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I'm looking over some specifications for black pipe. I understand why dimensional lumber is smaller than the stated dimensions, but why would the pipe manufacturers give you more volume than what is expected?

Does anyone know the history on this?

 Size | O. D. | I. D. | Wall Thickness
==================================================================
 1/4 in | 0.540 in | 0.364 in | 0.0880 in 
 3/8 in | 0.675 in | 0.493 in | 0.0910 in 
 1/2 in | 0.840 in | 0.622 in | 0.1090 in
 3/4 in | 1.050 in | 0.824 in | 0.1130 in 
 1 in | 1.315 in | 1.049 in | 0.1330 in
 1.25 in | 1.660 in | 1.380 in | 0.1400 in 
 1.50 in | 1.900 in | 1.610 in | 0.1450 in

...and so on.....

1 inch pipe used to have an inside diameter of 1 inch.

As metallurgy improved, the steels used to make the pipe got stronger and you therefore needed less metal for the same size at the same strength. So a manufacturer can sell a lighter pipe for the same price and make more profit.

As the fittings attached to the outside, the OD had to stay the same for compatibility with the old stuff. The manufacturers fiddled with the wall and the matching of the actual measurements with the named size was abandoned just as it had been at the sawmill.

Schedule 80 has thicker walls. The OD is the same for both 40 and 80, but the IDs for 80 is the same as the pipe size. Thus, 1/4" Schedule 80 has an ID of .25".

Schedule 80 has higher pressure and tensile strength ratings because of the thicker walls.

In US , ANSI refers back to ASTM A 530 : this is the specification that defines all pipe dimensions. It has nothing to do with strength ( or anything else) , strength is defined in the specific pipe specifications such as ' A53 , A120, A106 and a hundred more. Pipe is made so it will thread together , So every piece of one inch pipe threads into every other piece , Sch 10, Sch 40, Sch 240 etc, or stainless or copper, etc, all can thread together. Threading together has nothing to do with the ID ; the schedule defines the wall thickness which determines the ID. And "one inch" is the name of a size of pipe, it does not mean that any dimension is actually one inch. "Black iron" refers to a dark mill varnish ( may be a derivative of tar and/or asphalt ), the steel mill applies to pipe to prevent rust while in storage ; nobody wants to buy new pipe and have it delivered covered with rust. Mill varnish is not put on galvanized, stainless, etc. API also has a pipe specification with significant differences : minor variations in thread dimensions from ASTM A 530 , A very wide range of wall thicknesses , and a very wide range of strengths and some chemical restrictions for welding considerations and comprehensive quality provisions. API also has specifications for casing and tubing which are used exclusively in the "oil patch". This answer may have been a bit long but having been a voting member on ASTM A-1, committees 9 and 10 ( pipe and tube) and later on API Committee 5 ( line pipe . casing and tubing) I had to put in my 2 cents