Carbon steel is what most people picture when they say “steel.” It’s what laser-cutting services cut by volume, what structural engineers specify by default, and what buyers order in tons. A36 plate, mild steel sheet, A572 Grade 50 — these are the workhorses of fabrication. And in my 17 years of processing them, the density question almost always resolves to one number: 7,850 kg/m³.
Almost always. The reason I hedge is that carbon content genuinely moves steel density — by about 6–7 kg/m³ per 0.1% carbon — and the range across the 10xx series from 1008 to 1095 is about 60 kg/m³ wider than most tables show. For structural plate calculations, that range is irrelevant. For precision component design, it matters enough to look up.
This article gives you the correct density for every carbon steel grade you’re likely to encounter: plain carbon, HSLA structural, and the AISI 10xx series. It also covers the one question that comes up constantly in our quoting: why do A36 and A572 Grade 50 — materials with very different yield strength — have identical density?
Carbon Steel Density: The Reference Values You Need First
For all standard structural and fabrication carbon steel, use 7,850 kg/m³.
| Unit System | Carbon Steel Density (A36, Mild Steel, 1018) |
|---|---|
| kg/m³ (SI — engineering standard) | 7,850 |
| g/cm³ (scientific / laboratory) | 7.85 |
| lb/in³ (US customary — fabrication) | 0.284 |
| lb/ft³ (structural engineering) | 490 |
| g/mL (equivalent to g/cm³) | 7.85 |
Source: please refer to the Metal Density Chart.
Weight formula — SI: Weight (kg) = Length (m) × Width (m) × Thickness (m) × 7,850
Weight formula — Imperial: Weight (lb) = Length (in) × Width (in) × Thickness (in) × 0.284
Worked example — a standard 1,220 × 2,440 mm A36 plate, 10 mm thick:
Weight = 1.220 × 2.440 × 0.010 × 7,850 = 233.8 kg
That same plate in A572 Grade 50, A572 Grade 65, or 1018 bar stock: same formula, same density, same result. The reason is the subject of this article.
Why Carbon Steel Density Is 7,850 kg/m³ — The Chemistry Behind the Number
The iron baseline and what carbon does to it
Pure iron has a density of 7,874 kg/m³. This value comes directly from iron’s body-centred cubic (BCC) crystal structure at room temperature: a unit cell containing 2 atoms of iron (atomic mass 55.845 g/mol), with a lattice parameter of 2.866 Å.
Carbon dissolves into the iron lattice as an interstitial solute — it fits into the gaps between iron atoms rather than replacing them. Carbon’s atomic mass is 12.011 g/mol — approximately 21% of iron’s. When carbon occupies an interstitial site, it contributes very little mass but does expand the lattice slightly. The net effect is a small but measurable decrease in density as carbon content increases.
The gradient, derived from the density data in ASM Handbook Vol. 1:
Each 0.1 wt% increase in carbon content reduces steel density by approximately 6–7 kg/m³.
Applied across the AISI 10xx carbon range:
| Carbon (wt%) | Representative Grade | Approximate Density (kg/m³) | Change from Pure Iron |
|---|---|---|---|
| 0.0% | Pure iron | 7,874 | — |
| 0.08% | SAE 1008 | 7,869 | −5 |
| 0.18% | SAE 1018 | 7,861 | −13 |
| 0.25% | A36 (max) | 7,857 | −17 |
| 0.30% | SAE 1030 | 7,853 | −21 |
| 0.45% | SAE 1045 | 7,843 | −31 |
| 0.65% | SAE 1065 | 7,831 | −43 |
| 0.80% | SAE 1080 | 7,820 | −54 |
| 0.95% | SAE 1095 | 7,809 | −65 |
The practical conclusion: across the entire low-carbon structural range (0.05–0.30% C), density stays within 7,855–7,869 kg/m³ — essentially 7,860 rounded, and within measurement tolerance of 7,850. For fabrication weight estimates, using 7,850 for any grade from 1008 through A36 introduces an error of less than 0.3%.
A36 Density: The Standard Structural Plate Grade
ASTM A36 is the most widely used structural carbon steel in North America and the grade we laser-cut more than any other by tonnage.
Per ASTM A36/A36M, the composition limits are:
| Element | A36 Composition Limit |
|---|---|
| Carbon (C) | ≤ 0.25% (plates ≤ 20 mm) |
| Manganese (Mn) | 0.80–1.20% (plates > 20 mm) |
| Silicon (Si) | 0.15–0.40% |
| Phosphorus (P) | ≤ 0.04% |
| Sulfur (S) | ≤ 0.05% |
The density of A36 steel is 7,850 kg/m³ (7.85 g/cm³, 0.284 lb/in³).
Manganese (7,210 kg/m³) is slightly lighter than iron. Silicon (2,330 kg/m³) is substantially lighter. At the concentrations permitted by A36, these elements marginally pull density downward — but the effect is small enough that 7,850 kg/m³ is the correct working value for every A36 weight calculation.
A36 density across standard plate gauges — quick reference
| Thickness | Full Sheet Weight (1,220 × 2,440 mm) | Per m² |
|---|---|---|
| 3 mm | 70.1 kg | 23.6 kg/m² |
| 5 mm | 116.8 kg | 39.3 kg/m² |
| 6 mm | 140.2 kg | 47.1 kg/m² |
| 8 mm | 186.9 kg | 62.8 kg/m² |
| 10 mm | 233.7 kg | 78.5 kg/m² |
| 12 mm | 280.4 kg | 94.2 kg/m² |
| 16 mm | 373.9 kg | 125.7 kg/m² |
| 20 mm | 467.3 kg | 157.1 kg/m² |
| 25 mm | 584.1 kg | 196.3 kg/m² |
Formula: Weight (kg) = 1.220 × 2.440 × thickness (m) × 7,850
These numbers run in our quoting system for every A36 job. The 10 mm plate value — 233.7 kg per full sheet — is the one our operators and logistics team know by heart, because it’s the most common structural plate gauge on our tables.
A572 Density: Why Higher Yield Strength Does Not Mean Higher Density
This is the question I get more than any other from structural engineers ordering HSLA plate for the first time.
ASTM A572 is a high-strength low-alloy (HSLA) structural steel available in multiple yield strength grades: Grade 42, Grade 50, Grade 55, Grade 60, and Grade 65. Grade 50 is by far the most common in fabrication.
Here is the mechanical property comparison with A36:
| Property | A36 | A572 Gr. 42 | A572 Gr. 50 | A572 Gr. 65 |
|---|---|---|---|---|
| Minimum yield strength | 250 MPa | 290 MPa | 345 MPa | 450 MPa |
| Minimum tensile strength | 400–550 MPa | 415 MPa | 450 MPa | 550 MPa |
| Density (kg/m³) | 7,850 | 7,850 | 7,850 | 7,850 |
All four grades have identical density. A572 Grade 65 is 80% stronger than A36 in yield — and weighs the same per unit volume.
Why HSLA strength does not come with a density premium
A36 achieves its properties through plain carbon content (up to 0.25%) and a relatively simple iron-carbon-manganese chemistry. A572 achieves its significantly higher yield strength through microalloying — small additions of niobium (columbium), vanadium, and/or titanium at concentrations of 0.01–0.15%.
Per ASTM A572/A572M, the permitted microalloy additions are:
| Element | A572 Limit |
|---|---|
| Columbium (Nb) | 0.005–0.05% |
| Vanadium (V) | 0.01–0.15% |
| Titanium (Ti) | 0.06% maximum |
The densities of these elements are: niobium (8,570 kg/m³), vanadium (6,110 kg/m³), and titanium (4,507 kg/m³). Vanadium is notably lighter than iron; niobium is slightly heavier; titanium is substantially lighter. But all are present at 0.01–0.15% by weight — concentrations so small that their contribution to the bulk density is unmeasurable in practice.
The strength increase in A572 comes from grain refinement and precipitation hardening — the microalloy additions pin grain boundaries and form fine carbide/nitride precipitates that impede dislocation movement. This is a microstructural mechanism that delivers strength without adding mass. The alloy chemistry remains close enough to pure iron that the density stays at 7,850 kg/m³.
The practical implication: When a structural engineer specifies A572 Grade 50 to save weight — using a stronger steel to design with thinner sections — the weight saving comes entirely from the thinner sections, not from any density advantage. The material itself is no lighter per unit volume than A36. The structural efficiency gain is real; it comes from section geometry, not material density.
Density for Carbon Steel Across the AISI 10xx Series
The 10xx series covers the full range of plain carbon steel from soft deep-draw sheet to spring steel and tooling applications. Density decreases steadily with carbon content, but stays within a comparatively narrow band.
| Grade | C (%) | Primary Use | Density (kg/m³) | lb/in³ |
|---|---|---|---|---|
| SAE 1006 | 0.06 | Automotive body sheet (deep draw) | 7,871 | 0.2843 |
| SAE 1008 | 0.08 | Cold-rolled sheet, formed panels | 7,869 | 0.2842 |
| SAE 1010 | 0.10 | Sheet metal, light structural | 7,867 | 0.2842 |
| SAE 1018 | 0.18 | Bar, rod, plate — general machining | 7,861 | 0.2839 |
| SAE 1020 | 0.20 | Shaft stock, structural bar | 7,859 | 0.2839 |
| SAE 1025 | 0.25 | Plate and structural — close to A36 | 7,856 | 0.2838 |
| SAE 1030 | 0.30 | Axles, shafts, machine parts | 7,853 | 0.2837 |
| SAE 1040 | 0.40 | Bolts, studs, crankshafts | 7,846 | 0.2834 |
| SAE 1045 | 0.45 | Shafts, tooling, gears | 7,843 | 0.2833 |
| SAE 1060 | 0.60 | Springs, cutting edges | 7,831 | 0.2829 |
| SAE 1080 | 0.80 | Springs, rail, wire | 7,820 | 0.2825 |
| SAE 1095 | 0.95 | Blades, springs, high-carbon tooling | 7,809 | 0.2821 |
Source: ASM Handbook, Volume 1, physical properties of carbon and low-alloy steels.
The full range — from 1006 at 7,871 kg/m³ to 1095 at 7,809 kg/m³ — spans 62 kg/m³ or about 0.8% of the 1006 value. This is genuinely small. For freight estimates and structural calculations, using 7,850 for any 10xx grade introduces no meaningful error. For precision weight specifications on high-carbon components — blade stock, spring elements, bearing races — the grade-specific value is the right choice.
Data source: density of steel complete reference
1018 steel density: the machinability standard
SAE 1018 deserves specific mention because it’s the most common steel grade for machined components in North American shops — the cold-drawn bar and plate grade that machinists default to.
Density of 1018: 7,861 kg/m³ (7.86 g/cm³, 0.2839 lb/in³).
For practical purposes, 7,850 kg/m³ is the correct working value — the 11 kg/m³ difference introduces 0.14% error in weight calculations. The distinction matters only in precision weight budgets where the part weight callout on the drawing is verified post-machining. We’ve supplied 1018 bar stock as feedstock for machined components where the customer’s weight spec had a ±5 g tolerance on a 300-gram part. At that tolerance level, even 7,850 is accurate enough; the machining tolerance on the final geometry dominates the weight variation.
Carbon Steel vs. HSLA: The Structural Weight Calculation That Matters
The A36 vs. A572 comparison above establishes that density is identical. But the real design question is: if I switch from A36 to A572 Grade 50, how much weight do I actually save in a real structure?
The answer depends on what the design is governed by. Two scenarios:
Scenario 1: Strength-governed design
A gusset plate in a steel frame carries a tensile load of 800 kN. The designer needs to size the plate cross-section.
- In A36 (yield 250 MPa, density 7,850 kg/m³): Required area = 800,000 ÷ 250 = 3,200 mm²
- In A572 Gr. 50 (yield 345 MPa, density 7,850 kg/m³): Required area = 800,000 ÷ 345 = 2,319 mm²
Section area reduced by 27.5%. If plate thickness is fixed, the plate width reduces proportionally. The installed weight is 27.5% lower with A572 — not because A572 is lighter per unit volume (it isn’t), but because the stronger material requires less of it.
We’ve processed exactly this kind of optimisation for customers in the material handling and crane manufacturing sector, where every kilogram of structural self-weight reduces the net payload capacity. The switch from A36 to A572 Gr. 50 on a standard crane bridge girder reduced the main structural steel tonnage by approximately 22% on one project—the density of both materials in our quoting system: 7,850 kg/m³. The weight saving came entirely from the section redesign.
Scenario 2: Stiffness-governed design
A floor beam must not deflect more than L/360 under a specified live load.
In this case, the governing property is stiffness (Young’s modulus × moment of inertia), not yield strength. The Young’s modulus of A36 and A572 Grade 50 is essentially identical — approximately 200 GPa for both. Upgrading to the stronger steel does not reduce deflection unless the geometry of the section is also changed. A stronger but lighter (thinner) A572 section may actually deflect more than the heavier A36 section it replaced.
This is the honest limitation that structural engineers understand, but that doesn’t appear in most “why use HSLA?” articles. The material density is the same. The stiffness per unit mass is the same. Upgrading to A572 only saves weight when the design is strength-limited, not deflection-limited.
Carbon Steel Density in Laser Cutting: What We See Every Day
We process A36 and mild steel every working day. A few observations that only come from running production volume:
Sheet weight is the starting point for every job
Before nesting, before quoting cut time, before calculating gas consumption — we calculate sheet weight. For A36, it’s always 7,850 × length × width × thickness. The number goes on the job traveller, drives the pallet weight estimate, and determines the freight classification. A stack of twenty 10 mm A36 sheets (full size, 1,220 × 2,440 mm) weighs approximately 4,674 kg — just over 4.5 tonnes. That number goes to our logistics team before the sheets are moved from stock.
The scrap-to-parts ratio and its density implications
On a typical A36 structural bracket nesting job, we aim for 75–85% material utilisation. At 80% utilization on a batch of 10 mm plate:
- Sheet weight: 233.7 kg
- Parts weight: 233.7 × 0.80 = 186.9 kg
- Scrap (skeleton, drops, remnants): 46.8 kg
The scrap goes into the ferrous bin. At current ferrous scrap values, those 46.8 kg have a measurable value to job profitability on thin-margin work. Getting the density right is not just about customer specifications — it touches job economics.
When customers specify yield strength but forget density
This is the most common specification error we see on drawings. A customer designs an A572 Grade 50 structure specifically because it’s stronger than A36, and then calculates assembly weight using a “lighter steel” assumption that doesn’t exist in reality. Both materials are 7,850 kg/m³. The assembly weight is determined by section sizes and total material volume, which should be smaller in the A572 design, but only if the design was actually optimised for the higher yield strength. We’ve received assemblies designed in A572 that were dimensioned identically to A36 predecessors with no section reduction. The result: same weight, higher cost.
Frequently Asked Questions About Carbon Steel Density
Q: What is the density of carbon steel in kg/m³? 7,850 kg/m³ for all standard carbon and mild structural steel — A36, 1018, 1020, 1025, and equivalent grades. This value applies equally to low-carbon and medium-carbon grades for practical purposes. High-carbon steel (1080–1095) sits slightly lower, at approximately 7,810–7,820 kg/m³.
Q: What is the density of mild steel? 7,850 kg/m³. “Mild steel” refers to low-carbon steel with approximately 0.05–0.30% carbon — the same range covered by A36 and the common 10xx structural grades. The density is essentially that of pure iron (7,874 kg/m³) reduced slightly by the carbon content.
Q: Is the density of A36 the same as A572 Grade 50? Yes — both are 7,850 kg/m³. A572 achieves its higher yield strength through microalloy additions (niobium, vanadium, titanium) at concentrations of 0.01–0.15%, which are too small to measurably change the bulk density. The two grades are indistinguishable in any weight calculation.
Q: What is the density of 1018 steel? Precisely, 7,861 kg/m³ — about 11 kg/m³ above the round 7,850 figure. For all practical fabrication calculations, use 7,850. The 0.14% difference matters only in precision weight specifications with tight tolerances.
Q: Does heat treatment change carbon steel density? Not for practical purposes. The microstructural transformation from ferrite-pearlite to martensite changes density by less than 1–2%. For room-temperature weight calculations, use 7,850 kg/m³ regardless of the heat treatment condition the part will receive.
Q: Is carbon steel denser than stainless steel? No — stainless steel is slightly denser. 304 austenitic stainless is approximately 7,930 kg/m³ vs. 7,850 for carbon steel, a difference of about 80 kg/m³ (1%). The reason is nickel: austenitic stainless carries 8–12% nickel (density 8,908 kg/m³), which pulls the alloy density above the iron baseline. For most fabrication calculations, the difference is within rounding tolerance. For large assemblies with mixed carbon and stainless content, use the material-specific values.
Q: What is the density of carbon steel in lb/in³? 0.284 lb/in³ for A36 and all standard structural carbon steel. Precisely: 7,850 kg/m³ × 0.000036127 = 0.2836 lb/in³, which rounds to 0.284.
References
- ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys, 10th Edition. ASM International, Materials Park, OH, 1990 (4th printing 1995). (AISI/SAE carbon steel classification; physical properties of low-, medium-, and high-carbon grades; iron crystal structure and phase transformation data; density values for the 10xx series.)
- ASTM A36/A36M, Standard Specification for Carbon Structural Steel, ASTM International, current edition. (Composition limits for A36 plate, shape, and bar; basis for the 7,850 kg/m³ standard structural density value.)
- ASTM A572/A572M, Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel, ASTM International, current edition. (Microalloy composition limits for Grade 42 through Grade 65; mechanical property minimums; basis for the density-equivalence discussion with A36.)
- American Institute of Steel Construction (AISC), Steel Construction Manual, 16th Edition. AISC, Chicago, IL. (Structural design tables for A36 and A572 Grade 50; section property tables using 7,850 kg/m³ density as the standard value for all carbon structural steel.)
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