Rod Guide question

Is there any way to actually measure a guide to get the size. Have a friend with a busted guide and he is wanting to replace it himself. Pretty sure it is an Alconite guide but how to you determine the size? It is on a 7' casting rod, third guide down from the tip.

Measure the iside diameter of the guide in millimeters. It's probably a No. 6 or No. 7. 6mm or 7mm

I was hoping it was a simple measurement. Thanks

yeah it's probably a 6 o 7 that is the usual suspect at that point of any rod.