A completely description for steel pipe dimension includes outer diameter (OD), wall thickness (WT), pipe length (Normally 20 ft 6 meter, or 40 ft 12 meters).
Through these characters we could calculate the pipe weight, how much pressure pipe could bear, and the cost per foot or per meter.
Therefore, that’s why we always need to know a right pipe size.

Steel Pipe Dimensions Chart (Size Chart)
Pipe Schedule Chart unit in mm as below, view here for Pipe Schedule Chart in inch.

ASME B36.10M
The standard covers the standardization of steel pipe dimensions and sizes. These pipes includes seamless or welded types, and applied in high or low temperature and pressures.
The pipe distinguished from tube (Pipe vs Tube), here the pipe is specially for pipeline systems, fluids (Oil and gas, water, slurry) transmissions. Use the standard of ASME B 36.10M.
In this standard, the pipe Outer Diameter smaller than 12.75 in (NPS 12, DN 300), pipe actual diameters is larger than NPS (Nominal Pipe Size)or DN (Nominal Diameter).

On the hand, for steel tube dimensions, the actual outside diameter same with pipe number for all sizes.

Why pipe size smaller to NPS 12 (DN 300) have different OD
First, we need to admit that all pipe size should be identified as nominal pipe size (NPS).

Second, for NPS 1/8 (DN 6) to NPS 12 (DN 300), these are based on a standard outside diameter (O.D.) This O.D. was originally selected to specify pipe dimension, but as the pipe always have an wall thickness and internal diameter (ID). For small pipe, there is a different size between OD and ID (Wall thickness), but as the pipe dimensions became larger, the OD and ID approximately to become equal.

As there is no such relation between the old standard thickness (O.D.) and Nominal Size, so both of them accepted by standard ASME B 36.10 M. 
Steel Pipe Schedule
In ASME B36.10M, Pipe schedule identified as Standard (STD), Ex-Standard (XS), or Double Extra Strong(XSS); Or with Schedule No. 5, 10, 20, 30, 40, 80, 120, 160.