More ordering mistakes are made on tube than on any other stock form, and nearly all of them come from one confusion: tube and pipe are sized by completely different conventions, and the same nominal number means different physical dimensions in each.
Independent reference. This is an independent, non-commercial reference on metal forms, grades and mill processes. It sells nothing, quotes nothing and takes no orders, and it is not affiliated with any metal supplier, distributor or mill. See about this reference.
Tube versus pipe
Tube is specified by its actual outside dimension and its wall thickness. A 1 inch round tube measures 1.000 inch across the outside. A 2 x 2 x 1/8 square tube measures two inches across the flats with an eighth-inch wall. Change the wall and the outside dimension does not move; the bore does.
Pipe is specified by nominal pipe size and schedule. The nominal size is a historical label rather than a measurement — 1-1/4 inch schedule 40 pipe has an outside diameter of 1.660 inch — and the schedule number sets the wall. The outside diameter is held constant across all schedules of a given nominal size, so heavier schedules eat into the bore. This is deliberate: it lets fittings and threads stay interchangeable regardless of wall.
The practical consequence is that a 1 inch tube and a 1 inch pipe are not interchangeable, will not fit the same clamps, and will not slide inside one another. Specify which you mean, every time.
The tube categories
- Ornamental tube — welded, polished on the outside, made to a cosmetic standard. Handrail, guardrail, display fixtures, furniture. The weld is sound but the wall tolerance and mechanical properties are not certified for structural use.
- Structural tube — welded, made to a structural specification with certified mechanical properties, supplied unpolished. Frames, supports, load-carrying members.
- Mechanical tube — made to close dimensional tolerances for machining and assembly rather than for appearance or for a structural code.
- Sanitary tube — made to a hygienic standard with a controlled interior surface finish, for dairy, brewing, food and pharmaceutical process lines. Sized by outside diameter with a light wall.
- Seamless tube — extruded or drawn over a mandrel with no weld seam at all; see tube drawing. Specified for high pressure and for applications where a weld line is unacceptable. The equivalent carbon-steel product is discussed in the DOM tubing note.
Wall thickness
Stainless tube wall is quoted either as a decimal or as a gauge number, and the gauge series here is again its own. Thin-wall ornamental tube commonly runs 16, 18 or 20 gauge. As a feel for it: a handrail in 16 gauge is solid under the hand, and the same rail in 20 gauge will dent if it is leaned on hard. Wall thickness, not diameter, is what usually decides whether a tube assembly feels cheap.
Grades
304 covers general interior and sheltered exterior use. 316 is the specification wherever chloride is present — coastal handrail, pool enclosures, marine fittings — and the extra cost is small next to the cost of replacing pitted rail. 409 and 439 appear in exhaust tubing, where the requirement is heat resistance rather than appearance. Grade selection is covered on the stainless steel page.
Cutting and joining
Stainless tube is best cut with an abrasive-free method — a fine-tooth bandsaw or a cold saw — since a chop saw's abrasive disc embeds particles that later rust. Cutting always leaves an internal burr, which matters on anything carrying fluid and matters aesthetically on anything a hand can reach into.
Welded tube joints in handrail are normally ground flush and re-grained to match the surrounding finish, which is skilled work; on a polished No. 4 rail the join should be invisible. For fluid systems, orbital welding with an internal purge is the standard method, because an unpurged weld root oxidises into a rough, corrosion-prone interior surface that cannot be reached afterwards.
Ordering checklist
- Tube or pipe?
- Outside dimension (or nominal size and schedule, for pipe)
- Wall thickness, as a decimal
- Grade — 304, 316 or other
- Category — ornamental, structural, mechanical or sanitary
- Exterior finish, if it will be seen
- Length, and whether ends must be square-cut