Let's don't talk pipe sizes........
Water and gas pipe for houses is based on ID of std wall thickness pipe. PVC is something else. Pipe used in the oilfield is based on OD (which kind makes sense), but then oilfield barrels are 42 gallons instead of 55 gals like std imperial measure. So now lets change all this to metric and it would really be fun.
Drill bits in the oilfield are measured as the hole size they drill in inches. Probably 20 years ago, API (American Petroleum Institute) said they had to have the metric size printed on the box in parenthesis. No one ever knew the metric hole size.