1.Off the top of my head
例句：Off the top of my head, this phone would be worth £300
2.Knowing him, he will be late/angry/excited
例句：Knowing Bryan, he will probably take the train, he hates driving.
3.If I had to take a guess, I’d say
例句：If I had to take a guess, I’d say he was over 40 years old
4.It’s difficult to say, but I think…..
例句：It’s difficult to say, but I think anyone can won this match now.
5.I wouldn’t be surprised if……
例句：I wouldn’t be surprised if he leaves his job, he is too stressed for the kind of work he does.
例句：I bet he’s gonna ask me out tonight, he just can’t stop staring
7.We’re talking maybe…..
例句：We’re talking maybe 40000 sq feet to build this house upon.
8.It’s somewhere in the ballpark of …
例句：She gets paid somewhere in the ballpark of 10 million dollars to host that show
9.Your guess is as good as mine
例句：When someone says to you ..”Don’t even know if Mary will show up for the wedding..” and you have no idea , you just say..”well, Your guess is as good as mine”