ASTM A36 vs A572 Steel Plate: Weight, Density & Grade Differences

An ASTM A36 plate and an ASTM A572 plate of identical dimensions weigh the same. ASTM A6/A6M, the general requirements standard both specifications inherit from, applies one density to rolled structural steel: 490 lb per cubic foot and 7,850 kg per cubic metre. It makes no distinction by grade, and neither does a weight calculation.

What separates the two is specified strength. A36 has a specified minimum yield strength of 36 ksi (250 MPa). A572 Grade 50, the most common of its five grades, has 50 ksi (345 MPa), 39% higher, achieved through microalloying rather than through carrying more material.

The weight question, settled

There is no asterisk on this. ASTM A6/A6M states that one cubic foot of rolled steel is assumed to weigh 490 lb and one cubic metre is assumed to have a mass of 7,850 kg, and it applies that across the product specifications it governs, which include A36/A36M, A572/A572M, A709/A709M and A992/A992M among many others.

Enter either grade into the Steel Plate Weight Calculator with the same dimensions and the answer does not move.

Why higher strength does not mean higher density

The intuition that stronger steel must be denser is reasonable and wrong, and understanding why is the most useful thing in this comparison.

Density is set by what the atoms are and how closely they pack. Steel is overwhelmingly iron, and the crystal structure of ferrite and pearlite in a hot-rolled structural steel is the same whichever of these two specifications the plate was ordered to. Changing a fraction of a percent of the chemistry changes the packing barely at all.

Strength in a structural steel is set by something else entirely: grain size, precipitates, dislocation density and the processing route. A572 is a high-strength low-alloy steel, and its full title names the mechanism, Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel. Small, deliberate additions of columbium (niobium) and vanadium form fine precipitates that pin grain boundaries and restrict dislocation movement. The result is a finer grain and higher yield strength at almost the same composition by mass.

Those additions are measured in hundredths of a percent. Their effect on density is far smaller than the difference between published reference values for plain carbon steel, which the density work on this site puts at under 0.4% across the ordinary range.

The specifications side by side

Two details in that table are worth drawing out.

A572 is a family, not a grade. “A572” on a drawing without a grade number is incomplete. The five grades span 42 to 65 ksi minimum yield, a range of more than 50% between the ends. Grade 50 is the common one, to the point where people say A572 and mean A572-50, but the order needs the number.

A36 specifies a tensile range, not just a minimum. Its 58 to 80 ksi has an upper bound as well as a lower one. That matters for some applications and it is a genuine difference in how the two specifications are written.

Availability and thickness

Both are widely stocked as plate, and both are held by most service centres in common sizes. Two practical differences show up on enquiries.

Thickness range. SSAB lists A36 as applicable from 0.100 to 6.00 in (2.54 to 152 mm) and A572 Grade 50 from 0.100 to 4.00 in (2.54 to 102 mm). Very heavy plate is more likely to be available in A36.

Stock depth. A36 is the default carbon plate in North American service centres and is typically held in the widest range of sizes. A572 Grade 50 is stocked in the common structural sizes but the size list can be shorter, particularly at the thin and heavy ends. Standard steel plate sizes covers which dimensions exist as stock; check size and grade together rather than in sequence.

Plate is also frequently supplied dual certified, meeting more than one specification at once. A plate certified to both A36 and A572 Grade 50 is common, because a steel that meets the higher requirement also meets the lower one. That does not change its weight either.

Worked example: the same plate in both grades

Known values: 20 mm × 2500 mm × 6000 mm, carbon steel at 7,850 kg/m³.

V = 0.020 × 2.500 × 6.000 = 0.300 m3
m = 0.300 × 7,850 = 2,355.0 kg (5,192 lb)

Result: 2,355.0 kg in A36 and 2,355.0 kg in A572 Grade 50. Interpretation: the grade field in a weight calculation is there to select a density, and these two share one. Reverse check: 15.00 m² × 20 mm × 7.85 kg/m² per mm = 2,355.0 kg.

Reference: common plate weights, valid for both grades

Because the density is shared, one table serves both specifications.

The shortcuts work in both grades too: pounds per square foot is the thickness in inches multiplied by 40.833, and kilograms per square metre is the thickness in millimetres multiplied by 7.85.

Dual certification

A single plate frequently carries more than one specification on its certificate. A plate certified to both A36 and A572 Grade 50 is common, and it is not a compromise between the two.

The logic is straightforward. A36 sets a minimum yield of 36 ksi and a tensile range of 58 to 80 ksi. A572 Grade 50 sets a minimum yield of 50 ksi and a minimum tensile of 65 ksi. A heat of steel that meets the A572-50 requirements also meets the A36 minimum yield and falls inside the A36 tensile range, so one plate can legitimately be certified to both.

For a buyer this has two consequences worth knowing.

It widens what can be supplied against an A36 order, because A572-50 material qualifies. That is generally helpful for availability.

It does not change the weight, the tolerances or the calculation. Dual certified plate is ordered to a nominal thickness like any other, inherits the same A6/A6M tolerances, and calculates at the same density.

Where a design is written against a specific grade, check whether dual certified material is acceptable under the project specification before assuming it is. That is a specification question for the responsible engineer, not a stock question.

Worked example: what strength does to the arithmetic

This example is arithmetic, not engineering. It shows what a thickness change does to weight, nothing more.

Known values: 2500 × 6000 mm plate, carbon steel, at three thicknesses.

Interpretation: weight scales linearly with thickness, so a 20% thickness reduction is a 20% weight reduction, on every plate, in every grade. That is the entire mathematical content of the “higher strength saves weight” argument.

What the arithmetic cannot tell you is whether a thinner plate is acceptable. Section capacity does not scale linearly with thickness for every failure mode, buckling and deflection behave differently from yielding, connections and fatigue have their own limits, and the applicable design code governs. Whether a thinner section in a higher grade is adequate is a design calculation for the project engineer, not a substitution rule. Plate selection for structural or safety-critical applications should follow project specifications, applicable codes and qualified engineering review.

Nothing on this page, and nothing in a weight calculator, is a basis for substituting one grade for another.

Weldability, fabrication and cost

Three differences that do not appear in the weight but do appear in the job.

Welding. Both are commonly welded structural steels, and both have established procedures. A572 grades carry different chemistry limits and, at the higher grades, different carbon equivalent considerations, which can change preheat requirements and consumable selection. The welding procedure specification for the work governs, and it is written against the specific grade and thickness rather than against a general rule.

Forming. Higher yield strength means more force to bend and more springback for the same geometry. A press brake programme written for A36 does not transfer unchanged to A572-50.

Cost. A572 Grade 50 normally carries an extra over A36 at the mill, and the size of that extra moves with the market. It is small relative to the base price and it is not a fixed multiple, so it belongs in a current quotation rather than on a reference page. How to estimate steel plate cost from weight covers where grade extras sit in a price build-up.

Tolerances are identical

This follows from the same place as the density. Both specifications take their permitted dimensional variations from ASTM A6/A6M, so a 20 mm A36 plate and a 20 mm A572-50 plate of the same width are held to the same thickness tolerance: a fixed 0.01 in (0.3 mm) under nominal, and a tabulated amount over it that depends on thickness and width.

That means the gap between calculated and delivered weight behaves the same way in both grades. ASTM A6/A6M thickness tolerances covers the structure, and theoretical versus actual steel plate weight covers what it does to the scale reading.

A36 vs A572 FAQs

Is A572 heavier than A36? No. Both use the density ASTM A6/A6M adopts for rolled steel, 7,850 kg/m³ or 490 lb/ft³, so plates of the same dimensions weigh exactly the same.

What is the density of A572 steel? 7,850 kg/m³ (7.85 g/cm³, 0.2836 lb/in³, 490 lb/ft³), the same nominal value used for A36 and for structural carbon steels generally. That is a nominal engineering value for calculation, not a measurement of a particular heat.

What is the difference between A36 and A572 Grade 50? Specified minimum yield strength, mainly: 36 ksi against 50 ksi. A36 is a plain carbon structural steel; A572 is a high-strength low-alloy steel strengthened by small columbium and vanadium additions. Density, and therefore weight, is the same.

Can I substitute A572 for A36? That is a design question, not a materials one, and it has to be answered against the project specification and the applicable code by whoever is responsible for the design. A higher specified minimum yield does not on its own make a substitution acceptable.

Which grade should I enter in a weight calculator? Either. They share a density, so the result is identical. If your calculator offers a generic carbon steel or mild steel option, that works too.

One density, two specifications

The weight comparison between A36 and A572 has a short answer, and it is worth stating plainly because so much published material implies otherwise: there is no difference. The standard that governs both applies one density, and a weight calculation has nothing else to work with.

The difference between the grades is specified strength, and it lives in the design calculation rather than in the mass. Calculate the plate once in the Steel Plate Weight Calculator, use the answer for both, and take the grade decision from the project specification.

Strength values in this article are the specified minimums from the respective ASTM specifications, cross-checked against published grade data from SSAB for ASTM A36 and SSAB for ASTM A572 Grade 50. The density basis is taken from ASTM A6/A6M. Confirm the current edition of any specification before relying on it.