Wall Thickness Tolerance on Steel Tube: What the -10% Band Means

Two bundles can both be labelled 50x50x2.0 mm and still hand you a 10% difference in steel. Wall thickness is the one dimension a standard lets move — here is what the tolerance band really is, what it costs you in tons, and how to close the gap before the order ships.

Why wall thickness is the dimension that starts arguments

Outside dimensions are easy to hold. The mill sizes the forming rolls, and every piece leaves the line at 50 mm across, because that number is set by the tooling and does not drift. The wall is different: it is formed from strip of a nominal thickness, welded, then squeezed through a sizing stand, and the finished wall lands somewhere inside a tolerance band that the product standard allows. Nobody is cheating when that happens — the band is written into the standard.

So the honest question is not "is the wall 2.0 mm?" but "which part of the band did this bundle land in, and is that the part I paid for?" Wall thickness is the variable that decides two things at once: how many tons actually arrived, and how much load the section can carry.

What the standards actually allow

The published limits below are typical for cold-formed welded hollow sections. Editions differ, so the only binding figure is the one printed in the edition named on your purchase order — treat this table as orientation, not as a substitute for the standard text.

Read that table again and notice what it does not say: it does not say the wall will be 2.0 mm. It says the wall will be somewhere between roughly 1.8 and 2.2 mm. A bundle at 1.82 mm is compliant. A buyer who assumed 2.0 mm and priced the job on 2.0 mm has just absorbed the difference.

What the minus side costs you in tons

The weight effect is not intuitive, because the loss is on a section that still measures 50x50 outside. Work through the numbers for square tube using kg/m = 4 x (a - t) x t x 0.00785:

On a 25-ton order that is roughly 2.3 tons of steel that does not exist. It also cuts bending capacity by nearly the same proportion, because for thin-wall hollow sections the section modulus moves almost linearly with wall thickness. In a furniture frame that shows up as dents during handling; in a structural member it shows up in the load calculation; in a threaded pipe it shows up as a joint that will not hold pressure.

This is the practical reason experienced buyers specify a minimum actual wall rather than a nominal one, and it is the reason a quotation should state which basis it is priced on. Two suppliers quoting "50x50x2.0" can be quoting two different products.

Why product lands on the minus side

Strip is the dominant cost in a tube, so the tolerance band is where a mill recovers yield. Rolling to the minus side is legal, it is standard practice across every producing country, and it is invisible in a photograph. The buyer's protection is not to demand impossible accuracy — it is to specify the band and verify it. A sensible order says "2.0 mm nominal, minimum actual 1.90 mm" and gets a supplier who can hold it. An order that says only "2.0 mm" gets whatever the band allows, and no one has broken a promise.

Four checks that settle it on arrival

1. Micrometer, not the eye. Measure four points around each face at both ends of three pieces per bundle, and write down the minimum and maximum. Five minutes per bundle, and it converts a suspicion into a number.
2. Weighbridge against theoretical. Compare the scale ticket with the calculated weight of the same piece count. A gap beyond 3% needs an explanation — packing adds weight, thin wall removes it.
3. Photograph the cut end. A caliper on a cut face is the most honest view of the wall you can get, and it is the image that ends a dispute quickly in either direction.
4. Divide bundle weight by piece count. If the tag says 2,150 kg for 120 pieces, that is 17.9 kg per piece — compare it with the nominal kg/m times the length and the answer is immediate.

How to write it on the purchase order

Both columns describe the same product on paper. Only one of them gives you a number to hold someone to.

Where a thin wall actually hurts

• Welding: at 1.8 mm effective wall, heat input that was fine at 2.0 mm starts to burn through, and the welder compensates by running colder — which is where porosity begins.
• Threading and coupling: pipe wall carries the thread depth. A minus-tolerance wall leaves fewer full threads and a joint that leaks under pressure test.
• Hot-dip galvanizing: the zinc reacts with the steel surface, and a thin wall plus a heavy coating changes the finished dimension and can distort light sections.
• Structural and racking use: capacity tables are built on nominal wall. Anything below it is uncounted capacity you are spending without knowing.
• Furniture and handling: thin wall dents at the fork pocket and at the strap line, and a dented tube is a rejected tube at your customer's receiving dock.

How this is handled in our yard

We buy from mills and from the Lecong market, and the wall is checked before a bundle is quoted: micrometer readings at four points, minimum and maximum recorded per bundle, and the readings reported to the buyer on request rather than only when asked twice. Where a buyer needs a floor rather than a nominal figure, we say plainly whether the stock in front of us can hold that floor — and if it cannot, the answer is no before the order, not a credit note after it.

For export orders the same readings go into the loading record alongside the bundle tags and the weighbridge ticket, so the thickness conversation is closed before the container door shuts.

Wall thickness requirements, tolerances and inspection scope are confirmed against each purchase list and the standard edition named on it. Figures in this article are worked examples for orientation.

Send the spec, not just the size

Send your purchase list with section, nominal wall, the minimum you can accept, grade, length and quantity, and we will reply with the itemized weights, the actual wall we can supply against current stock, and the container split. Product pages with full size ranges: galvanized square tube, black square tube, ERW steel pipe. Or send your purchase list and we will check the wall against stock before quoting.