DN does not exactly correspond to a size in millimeters, because ISO 6708 defines it as being a dimensionless specification only indirectly related to a diameter. The ISO 6708 sizes provide a metric name for existing inch sizes, resulting in a 1:1 correlation between NPS and DN sizes. ISO 6708 does not include values for "DN 6" or "DN 8", however ASME B36.10M list the "DN 6" and "DN 8" . Also, the European Standard [https://www.techstreet.com/standards/din-en-12516-1?product_id=2027332 EN 12 516-1] (Industrial valves - Shell design strength - Part 1: Tabulation method for steel valve shells) specifies the dimensions "DN 6" and "DN 8", respectively their equivalents NPS  "and NPS ". 
Tolerance:  The tolerance on pipe OD is + (0.0156) inch (), − (0.0312) inch ().
As per [https://www.asme.org/products/codes-standards/b3610m-2018-welded-seamless-wrought-steel-pipe ASME B36.10M -2018] Pipe wall thickness are rounded to nearest , while converting wall thickness from inch to millimetre.