“Fine by me” and “Fine with me” are both common ways to say that something is acceptable in English. They are usually used in spoken English as a response to someone suggesting an idea.
“Fine with me” is slightly more common than “Fine by me” but there is no real difference between them.
Fine by me
In my experience “Fine by me” is more commonly used when somebody is checking or confirming something with someone.
I just wanted to check with you that dinner is at Rafael’s tonight.
Yes, that is fine by me.
But “fine by me” can also be used in more of a general use of “it is acceptable to me”(not being prompted to answer or confirm) as you can see in the famous Friends scene with Ross and Rachel where Ross shouts “FINE BY ME”
Fine with me
“Fine with me” is more common and is another way to say something is “ok with you” and you are happy with a solution or plan.
We will get sandwiches for lunch.
Yeah, that is fine with me.
In this case, somebody is not checking with you but stating a fact and you do not need to give your permission.
Is it OK to say “it’s fine”?
Yes, it is ok to say “it’s fine” although “it’s fine” can sometimes sound a little bit passive-aggressive. This means that you are presenting that you are satisfied with the situation but in reality, you are not. It all depends on tone.
Are you happy with your new job?
What is a formal way to say “fine with me”?
If you do not want to say “fine with me” you can use the following phrases:
It is acceptable to me
I agree with your proposal.
That sounds agreeable to me(very formal).
These phrases are quite formal and not so common in spoken English. They are more common in writing.
Is “perfectly fine” grammatically correct?
Yes, “perfectly fine” is grammatically correct.
We use “perfectly fine” when someone wants to say that they are happy with the current situation and they want you to stop asking more questions.