Standard Diameter to wall thickness Ratio - The "SDR" number is a ratio between the diameter and wall thickness. e.g. If you have a 250mm pipe which is SDR 17.6 then the wall thickness is 250/17.6 = 14.2mm. So the bore of the pipe is 250- 14.2mm -14.2mm= 221.6mm.
The outside diameter is usually controlled closely, it's the inside diameter that normally fluctuates within a manufacturing tolerance.