Sounds like an easy question but a friend got fined for 1 day overstay even though it was only a 30 stay stay, seems like when you enter Thailand they count that as 1 day already, is this the case, or did they get it wrong?

They got it right. The arrival day counts as day 1.

So with my flights i arrive on the 6th of April and leave on the 5th of May that will be 30 days?

So with my flights i arrive on the 6th of April and leave on the 5th of May that will be 30 days?

Yes, you will be 'admitted until' May 5th (30 days) and have until midnight on the 5th to leave.

Edited January 30, 201610 yr by elviajero

I leave at 9.30pm so that's good, thanks for your help.