Door measurement question

I realize this may be a very basic question, but I'm new to all of this.

I finally fixed an exterior doorway that had a screen door in it at some point ( there were marks where door hinges must have gone).

I now need to install a screen door. I looked at stock at Lowes and Home Depot and found that they had 30" doors. I measured the doorway I'm trying to complete and it is 29.5"

Are those measurements rounded off like the way a 2"x4" is?

The thing to look at is the rough opening the thing is designed to fit in.

Screen doors come in standard sizes....30, 36, with 32 and 34 less common. Virtually all screen doors you will find in a home store have and adjustable width, within a range, so they can be installed to fit your particular trim. A 30" door should work for you.

IF there is a door in the doorway, then it is a 30" door, since they do NOT make 29 1/2" doors.

If there are hinge marks on the existing frame, the old door was a wood screen door. Aluminum/vinyl screen doors are pre-hung.