When it comes to steel tubing sizes, hollow structural sections (HSS) and standard pipe use different sizing conventions. Understanding how each product is sized can help prevent inaccurate specifications, ordering, and fit-up issues. Hollow structural sections and pipe piling use actual outside dimensions for sizing. Standard pipe is designated by nominal pipe size (NPS), which does not correspond directly to any actual pipe dimension. As a result, specifiers should verify dimensions carefully when transitioning between HSS and pipe products.
How Steel Tubing Sizes Are Defined Across Product Categories
The American Society for Testing and Materials (ASTM) A500 specification covers cold-formed hollow structural sections (HSS) in square, rectangular, and round profiles. Square and rectangular HSS, for example, use the actual outside dimensions of the section for sizing. A 4x4x1/4 HSS has an actual outside dimension of 4 inches on each face. Similarly, round HSS uses the actual outside diameter as its size designation. Standard pipe, however, uses a different system entirely.
Nominal pipe size (NPS) is a legacy designation that does not correspond to any actual dimension on the pipe. In contrast, a 2-inch NPS pipe has an actual outside diameter of 2.375 inches. That distinction matters when connecting tubing to pipe fittings, designing penetrations, or coordinating with mechanical trades. A side-by-side reference covering both HSS and standard pipe makes those distinctions clear before engineering contract documents go out to bid causing problems on site.
Size Ranges by Product Type
Square HSS specified to ASTM A500 Grade C has outside dimensions which range from 0.5 inches to 22 inches, with wall thicknesses from 0.065 inches to 1 inch. Rectangular HSS covers an equivalent range of width and depth combinations within those same wall thickness limits. Bull Moose Tube provides round HSS runs from 0.84 inches to 18 inches in outside diameter, with wall thicknesses up to 3/4” inch.
Wall Thickness, Tolerances, and What They Mean for Design
Specifying the appropriate wall thickness is important because manufacturing tolerances can affect the actual dimensions available for structural design. Specifically, ASTM A500 allows a wall thickness tolerance of plus or minus 10 percent of the nominal value. Hence, manufacturers have produced HSS with a design thickness of up to 10% less than the nominal thickness required by the standard. This reduction led to recommendations made jointly between AISC and the Steel Tube Institute (STI), leading to provisions (AISC 360) requiring a reduction in the nominal thickness of all HSS members by 7% (for a design wall thickness of 0.93 times the nominal thickness) for section property calculations.
The A500 section properties resource provided from the Steel Tube Institute, documents the full size range alongside the tolerance and design thickness information engineers need. A relatively new premium HSS specification, ASTM A1085, tightens up this wall thickness tolerance to -5% and adds a new mass tolerance of -3.5%. These tighter restrictions better align HSS tolerances with other structural members and eliminate the need for the 0.93 factor in calculations. These improvements result in a more efficient HSS design since the actual thickness stays reliably close to the nominal value. Therefore, A1085 removes that calculation step and simplifies the design process where section property consistency matters.
Bull Moose Tube produces HSS to both A500 and A1085 at their Domestic manufacturing facilities nationwide. Section property data for all available sizes is in Bull Moose's technical documentation. Those section properties cover area, weight, moment of inertia, section modulus, surface area and radius of gyration.
Contact us to learn more about available steel tubing sizes, specs, or delivery options for your next project. Our Sales and Technical team will be happy to help and collaborate.