Mill Certificate Weight vs Theoretical Steel Weight: What Buyers Should Check

A mill certificate does not certify the weight of your steel. It certifies chemistry and mechanical test results for a heat of steel, and it identifies the products made from that heat by their nominal sizes. ASTM A6/A6M requires the test report to state the heat number, the heat analysis and the nominal sizes; it does not require a measured thickness, a measured area or a mass.

So when a delivered load does not match the calculated tonnage, the certificate is the wrong document to reach for. The documents that settle weight are the purchase order, which says what basis the order was priced on, and the weighbridge ticket, if the order was priced on actual weight at all.

Three documents, three different jobs

Reading the wrong one for the wrong question is the root of most weight disputes. A certificate matching the order is not evidence that the tonnage was right, and a weighbridge reading differing from the calculation is not evidence that the certificate was wrong.

What an inspection document actually is

In European practice the document types are defined by EN 10204, currently in its 2004 edition, which superseded EN 10204:1991. It specifies the types of inspection documents supplied for the delivery of metallic products: plates, sheets, bars, forgings and castings, whatever their method of production. It is used alongside the product specification, which sets the technical delivery conditions.

There are four types, split by whether the inspection was specific to your order.

Type 2.1, declaration of compliance with the order. The manufacturer declares that the products supplied comply with the order, without including test results.

Type 2.2, test report. The manufacturer declares compliance and supplies test results based on non-specific inspection, meaning inspection carried out under the manufacturer’s own procedures rather than on the material in your consignment specifically.

Type 3.1, inspection certificate. The manufacturer declares compliance and supplies test results from specific inspection, with the test unit and tests defined by the product specification, regulation or order. It is validated by the manufacturer’s authorised inspection representative, independent of the manufacturing department.

Type 3.2, inspection certificate. Prepared jointly by the manufacturer’s authorised inspection representative and either the purchaser’s authorised inspection representative or an inspector designated by official regulations, with test results supplied.

Type 3.1 is the common requirement for structural plate. Type 3.2 is specified where third-party or purchaser witnessing is required.

Note what the definitions do not mention. None of the four types is a statement about the mass of the delivered material. They are statements about conformity and test results.

In North American practice the equivalent document is usually called a mill test report or MTR, and its required content comes from ASTM A6/A6M rather than from a separate document standard.

What ASTM A6/A6M requires on the test report

A6/A6M sets out what the report must carry, and the list is worth reading closely by anyone reconciling a delivery.

It requires the applicable product specification designation, including year-date and whichever of grade, class and type were specified on the purchase order. It requires the heat number, heat analysis and nominal sizes. It requires tension test results appropriate to qualify the shipment. For products produced from coil, both the manufacturer and the processor must be identified.

It also makes a point that surprises people: a signature is not required on the test report. The document must clearly identify the organisation submitting it, and that organisation is responsible for the content regardless of whether anyone signed. An electronically transmitted report has the same validity as one printed at the certifier’s facility.

The phrase that matters for weight is nominal sizes. The report records the size the plate was ordered to, not a measurement of the plate in front of you. That is entirely appropriate for what the document is for, and it means the certificate can never confirm or refute a weight calculation.

What the plate itself carries

A6/A6M also specifies what must be marked on the plate. Required plate markings are the applicable ASTM designation, the letter “G” or “MT” where applicable, the applicable grade, the heat number, size and thickness, and the name, brand or trademark of the manufacturer, or of the processor for plates produced from coil.

The heat number is the link. It appears on the plate and on the certificate, and matching them is the single most important check on receipt. Without that match the certificate is a piece of paper about some other steel.

The size and thickness marked on the plate are, again, the nominal ordered size.

Which weight basis does the order use

This is the question the purchase order has to answer, and it should be answered before the steel is rolled rather than after it arrives.

Theoretical weight basis. The order is priced on the calculated nominal weight: dimensions from the order, density from the standard. Both parties can compute it independently and it does not change if the plate rolls heavy or light. It is common for plate ordered to thickness.

Actual weight basis. The order is priced on a measured weight, normally from a weighbridge. It follows what was delivered.

Neither is universal, and the same supplier may use different bases for different products. A quotation on one basis is not directly comparable with a quotation on the other.

ASTM A6/A6M keeps the distinction visible in its own structure: plate ordered to thickness is governed by thickness tolerances, while plate ordered to weight is governed by a separate table of permitted weight variations, expressed as percentages of the specified weight per square foot and applied to the average weight of a lot rather than to individual plates.

Worked example: reconciling a delivery

Known values: 24 plates at 10 mm × 1500 mm × 3000 mm, carbon steel at 7,850 kg/m³. Weighbridge net reads 8,732 kg.

Theoretical weight:

m per plate = 0.010 × 1.500 × 3.000 × 7,850 = 353.25 kg
lot         = 24 × 353.25 = 8,478.0 kg

Difference:

8,732 − 8,478 = 254 kg, which is +3.00%

Interpretation: the delivery is 3.00% heavier than the calculated figure. That is comfortably inside the conforming range for this plate, which under EN 10029 class A runs from about 5% under to about 11% over once thickness, width and length tolerances are counted together. Nothing is wrong. Reverse check: 254 ÷ 8,478 = 0.0300.

What to do next depends entirely on the purchase order. On a theoretical weight basis, the invoice is 8,478 kg and the weighbridge reading is interesting but not billable. On an actual weight basis, the invoice is 8,732 kg and the difference at, say, 1.20 per kg is 305.21 of genuine cost that the estimate did not carry.

Theoretical versus actual steel plate weight covers where that 3% comes from physically. This article is about which piece of paper decides who pays for it.

A receiving check that works

Six items, in order. The first three take a minute and catch most problems.

Match the heat number. Compare the marking on each plate against the certificate. If they do not match, stop there; nothing else on the certificate applies to this plate.

Check the specification and grade. The certificate should name the product specification with its year-date, and the grade, class or type as ordered. A certificate to a different grade, or to an older edition than specified, is a discrepancy to raise before the plate is cut.

Check the nominal sizes against the order. Thickness, width and length as ordered. This is a paperwork check, not a measurement.

Measure a sample where it matters. If thickness is critical, or if the order specified a tolerance class such as EN 10029 class C, measure it where the applicable standard says to measure it: more than 25 mm from any edge under EN 10029, or 3/8 to 3/4 in (10 to 20 mm) from the longitudinal edge under ASTM A6/A6M. Measuring in the wrong place produces a number that cannot be compared with the tolerance table.

Read the test results against the specification. Chemistry and mechanical results should meet the limits in the product specification. This is what the certificate exists for.

Reconcile weight against the agreed basis. Not against the certificate, which does not carry one, and not against an assumption.

Where buyers go wrong

Treating the certificate as a weight document. It records nominal sizes. It was never intended to confirm mass and it cannot.

Comparing a weighbridge reading to a calculator result and calling the difference an error. The two numbers measure different things. A difference inside the tolerance band is the normal outcome.

Accepting a type 2.2 where a 3.1 was specified. The difference is specific versus non-specific inspection, and independent validation. A type 2.2 test report carries results from the manufacturer’s own procedures rather than from inspection specific to your order.

Accepting a certificate from an intermediary that has been altered. EN 10204 requires an intermediary to pass on either the original or an unaltered copy of the manufacturer’s inspection documents, accompanied by suitable identification of the product to maintain traceability. A retyped certificate on a stockholder’s letterhead does not meet that requirement.

Assuming a missing signature invalidates a report. Under A6/A6M a signature is not required; the report must identify the submitting organisation, which is responsible for its content.

Leaving the weight basis unstated on the purchase order. This is the one that costs money, and it is free to fix. One line on the order.

Plate selection and acceptance for structural or safety-critical applications should follow project specifications, applicable codes and qualified engineering review. The checks above are procurement and dimensional checks, not a substitute for the inspection regime the project requires.

Mill certificate FAQs

Does a mill certificate show the weight of the steel? Not as a certified value. ASTM A6/A6M requires the heat number, heat analysis and nominal sizes on the test report. Any weight shown alongside is normally a calculated theoretical figure or a shipping weight rather than a certified property.

What is the difference between EN 10204 3.1 and 3.2? A type 3.1 certificate is validated by the manufacturer’s own authorised inspection representative, independent of the manufacturing department. A type 3.2 is prepared jointly by that representative and either the purchaser’s authorised representative or an inspector designated by official regulations.

What should I check first when steel arrives? The heat number on the plate against the heat number on the certificate. Without that link, nothing else on the document applies to the material in front of you.

My delivery weighs more than the calculator said. Is that a problem? Usually not. Calculated weight uses nominal dimensions, and plate is rolled and cut inside a tolerance band with more room above nominal than below it. Check the difference against the conforming range for your size and standard, then check which weight basis the purchase order uses.

Should I ask for actual weight or theoretical weight billing? Whichever you can plan around, but settle it on the order rather than after delivery. The important thing is that both parties are quoting and invoicing on the same basis, because a theoretical-weight price and an actual-weight price for the same plate are not comparable.

Ask the document that knows

A mill certificate is an excellent document for the thing it was designed to prove: that a specific heat of steel, traceable by number to the plate in your yard, met the chemical and mechanical requirements of the order. It is silent on mass by design.

Calculate the theoretical weight from the nominal dimensions on the order with the Steel Plate Weight Calculator, state on the quotation that it is theoretical, and settle the commercial basis for weight in writing before the order is placed. Then, when the weighbridge disagrees with the calculation, the answer is already written down.

Inspection document definitions in this article are taken from EN 10204:2004, Metallic products: types of inspection documents. Test report content and plate marking requirements are taken from ASTM A6/A6M, Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling. Confirm the current edition of either before relying on it for a contract.