Sorry I mean to say nut...

So like a 10mm X 1.25 describes the inside diameter of the
hole and the 1.25 describes the thread( I think) ...

I'm taking off the struts on a civic, the manual it says a 10M X 1.25 lock nut,

But I use a 14mm socket to get it off the bolt. Is there a way
I can calculate this, or is it a trial and error thing?