Pipe and tube are both round and hollow, and they are not interchangeable. The difference is in how each is specified. Pipe is sized by a nominal bore that refers to what flows through it, with the outside diameter fixed and the wall varied to suit. Tube is sized by its outside diameter and its wall, which together define both the outside and the bore.
That single difference decides which one a designer reaches for. If the requirement is flow through a nominal passage, pipe makes sense. If the requirement is a specific outside dimension that has to fit inside something else, tube makes sense, because its outside is the controlled number.
Wall is the number that governs almost everything else. It sets how much heat the part can take before it distorts, how much a clamp can squeeze it before it marks, and how much material there is to weld to. On a fabricated assembly the wall is often chosen by the welder rather than the engineer, because a wall that is too thin for the process will fail at the joint regardless of the design.
Thicker wall is not automatically better. It carries more heat away from the weld pool, needs more current to fuse, and pulls harder as it cools. On thin-wall titanium the whole technique is built around getting in and out before the wall overheats, and adding wall to solve a welding problem often makes the distortion worse rather than better.
Tube is made to a controlled outside diameter and a controlled wall, and both are held to a tolerance that the buyer can specify. Pipe is made to a nominal bore with the wall defined by a schedule, and the actual bore can vary within the allowance. Neither is more accurate in general; they control different dimensions.
This matters when a part has to slip inside another, or pass through a hole, or sit in a clamp. In those cases the outside diameter is the functional dimension and tube is the sensible choice. Where the part has to mate with a threaded fitting or a standard flange, the pipe dimension is what the fitting expects.
Exhaust systems are built from tube because the outside diameter is what has to fit. A system runs close to bodywork, subframes and heat shields, and the clearance that matters is measured on the outside of the pipe. Wall is then chosen thin enough to keep weight down and thick enough to weld reliably.
Tube also bends predictably, which is the other half of the reason. A mandrel bender works on the outside diameter and the wall together, and the resulting bend keeps its section. That predictability is what allows a multi-bend system to be designed on a screen and built to match, and it is why the bent tube assemblies on this site can be repeated.
Pipe comes into its own where the connection is standardised: a threaded port, a flanged joint, a boss welded into a tank. In those places the fitting defines the interface and the pipe dimension is simply what matches it. Fabricators who build tanks, catch cans and fluid hardware work in pipe dimensions for exactly that reason.
Pipe is also convenient where the bore matters more than the outside, such as a drain or a feed line where a minimum passage has to be guaranteed. In that case the nominal bore is the requirement and the wall is whatever the schedule provides.
Joining pipe to tube is a normal fabrication problem and it is almost always solved with a transition piece rather than by forcing a fit. The two products rarely share an outside diameter, so a short adapter turned or formed to suit both is the clean answer, and it gives the welder a joint with matched wall on each side.
Where an adapter is not possible, the joint has to be designed around the mismatch: a sleeve, a flange, or a socket. What does not work is closing a gap with filler alone. A joint with a large, uneven gap at the root is where lack of fusion starts, and no amount of cap pass hides it.
A wall callout without a tolerance is an invitation to argue. The drawing should say whether the number is nominal, minimum or subject to a percentage, because the difference changes what the supplier can deliver and what the inspector should accept. It should also say whether the wall is measured on the straight or at the bend.
Bending thins the outside of the bend and thickens the inside, so a wall measured at the extrados of a tight bend is not the wall that was ordered. If the drawing is silent on where to measure, the measurement will be taken wherever it is convenient, which is usually the thinnest point.
The wall decides how much heat the joint can absorb, so the welding procedure follows the wall rather than the other way around. A joint between a thin tube and a thick flange pulls heat into the flange and starves the tube, which is why those joints are usually biased toward the heavier side and why a heat sink is sometimes added behind the thin member.
Getting this right is what allows a mixed-wall assembly to be welded without cracking at the toe. It is also why the stainless TIG welding pages treat wall as the first input to the procedure rather than a detail on the drawing.
The product type, the controlled dimension and the wall tolerance decide what can be built, and all three need to be on the enquiry.
Most of the confusion around pipe and tube disappears once the functional dimension is named. Decide whether the outside has to fit or the bore has to flow, and the product follows. The exhaust pipe range is built to outside dimensions, the cut and formed sections show how wall behaves at a joint, and the fabricated components pages cover mixed-wall work. Wall selection on thin material is set out under stainless welding, and the customization process starts from the drawing or sample that carries these callouts.
