Density of Fe: The Element Iron’s Density in All Common Units

Fe is the chemical symbol for iron, derived from the Latin ferrum. It is element number 26 on the periodic table, a 3d transition metal, and the basis of every steel and cast iron alloy in structural engineering.

The density of Fe as a pure element is 7,874 kg/m³ — 7.874 g/cm³, 0.2843 lb/in³, measured at 20°C in the α-iron (body-centered cubic) phase.

This article covers the Fe density value completely: every unit system with exact conversions, the crystallographic and atomic basis for the number, iron’s position on the periodic table relative to its 3d transition metal neighbors, and the precise measurement conditions that define the reference value. If you have arrived here from a chemistry course, a materials database, or a periodic table reference, this is the complete picture.

Density of Fe: The Reference Value in Every Unit System

The standard reference density for the element Fe at 20°C, α-phase, is derived from X-ray crystallographic measurement of the body-centered cubic (BCC) unit cell. All values below are exact conversions of the 7,874 kg/m³ reference.

SI and metric units

US customary and imperial units

Conversion arithmetic — showing the work

The most commonly needed conversions, with the exact calculation:

kg/m³ to g/cm³: 7,874 kg/m³ × (1,000 g/kg) × (1 m³ / 1,000,000 cm³) = 7,874 × 10⁻³ = 7.874 g/cm³ ✓

kg/m³ to lb/in³: 7,874 kg/m³ × (1 lb / 0.453592 kg) × (1 m / 39.3701 in)³ = 7,874 × 2.20462 × (1/61,023.7) = 7,874 × 3.61273 × 10⁻⁵ = 0.28434 lb/in³ ✓

kg/m³ to lb/ft³: 7,874 × 0.062428 = 491.4 lb/ft³ ✓

The lb/in³ value is often rounded to 0.284 in engineering practice. The rounding error is 0.12% — negligible for freight and weight calculations, but worth carrying the full 0.2843 for precision mechanical applications.

Where the Fe Density Value Comes From — The Atomic Basis

The density of Fe as an element is not measured by weighing a chunk of iron and dividing by its volume. It is derived from the crystal structure, using the measured lattice parameter and the known atomic mass. This is one of the most direct connections between atomic-scale physics and macroscopic material properties in all of chemistry.

Iron’s body-centered cubic crystal structure at room temperature

At room temperature, elemental iron exists as α-iron — a body-centered cubic (BCC) lattice. In a BCC structure:

• One atom occupies the center of the cube
• Eight atoms occupy the corners, each shared among 8 adjacent unit cells
• Effective number of atoms per unit cell: 1 + (8 × 1/8) = 2 atoms

The lattice parameter (edge length of the cubic unit cell) for α-iron is a = 2.866 Å = 2.866 × 10⁻¹⁰ m, measured by X-ray diffraction.

The density calculation from first principles

Step 1: Unit cell volume V = a³ = (2.866 × 10⁻¹⁰)³ = 2.354 × 10⁻²⁹ m³

Step 2: Mass of atoms in one unit cell. Iron’s standard atomic weight is 55.845 g/mol (the IUPAC 2021 value, reflecting the natural isotopic composition of iron).

Mass per unit cell = 2 atoms × (55.845 g/mol) ÷ (6.02214 × 10²³ atoms/mol) = 2 × 9.2736 × 10⁻²³ g = 1.8547 × 10⁻²² g = 1.8547 × 10⁻²⁵ kg

Step 3: Calculated density ρ = mass ÷ volume = 1.8547 × 10⁻²⁵ kg ÷ 2.354 × 10⁻²⁹ m³ = 7,880 kg/m³

Step 4: Measured vs. calculated The measured density of pure iron is 7,874 kg/m³ — 6 kg/m³ below the calculated lattice value. The discrepancy reflects lattice vacancies (Schottky defects — vacant atomic sites that are present in every real crystal at non-zero temperature) and the slight thermal expansion of the lattice at 20°C above absolute zero. These are small but real effects.

The agreement between the crystallographic calculation (7,880) and the measured value (7,874) is better than 0.1% — confirmation that the BCC lattice parameter and Avogadro’s number together explain all of iron’s density essentially.

The role of iron’s isotopic composition in the atomic mass

Iron’s standard atomic weight of 55.845 g/mol is not a fixed atomic mass — it is the natural abundance-weighted average of four stable isotopes:

Weighted average: (53.9396 × 0.05845) + (55.9349 × 0.91754) + (56.9354 × 0.02119) + (57.9333 × 0.00282) = 55.845 u

The dominant isotope ⁵⁶Fe (91.75% of natural iron) controls the atomic mass almost entirely. The density of a hypothetical pure-⁵⁶Fe sample would be fractionally different from 7,874 kg/m³ — by less than 0.02% — which is undetectable in any practical measurement.

For all engineering and materials science purposes, the isotopic composition of natural iron is constant regardless of source, and the 55.845 g/mol atomic weight is fixed.

Fe on the Periodic Table — Why Its Density Is What It Is

Iron’s density of 7,874 kg/m³ is not arbitrary. It sits within a pattern that runs across the 3d transition metals of Period 4 — a pattern driven by atomic mass, electron configuration, and crystal structure.

Iron’s electron configuration and metallic bonding

Iron has the electron configuration [Ar] 3d⁶ 4s² — argon core plus 6 electrons in the 3d shell and 2 in the 4s shell. In metallic iron, the 4s electrons and part of the 3d electrons delocalize into the conduction band, forming the metallic bond that holds the crystal together.

Iron’s 3d subshell at half-fill-plus-one (six electrons vs. five for manganese’s half-filled 3d⁵) produces strong metallic bonding with the BCC structure favored by elements in the middle of the 3d transition series. The bond strength and atomic packing determine the lattice parameter; the atomic mass determines the mass per unit cell. Both together determine density.

Density trend across Period 4 transition metals

The 3d transition metals of Period 4 show a general density increase from Sc to Os (in the heaviest transition metal rows) as atomic mass rises faster than atomic volume increases. Within the Fe-group neighborhood:

Iron sits at a density step up from manganese and a step below cobalt and nickel. The reason Mn (54.938 u) is less dense than Fe (55.845 u) despite similar atomic mass: manganese’s complex cubic crystal structure (α-Mn has 58 atoms per unit cell in four distinct atomic sites) packs atoms less efficiently than iron’s clean BCC structure. Crystal structure efficiency, not atomic mass alone, determines density.

Cobalt and nickel are both denser than iron despite similar atomic masses (Co: 58.933 u, Ni: 58.693 u) because their FCC and HCP structures pack more efficiently than BCC — 74% of space filled vs. 68% for BCC — and their slightly higher atomic masses compound the effect.

Iron’s position — BCC structure, atomic mass 55.845, density 7,874 kg/m³ — is precisely what the crystal structure and atomic mass predict. There is no anomaly. The number is correct.

Measurement Conditions for the Density of Fe — Precision Matters

The value 7,874 kg/m³ is specified under standard conditions. For precision work in chemistry or materials science, knowing exactly what those conditions are is essential.

Temperature: 20°C (293.15 K)

The density of any solid decreases as temperature rises — thermal expansion increases the unit cell volume without adding mass. Iron’s coefficient of thermal expansion is approximately 11.8 × 10⁻⁶ /°C in the α-iron range.

At temperatures other than 20°C, iron’s density is approximately:

Note: Some references use 25°C (298.15 K) as the standard temperature. The density at 25°C is approximately 7,870 kg/m³ — 4 kg/m³ below the 20°C value. This small difference is within round-off for most applications.

Crystal phase: α-iron (BCC), not γ-iron or δ-iron

The reference value always corresponds to α-iron — the stable BCC phase at room temperature and pressure. Iron also exists as:

• γ-iron (FCC austenite): 912°C to 1,394°C — less dense than α-iron (~7,650 kg/m³ at the transition temperature)
• δ-iron (BCC): 1,394°C to 1,538°C — lowest density solid phase (~7,500 kg/m³)

If a materials database or textbook lists iron density without specifying the phase, it is reporting the α-iron (room temperature) value. That is always the correct interpretation.

Purity: pure elemental Fe

The 7,874 kg/m³ value applies to pure iron — effectively 100% Fe with no dissolved alloying elements. Real commercial iron always contains trace impurities:

Each step down in Fe content — from dissolved carbon reducing density slightly, to precipitated graphite reducing it dramatically — moves the density away from 7,874. The element value is the ceiling, not the engineering number.

Fe Density in the Context of Common Engineering Elements

Positioning Fe among the elements most often encountered in materials selection:

Iron is lighter than cobalt, nickel, copper, and all the heavier metals — but substantially denser than aluminum (0.34×), titanium (0.57×), and chromium (0.91×). This positioning explains why stainless steel — which dilutes iron with 10–20% chromium and 8–12% nickel — ends up with a density close to iron: Cr pulls it down slightly, Ni pushes it up slightly, and the net result (7,930–7,980 kg/m³ for austenitic grades) is almost identical to pure Fe.

The molybdenum comparison (10,280 kg/m³, 1.31× Fe) explains why adding 2–3% Mo to 316 stainless raises its density above 304 stainless. Molybdenum is 30% denser than iron, and even at 2% concentration, it measurably shifts the bulk density upward.

Frequently Asked Questions About the Density of Fe

Q: What is the density of Fe in g/cm³?

7.874 g/cm³. This is the standard reference value for pure iron (α-phase, BCC crystal structure) at 20°C, from NIST Standard Reference Data and the CRC Handbook of Chemistry and Physics. It is the value found on most periodic tables and chemistry references.

Q: What is the density of Fe in kg/m³?

7,874 kg/m³. Exact conversion: 7.874 g/cm³ × 1,000 = 7,874 kg/m³. This is the SI standard unit of density and the value used in engineering weight calculations.

Q: What is the density of Fe in lb/in³?

0.2843 lb/in³ (commonly rounded to 0.284). Full calculation: 7,874 kg/m³ × (1 lb/0.453592 kg) × (1 m/39.3701 in ³ = 0.28434 lb/in³.

Q: Why is iron’s chemical symbol Fe and not Ir?

The symbol Fe derives from ferrum, the Latin word for iron. The Latin-based symbols for several common metals predate the systematic IUPAC naming conventions — other examples include Cu (cuprum, copper), Pb (plumbum, lead), Au (aurum, gold), and Ag (argentum, silver). The symbol Fe has been standardized internationally and appears on every modern periodic table.

Q: Is the density of Fe the same as the density of steel?

No, but close. Pure iron (Fe) is 7,874 kg/m³. Structural carbon steel (A36) is approximately 7,850 kg/m³ — about 0.3% less, because dissolved carbon (lighter than iron at the atomic level) slightly reduces the alloy density. For engineering purposes, iron and low-carbon steel are treated as identical in density. For exact calculations, use the grade-specific value: 7,874 for the pure element, 7,850 for structural steel.

Q: At what temperature does the density of Fe change significantly?

Iron’s density changes measurably at two important temperature points. At 912°C, iron undergoes an allotropic transformation from BCC (α-iron) to FCC (γ-iron), and density drops from approximately 7,668 to approximately 7,650 kg/m³ — a small decrease despite the phase change, because the FCC unit cell at that temperature expands beyond what FCC packing efficiency alone would predict. For engineering calculations at room temperature, 7,874 kg/m³ applies regardless of what heat treatment the iron or steel part will receive.

References

1. National Institute of Standards and Technology (NIST) WebBook, Standard Reference Data — Iron (Fe), available at webbook.nist.gov. (Authoritative elemental density, crystal structure, and physical property data for Fe; source of the 7,874 kg/m³ reference value; BCC lattice parameter a = 2.866 Å.)
2. Haynes, W.M., ed., CRC Handbook of Chemistry and Physics, 104th Edition. CRC Press, Boca Raton, FL, 2024. (Physical constants of inorganic compounds; elemental properties table — Fe density, atomic weight 55.845; isotopic composition of iron.)
3. IUPAC Commission on Isotopic Abundances and Atomic Weights, Atomic Weights of the Elements 2021, Pure and Applied Chemistry, 2022. (Standard atomic weight of Fe: 55.845 u; isotopic abundances: ⁵⁴Fe 5.845%, ⁵⁶Fe 91.754%, ⁵⁷Fe 2.119%, ⁵⁸Fe 0.282%.)
4. ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys, 10th Edition. ASM International, Materials Park, OH, 1990 (4th printing 1995). (Iron crystal structure and phase transformation temperatures; α-, γ-, and δ-iron phase densities; commercial iron alloy properties showing density departure from pure-element value.)

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Dr. Liu Che

Dr. Liu Che holds a Ph.D. in Engineering from Dalian University of Technology and completed postdoctoral research at the Mechanical Engineering Postdoctoral Fellowship of Harbin Institute of Technology. His research focuses on core algorithms for large-scale laser equipment integration, laser cladding remanufacturing, and real-time intelligent monitoring systems for processing. He possesses profound academic expertise in cutting-edge fields such as laser beam spatial shaping and the mechanism analysis of laser processing of special alloys. He has led numerous solutions to technical challenges in cutting and surface modification of high-hardness materials in the heavy machinery industry, demonstrating strong theoretical foundations and outstanding engineering implementation capabilities.

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