under upper-intermediate would even want to know what there fide is - as we don’t usually expect players below this rating to attend even nationally rated tournaments, let alone FIDE rated ones…. I’m around 2000 on chesscom, slightly above right now, and I’m usually one of the weaker players at ECF tournaments :/
Blitz chess.com ROUGHLY minus 500points , that's your FIDE rating
Rapid chess.com ROUGHLY minus 600, 700 points , that's your FIDE rating.
That will show your FIDE classical ROUGHLY rating strength.
Of course it varies and can be completely different, but if you never played FIDE that's some information you can at least have some idea on .
Also 1400 FIDE is a bare minimum rating ! And before that you pretty much don't exist!
It's difficult to compare, since the player pools are completely different, but your chess.com rapid rating minus about 500 or 600 points is probably a decent estimate of FIDE rating as @ESP-918 says.
For some people it's "only" a few hundred points lower. It depends on many factors.
That said, FIDE ratings aren't common where I live in the USA because most people here have USCF Ratings instead and FIDE only for international events. I'm currently about 2100 rapid on chess.com now and about 1700 USCF rating now. Ironically, I was about 150 points higher in USCF back when I was only about 2000 rapid, but that's probably because of not having gone to many OTB rated events lately which have helped my rating, so I'll probably have a while before I go back up again.
Another thing to consider is USCF tends to be 100 elo higher than fide ratings
ECF (if you are English op) is about the same as fide ratings
There is surely some formula.
I want to know approximately. I'm looking for an answer like this
400 + 20x years you played + 0.5 times hours you study
Someone check my maths for me please
I've been playing 2.5 years at about 4 hours a day
400 + (20*2.5 = 50) = 450
4/2 = 2
450*2 = 900 rated? Is that right? 😂
My rapid is 655