Pipe sizes can be pretty confusing. That's because sometimes people talk about the outside diameter (OD) while others talk about the inside diameter (ID). To overcome this confusion, the manufacturers introduced a sizing system that gives each size a name instead of a measurement. So pipes are given a 'Nominal Bore' size (NB). Here's how it works - every NB size gives you only a rough idea of the inside measurement. It's only rough because it depends on the wall thickness - it's the OD that stays constant.
For example, pipe that was once called 1" is now called 25NB. This size will always have an OD of 33.7mm, and comes in a range of different wall thicknesses (WT). If it has a 'Medium' WT (3.2mm), then the actual ID will be 33.7 - 3.2 - 3.2 = 27.3mm