Is pipe diameter measured interior or exterior? E.g. on screwfix.com there is a pipe 100mm is that interior...

E.g. on screwfix.com there is a pipe 100mm is that interior diameter or exterior diameter?
How to properly flash octagon gable vent?
Answers:    Standard pipe is measured on the inside diameter, (ID) and tubing is on the outside diameter (OD). Metric or SAE are measured one and the same.
Do places exist where on earth you can...
It depends upon which type of pipe you buy. If you are buying copper pipe, which is similar to copper tubing it is measured in outside diameter. Most other pipes such as black iron galvanize steel, or PVC piping measure the inside diameter.As a rule of thumb, the difference between inside diameter and outside diameter is around an eighth of an inch. So half inch copper pipe is equal size as 3/8s inch black iron pipe.
If you are installing sewer pipe you have to pay attention because the vent piping is a different thickness than the drain piping so the connector you use for the drains will not fit on the vent even though both inside measurements are the same. The companies use the vent pipe fairly than just the drain piping is because it is smaller amount expensive.