What steel tube gauge actually means on a chair frame

Two frames can look identical in a photograph and differ by 30 per cent in steel weight. The difference is almost always wall thickness, hidden inside a tube you can only check by weighing it or cutting it.

Published by JPChairs · Chair manufacturer in Foshan, China · 2026-10-21

Gauge numbers run backwards

Gauge is one of the few specification habits that actively creates mistakes, because the number falls as the wall gets thicker and because two different tables are both called wire gauge.

• Conversion: under Birmingham wire gauge 18 gauge is 1.2 mm, 16 gauge is 1.6 mm and 14 gauge is 2.0 mm.
• Two tables: the Birmingham and American lists differ by about 0.1 mm at several steps, which is 8 per cent of a 1.2 mm wall.
• How tube is quoted: outside diameter times wall. A 25 x 1.2 mm tube has a 22.6 mm bore.
• Our drawings: millimetres with a tolerance, 25 x 1.5 mm plus or minus 0.08, never a gauge number.
• Mill tolerance: rolled tube ships at roughly minus 0.05 to plus 0.08 mm, so a nominal 1.2 mm wall can legitimately arrive at 1.15.
• Profile: 25 mm round and 25 x 25 mm square carry similar weight, but square resists twisting better on a bench or banquet frame.

Wall thickness by chair type

There is no single right thickness. There is a right thickness for the way the chair is loaded, stacked and stored.

• Dining side chair that never stacks: 1.2 mm on 25 mm legs is enough to about 110 kg of user load.
• Stacking dining or banquet frame: 1.5 mm, because the stacked load runs straight down the front leg.
• Bar stool: 1.5 to 2.0 mm on the column, while 1.2 mm is plenty for the footring itself.
• Outdoor aluminium: 1.5 to 2.0 mm, extra metal bought as corrosion allowance rather than strength.
• Lounge frame carrying a mechanism plate: 2.0 mm at the four fixing points even when the rest of the frame is 1.5 mm.
• Weight reference: 25 mm round tube is 0.70 kg per metre at 1.2 mm, 0.87 kg/m at 1.5 mm and 1.13 kg/m at 2.0 mm.

What the extra 0.3 mm actually buys

Deflection, not breaking strength, is what a guest notices, and deflection is the property thickness buys fastest.

• Area: moving from 1.2 to 1.5 mm on a 25 mm tube adds 23 per cent of cross-section area, so 23 per cent more steel weight.
• Stiffness: the second moment of area rises about 20 per cent, so the same span deflects roughly 20 per cent less.
• The complaint: a chair that flexes 4 mm at the backrest feels cheap even though it will never break at that load.
• Cost: a 4.2 kg steel dining frame becomes about 5.1 kg, about 0.9 kg of tube at USD 1.0 to 1.3 per kg ex-works.
• Freight: on 400 chairs that is 360 kg of payload in a container carrying around 18,000 kg, roughly 2 per cent of the box.

Bend radius, ovality and wall thinning

Most frame failures start at a bend rather than in the middle of a straight run, so the bending specification matters as much as the tube size.

• Minimum radius: with a mandrel, about 2.5 times the outside diameter, so 62 mm for a 25 mm tube.
• Ovality: we reject above 8 per cent, measured as maximum diameter minus minimum diameter over the nominal diameter.
• Thinning: a tight bend thins the outer wall by 10 to 18 per cent, so a 1.2 mm wall can measure 1.0 mm exactly where stress peaks.
• Springback: 1 to 3 degrees, cut into the tooling and re-proofed after the first 20 pieces of every production run.
• Wrinkles: creases on the inside of a bend are a structural rejection, not a cosmetic one, because that is where fatigue cracks begin.
• Position: plus or minus 1.0 mm, and on a KD frame that error becomes a bolt hole that will not line up.

Checking what actually arrives

None of this is expensive to verify, but it has to be checked against a reference rather than in the abstract.

• Weight: weigh three stripped frames against the approved sample and reject anything more than 5 per cent light.
• Ultrasonic gauge: reads to plus or minus 0.05 mm; take five points per leg, one of them inside the bend zone.
• Destructive: cut one frame per 1,000 pieces and measure the wall with a caliper at three points.
• Seam placement: the electric-resistance weld seam should run to the inside of the frame, away from contact and stacking faces.
• Drainage: outdoor frames need a 4 mm drain hole at the lowest point of every tube, or they rust from the inside out.
• Paperwork: ask for the mill certificate on any order above 500 units; it costs nothing and it is the document that settles a claim.

Lines for the specification

Four lines on the drawing remove almost every argument that comes later.

• Tube: diameter times wall in millimetres with a tolerance, never a gauge number.
• Forming: minimum bend radius and maximum ovality as a percentage.
• Reference weight: stripped frame weight with a plus or minus 3 per cent tolerance.
• Fixings: KD hardware named by thread size, because a 1.2 mm wall will not hold a threaded insert reliably through repeated assembly.
• The trade: the extra 0.3 mm costs about USD 1 per chair, which makes it the cheapest insurance in the drawing.

The metal frames we build to these figures are shown on the iron and steel chair frames, and the drawing and tolerance sheet we expect to receive with an enquiry is described on the OEM drawing requirements.