Fan bolt size

[Rounded_nut]

I'm trying to change my radiator fan but can't seem to get the right size socket on it, anyone have a second to see what size it is? I ask because I don't apparently have the right size, not sure if metric or standard. Thanks!

*update*
It's 9/16, turns out with my arm twisted around in there my perception of straight was way off and I was trying to fit the socket at an angle. Oh well, new fan is on now.