Rectangular tube corner radius is the rounded arc where two flat faces of the section meet. It is described by two values: the outside corner radius (Ro), measured on the outer surface, and the inside corner radius (Ri), measured on the inner wall. Wall thickness locks the two together, because Ri = Ro − t.
That single dimension decides whether your tube seats in a fitting, slides inside another section, or leaves a visible gap on an exposed rail. Yet it appears on no size chart.
A fabricator in Ghent found that out last spring. His 120 x 60 x 3.0 mm stainless sections met every dimension on the drawing. The machined corner brackets did not close. The gap was under two millimetres, invisible on a spec sheet, and enough to stall a finished balustrade run for a week of grinding and weld repair.
This guide prevents that. We form, weld, and finish stainless rectangular sections on our own Wenzhou lines, so this is how corner radius behaves on a production floor, not in a textbook. You’ll learn what corner radius is, what ASTM A554 permits, how it compares across EN, GB, JIS, and A500 systems, why it breaks assemblies, how to measure it, and how to put it on a purchase order. For the wider context, start with our complete stainless steel rectangular tube guide.
Key Takeaways
- Corner radius has two values, outside (Ro) and inside (Ri), linked by wall thickness: Ri = Ro − t. A drawing that says only “rounded corners” specifies nothing.
- Usable flat width = outside dimension − 2 × Ro. As the radius grows, the flat face you can weld, clamp, or bolt to shrinks, even when the nominal size never changes.
- ASTM A554 sets a maximum corner radius by wall thickness, rising from 2.4 mm (3/32 in.) on thin walls to 19.1 mm (3/4 in.) on heavy walls. A mill that delivers tighter than the maximum is fully compliant.
- Cold-formed stainless tube typically lands between 1.5 t and 3.0 t. Laser-welded ASTM A1127 sharp-cornered profiles (SCP) exist when a round corner is not acceptable.
- Corner radius cannot be calculated from wall thickness alone. Material flow, forming pressure, weld position, and sizing all shift the finished arc, which is why it is inspected rather than assumed.
What Is Rectangular Tube Corner Radius (the R Angle)?
Corner radius, called the R angle in trade language, is the arc joining two adjacent flat faces of a square or rectangular tube. It’s not a chamfer cut after the fact and not a fillet added later. The forming process creates it, so every mill produces some version of it and no mill can eliminate it without changing the manufacturing route.
Two numbers describe it, and buyers conflate them constantly. The outside radius (Ro) governs mating clearance, bracket seating, and polishing access. The inside radius (Ri) governs insert fit, cleaning access, and telescoping compatibility. Get the pair right and parts assemble. Get either wrong and parts meet nominal size and still fail.
Inside Radius vs Outside Radius: Ri = Ro − t
One equation resolves most corner-radius confusion: Ri = Ro − t, equivalently Ro = Ri + t. The inside radius is always smaller than the outside radius by exactly the wall thickness, because the wall keeps its thickness on both faces of the bend.
State that relationship on any drawing. A callout of “R3” is ambiguous. A callout of “outside corner radius 3.0 mm max” is not. A bracket seated on the outside cares about Ro, while a machined insert sliding into the bore cares about Ri, so specify the one the assembly actually touches.
Flat Width and the Limits of a Size Chart
The usable flat width of any face follows from the outside radius: flat width = outside dimension − 2 × Ro. This is the surface a welder, a clamp, or a bolt actually works with, and it explains why two tubes of identical nominal size behave differently in the same fixture.
A signage contractor learned this on a large order of 50 x 25 x 1.5 mm tube. One shipment passed the across-flats check but arrived with a larger corner radius than the approved sample, so the bracket flanges overhung the flats by roughly a millimetre each side and the weld prep no longer sat flush. The fix was a maximum radius written into the next order and verified against a sample.
Wall thickness alone cannot give you that number, because material flow during forming, forming pressure, weld position, and sizing through the Turks-head all move the finished arc. Two suppliers can quote the same size and wall and deliver visibly different corners. Corner radius is a measured dimension, like across-flats or wall, and it belongs in the inspection plan rather than the estimator’s spreadsheet.
How Corner Radius Is Formed (and Why It Varies)
Every welded rectangular section is roll-formed from flat strip and welded closed along a seam. The corners are the parts bent through the tightest arcs, so they carry the most variation. For the forming depth behind the weld, see our guide to how welded rectangular tube is made.
Directly formed tube is typically held to an outside radius of roughly 1 to 2 times the wall. Redrawn or sized tube passes through an additional die, which can tighten or open the radius depending on tooling condition, so a mill running worn tooling produces a slightly different radius than one running fresh tooling. The takeaway for a buyer: a radius that was acceptable on your first order is not guaranteed on the next unless it is written as a specification limit.
Rectangular Tube Corner Radius: What ASTM A554 Allows
For stainless mechanical tubing, the governing specification in North America is ASTM A554, which covers round, square, rectangular, and special shapes. It sets a maximum inside corner radius that increases with wall thickness. The table below reflects the values in wide circulation and aligned with the standard; confirm every band against the current edition text before you rely on it in a specification.
| Wall thickness | Maximum inside corner radius |
|---|---|
| 0.020–0.049 in. (0.51–1.24 mm) | 3/32 in. (2.4 mm) |
| Over 0.049–0.065 in. (1.24–1.65 mm) | 1/8 in. (3.2 mm) |
| Over 0.065–0.083 in. (1.65–2.11 mm) | 9/64 in. (3.6 mm) |
| Over 0.083–0.095 in. (2.11–2.42 mm) | 3/16 in. (4.8 mm) |
| Over 0.095–0.109 in. (2.42–2.77 mm) | 13/64 in. (5.2 mm) |
| Over 0.109–0.134 in. (2.77–3.40 mm) | 7/32 in. (5.6 mm) |
| Over 0.134–0.156 in. (3.40–3.96 mm) | 1/4 in. (6.4 mm) |
| Over 0.156–0.200 in. (3.96–5.08 mm) | 3/8 in. (9.5 mm) |
| Over 0.200–0.250 in. (5.08–6.35 mm) | 1/2 in. (12.7 mm) |
| Over 0.250–0.375 in. (6.35–9.53 mm) | 3/4 in. (19.1 mm) |
A Maximum, Not a Target
Read this as an envelope, not an instruction. ASTM A554 permits a radius up to the value shown, so a mill delivering a tighter corner than the maximum is fully compliant, and often preferable. The table does not oblige anyone to hit a particular radius.
Because the tabulated limit is an inside radius, convert with Ro = Ri + t before comparing it to a fitting that seats on the outside. If your assembly touches the corner, state plainly whether you mean inside or outside radius and confirm the standard’s wording for the edition you buy to.
Trade rules of thumb fill the gap when a drawing is silent, and they work as a sanity check rather than a specification. Cold-formed steel and stainless tube is commonly cited at Ro = 2 t and Ri = 1 t, with most references converging on an outside radius of 1.5 t to 3.0 t. Treat them as commercial generalisations: they predict a range, and they do not certify a lot.
For the grades, testing regime, and the full A554 tolerance set, our ASTM A554 rectangular tube explainer is the reference. This article stays on geometry and fitment.
Corner Radius Across Standards: A554, EN, GB, JIS and A500
Corner radius is defined differently by different standards, which is a real hazard for anyone sourcing across borders. The comparison below puts the main systems side by side. Verify each value against the current edition before writing it into a contract.
EN 10219-2 (cold-formed hollow sections), inside radius by wall thickness:
| Wall thickness t | Inside corner radius Ri |
|---|---|
| t ≤ 6 mm | 1.6 t |
| 6 < t ≤ 10 mm | 2.0 t |
| 10 < t ≤ 12.5 mm | 2.4 t |
| 12.5 < t ≤ 16 mm | 2.8 t |
| t > 16 mm | 3.0 t |
The European tables express the inside radius as a multiple of wall thickness, so conversion to an outside radius is a single addition: Ro = Ri + t. Hot-finished sections under EN 10210-2 carry larger radii than cold-formed ones, broadly 1.8 t to 3.2 t.
The other systems round out the picture. GB/T 3094 and GB/T 6728 typically land between 1.5 t and 3 t. JIS G3466 is tighter, broadly 1.2 t to 2.0 t. ASTM A500 covers carbon structural tube rather than stainless, and AISC 360 gives structural designers a design assumption of Ri = 1.5 t or 0.200 in., whichever is greater. Treat A500 as a separate product, included for completeness rather than as a substitute.
The Sharp-Corner Route: ASTM A1127
Every standard above assumes a rounded corner, but some architecture does not want one. A glazed facade or a machined joint may need a corner close to square. That is what ASTM A1127/A1127M covers: laser or laser-hybrid welded stainless square, rectangular, and special-shape structural tube, supplied either as a sharp-cornered profile (SCP) or with built-up rounded corners. Sections reach 36 in. (914 mm), corner welds are permitted, and grades follow A240, A276, or A479.
If your design cannot tolerate a visible round corner, SCP is the answer, and it is a different manufacturing route rather than a tighter setting on a conventional line. Confirm with your supplier that the profile is genuinely offered before designing around it.
Why Rectangular Tube Corner Radius Breaks Fabrication
This is the reason the query exists. A buyer doesn’t search for corner radius out of curiosity. They search because something didn’t fit. The answer is almost always the same: two sections with identical nominal size and different corner radii will not mate, and the corner was never specified.
| Failure mode | What goes wrong | What prevents it |
|---|---|---|
| Handrail fitting will not seat | The connector is machined for a tighter outside radius | Specify an outside radius maximum; buy the fitting first |
| Telescoping joint binds | Inside radii mismatch, or the internal weld bead blocks the bore | Match Ri on both sections; grind the bead or use a seamless route |
| Mandrel bend cracks or twists | Weld seam sits too close to the corner; the corner is work-hardened | Position the weld away from the corner; tell the mill the tube will be bent |
| Visible gap at a bracket | Flat width shrinks as the radius grows | Specify a maximum radius; verify against an approved sample |
| Finish cannot be polished | A tight radius blocks tool access at the joint | Loosen the radius, or choose a finish that suits the corner |
Handrail and Balustrade Fittings
Commercial railing connectors are machined for a specific corner radius. A rectangular-tube 90-degree connector with a rounded inner corner accepts a tube whose outside radius matches. If the tube’s radius is larger, the fitting will not seat, and the result is gaps, forced grinding, and visible weld repairs, which is unacceptable on an exposed architectural run.
This is the most common corner radius handrail fitting failure we are asked about, and it is entirely preventable. Buy the tube and the fitting against the same radius number, or send us the fitting with the drawing and we will confirm the corner condition before production. Our rectangular tube for handrails and balustrades guide covers the railing-specific detail.
Telescoping and Sliding Fits
Telescoping assemblies add a variable a single-section job never sees: the internal weld bead. Even when the inside radii are compatible, the weld bead along the bore can block the inner tube from sliding. It has to be ground or milled out, or the assembly has to use a seamless or DOM route instead. Inner and outer sections must also have matching geometry, and purpose-made telescoping tube is often welded on the corner with a tighter radius than standard square tube. One note for mixed-material designs: aluminium extrudes to tighter, sharper corners than formed steel ever will.
Ana, a furniture-hardware designer in Valencia, learned the bead lesson directly. An adjustable display frame that telescoped on paper locked solid on the first prototype. The inner and outer radii were close enough, but the weld bead on the outer section caught the inner tube at about 60 percent extension. Switching the outer member to a seamless route, which has no internal bead, solved it without changing the outside dimensions.
Bending, Weld Seam and Polishing Access
Bending a rectangular section is harder than bending a round one because the tube resists bending on its flat faces. A mandrel supports the bore through the bend, and radius affects how the mandrel seats. A weld seam positioned too close to a radiused corner raises the risk of mandrel misfit and seam cracking, and formed corners are work-hardened, which raises cracking risk when they are bent again to a tight radius. If your process bends the tube after delivery, tell the mill.
Tool access follows the same logic. A tight external radius restricts brushing, grinding, and polishing at corner joints, and on a mirror or brushed architectural finish that difference is visible. Our stainless steel rectangular tube surface finish guide covers what each finish demands at the corner.
Corner Radius in Structural Design: What It Affects
Corner radius is geometry, and geometry feeds structural behaviour, with a clear boundary attached: this article supplies the geometry, and section design (buckling, fatigue life, allowable stress) is the engineer’s task, for which AISC 360 and the relevant stainless design guidance are the reference.
Two effects are worth naming. Local buckling depends on flat width, which narrows as the radius grows, so two sections of the same nominal size can buckle differently. And cold-formed corners carry residual stress, which tight radii concentrate more sharply, so the inside corner can become the fatigue-critical location in a cyclically loaded structure. That tension between appearance and fatigue performance is a design decision: state the geometry, hand the numbers to the engineer, and let the analysis govern.
How to Measure Rectangular Tube Corner Radius
Measurement is where a specification either holds or collapses. A drawing callout that can’t be proven is a hope, not a requirement. Three approaches cover almost every situation.
R Gauge (Light-Gap Method)
An R gauge, also called a radius gauge, is the standard field check. The gauge’s curved surface is offered to the corner, and when no light gap remains between the gauge and the tube, the gauge value is the corner radius. A circle template is the low-cost equivalent for a quick visual check. Neither gives a number to two decimal places, and neither needs to for most receiving checks.
Profile Projector, CMM and 3D Scanning
When a callout must be proven with data, the tool changes. A profile projector shines a magnified shadow of the corner onto a screen for direct measurement. A coordinate measuring machine (CMM) captures the arc numerically. A 3D scanner captures the whole section, which suits first-article inspection on a custom profile. Pick the method to match the tolerance you wrote.
What the Mill Checks Before Shipment
Finished rectangular sections are dimensionally gauged at final QC before shipment, and corner radius belongs in that check alongside across-flats, wall thickness, straightness, and twist. That is the practical difference between a mill that inspects geometry and one that ships on a size label. Ask any supplier what they measure at final QC: the answer tells you whether the MTR you receive reflects the section in the crate.
How to Specify Corner Radius on a Drawing and RFQ
Understanding corner radius solves nothing unless it reaches the order. The final step is writing it down in a form a mill can quote and an inspector can verify.
Three Callout Styles
- Maximum radius, “Outside corner radius 3.0 mm max.” Use this when the only risk is a corner too generous for a fitting.
- Nominal with tolerance, “Outside corner radius 3.0 mm ± 0.5 mm.” Use this when the corner must sit in a window.
- Approved sample, “Corner profile to approved sample, revision A.” Use this when appearance governs.
Always state whether the number is inside or outside radius. Never write “rounded corners” alone.
The RFQ Line
A complete order line carries six elements plus certification: size (80 x 40 mm), wall (3.0 mm), grade (ASTM A554 MT-316L), finish (320 grit brushed), corner condition (outside radius 4.0 mm max), length and quantity (6 m lengths, 40 pieces), and certification (EN 10204 Type 3.1 MTR).
A worked example: “Rectangular tube, 80 x 40 x 3.0 mm, ASTM A554 MT-316L, 320 grit brushed, outside corner radius 4.0 mm max, 6 m lengths, 40 pieces, EN 10204 Type 3.1 MTR.” That line leaves nothing to interpretation. For the size and wall families you can order, see our full rectangular tube size chart.
Specify the Corner Before You Buy the Fittings
Buy the tube first and the fitting second, and you inherit whatever corner the mill happened to ship. Buy the fitting first and specify the tube to match it, and the assembly closes.
Ken, a facade contractor on a coastal project in New Zealand, reversed his usual sequence on a repeat order after a costly first run. He bought the machined brackets first, measured the corner they required, then wrote that maximum radius into the tube RFQ. The second shipment seated without a single ground joint. The change cost him nothing but the order in which he placed two purchase orders.
Frequently Asked Questions
What is the corner radius of a rectangular tube?
Corner radius is the rounded arc where two flat faces meet, described by an outside radius (Ro) and an inside radius (Ri). Wall thickness links them: Ri = Ro − t. It is created by the forming process and cannot be removed, only specified to a limit.
What is the difference between inside and outside corner radius?
The outside radius (Ro) is measured on the outer surface and governs bracket seating, mating clearance, and polishing access. The inside radius (Ri) is measured on the inner wall and governs insert fit, cleaning access, and telescoping. Ri is always smaller than Ro by the wall thickness.
What is the maximum corner radius for ASTM A554 tube?
ASTM A554 sets a maximum inside corner radius that rises with wall thickness, from about 2.4 mm (3/32 in.) on walls of 0.51 to 1.24 mm, up to 19.1 mm (3/4 in.) on walls over 6.35 mm. Delivering tighter than the maximum is fully compliant. Convert to an outside radius with Ro = Ri + t.
How do you measure the corner radius of square tube?
The fastest method is an R gauge using the light-gap method: press the gauge to the corner and read the value when no light shows through. For proven data, use a profile projector, a CMM, or a 3D scan. Square and rectangular tube share the same corner geometry.
What corner radius do handrail fittings require?
Handrail connectors are machined for a specific outside radius. The reliable approach is to buy the fitting first, measure the corner it accepts, and specify that radius, plus a maximum, on the tube order. Never assume a nominal size delivers a matching corner.
Is there a sharp-corner stainless steel tube?
Yes. ASTM A1127/A1127M covers laser or laser-hybrid welded stainless square and rectangular structural tube supplied as a sharp-cornered profile (SCP) or with built-up rounded corners, for facades and exposed structural work that cannot tolerate a visible round corner.
Conclusion: Specify the Corner, Not Just the Size
Rectangular tube corner radius is the dimension that decides whether a section fits, and it is the one dimension most orders leave blank. Close that gap with four habits.
State whether you mean inside or outside radius, because Ri = Ro − t and the two are not interchangeable. Give the number, not a vibe, as a maximum, a nominal with tolerance, or an approved sample. Specify it before you buy the fittings, since the fitting is the thing that has to close. And require the mill to gauge it, because a corner that is not measured is not controlled.
Do those four things and the Ghent problem disappears before the first drawing leaves the office. Our technical team will confirm the corner condition, grade, wall, finish, and lead time within 24 hours. Send us your drawing and the fitting you are using, and get the full order line back from a mill that forms, inspects, and finishes the geometry that decides your fit. Explore our stainless steel rectangular tube range or contact our technical team.
