# List of thermal conductivities
[Thermal conductivity](https://www.edgepedia.org/thermal-conductivity), given the symbol λ or k, is a material property indicating how effectively a substance carries heat; it is quantified as the amount of heat passing through a material's unit thickness, perpendicular to a unit-area surface, under a unit temperature gradient when conditions have reached steady state.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) The SI unit is W/(m K), also written W m⁻¹ K⁻¹; the Imperial unit is Btu/(hr ft °F), with 1 W/(m K) equal to 0.5779 Btu/(ft h °F).[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) • [2](https://publications.iupac.org/ci/2011/real_pages/July11CI/files/july11.pdf) Values span roughly five orders of magnitude across everyday materials, from about 0.02 W/(m K) for silica aerogel to 1000 W/(m K) for diamond.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) • [3](https://energyeducation.ca/encyclopedia/Thermal_conductivity)
| Key fact | Detail |
|---|---|
| Definition | Heat flow per unit area per unit temperature gradient under steady-state conditions; Fourier's law, \( \mathbf{J}_h = -\kappa \nabla T \)[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) • [4](https://acangi.github.io/lecture_notes_solids_thermal/lecture_10.html) |
| Unit | W/(m K) in SI; 1 W/(m K) = 0.5779 Btu/(ft h °F)[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Highest common solid | Diamond, 1000 W/(m K) at 25 °C; isotopically ultrapure single-crystal diamond reaches about 3100 W m⁻¹ K⁻¹[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) • [5](https://arxiv.org/abs/2609.24191) |
| Common metals | Copper 401, aluminum 205, iron 80 W/(m K) at 25 °C[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) |
| Water and air | Water 0.58 W/(m K); air 0.0262 W/(m K) at 25 °C[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) • [1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Insulators | Silica aerogel 0.02, cotton wool 0.029, fiberglass 0.04, cork board 0.043 W/(m K)[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Metals vs insulators | In pure metals heat is carried mainly by electrons; in insulators by lattice vibrations (phonons)[6](https://staibabussalamsula.ac.id/wp-content/uploads/2024/03/McGraw-Hill-Encyclopedia-of-Science-Technology-10th-Edition-Volume-18-SUR-TYP-staibabussalamsula.ac_.id_.pdf) |
## Representative values
The table below collects values for common materials at or near room temperature (25 °C unless noted). Values are approximate; conductivity varies with temperature, purity, alloying, density, and moisture.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html)
| Material | k, W/(m K) | Source note |
|---|---|---|
| Diamond | 1000[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | Single crystal; ultrapure isotopic material reaches ~3100[5](https://arxiv.org/abs/2609.24191) |
| Copper | 401[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | Engineering ToolBox example uses 215 for an aluminum pot and 17 for stainless steel[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Aluminum | 205[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | 215 W/(m K) in the ToolBox worked example[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Graphite | 168[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | Anisotropic; direction matters |
| Iron | 80[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | Ingot iron 47–58[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Stainless steel | 17[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | Alloying sharply lowers conductivity versus carbon steel |
| Ice (0 °C) | 2.18[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | About four times liquid water |
| Concrete, dense | 1.0–1.8[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | Aggregate type matters: quartzite mixes 4.1–3.1, basalt 2.0–1.9 W/m/°C[7](https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/pccp/04127/appb.cfm) |
| Glass | 1.05[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | Window glass 0.96[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) |
| Water | 0.58[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity) | 0.61 in another reference table[8](https://www.yorku.ca/menary/courses/phys1012/2021/exeq.pdf) |
| Softwoods (fir, pine) | 0.12[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | Wood Handbook values for softwoods run about 0.08–0.14, varying up to 20%[9](https://research.fs.usda.gov/download/treesearch/37440.pdf) |
| Air | 0.0262[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | 26.4 mW m⁻¹ K⁻¹ at 300 K in the NIST gas table[10](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926438) |
| Silica aerogel | 0.02[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) | Among the lowest of solid insulators |
## Gases
Gases conduct heat poorly because their molecules are far apart; typical values are 0.01 to 0.2 W/(m K). The NIST compilation, given at 100 kPa or the saturation vapor pressure if lower, reports at 300 K: hydrogen 186.6, helium 155.7, air 26.4, nitrogen 25.9, oxygen 26.5, carbon dioxide 24.7, and xenon 5.5, all in mW m⁻¹ K⁻¹.[10](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926438) Hydrogen and helium stand far above the other gases because their light molecules move fast. Gas conductivity rises with temperature: air goes from 0.0262 W/(m K) at 25 °C to 0.0333 at 125 °C and 0.0398 at 225 °C.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) Pressure has little effect near atmospheric conditions; the difference between the 100 kPa value and the low-pressure limit is generally under 1 percent.[10](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926438)
## Why the values differ: electrons, phonons, and structure
Heat in a solid is carried by electrons, lattice vibrations (phonons), or both. In pure metals the electronic contribution dominates, which is why good electrical conductors are good thermal conductors; the Wiedemann-Franz law links the two, \( K_e/\sigma = LT \), with the Lorentz number \( L = \pi^2 k_B^2/(3e^2) = 2.44 \times 10^{-8} \) W Ω K⁻².[11](https://lampz.tugraz.at/~hadley/ss1/book/transport/thermal.php) In insulators and semiconductors the electronic contribution is negligible and phonons carry the heat.[11](https://lampz.tugraz.at/~hadley/ss1/book/transport/thermal.php)
**Temperature shapes the curve.** In a clean insulating crystal, conductivity rises roughly as \( T^3 \) at very low temperature (limited by defects and boundaries), peaks at intermediate temperature, and falls as \( 1/T \) at high temperature as phonon-phonon umklapp scattering intensifies.[4](https://acangi.github.io/lecture_notes_solids_thermal/lecture_10.html) Conductivity goes to zero as temperature approaches absolute zero.[11](https://lampz.tugraz.at/~hadley/ss1/book/transport/thermal.php)
**Structure and composition matter as much as the substance.** Alloying drops conductivity sharply: stainless steel conducts at 17 W/(m K) against roughly 46 W/(m K) for plain steel.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) • [8](https://www.yorku.ca/menary/courses/phys1012/2021/exeq.pdf) Moisture dominates soil and porous building values: ground conductivity climbs from 0.33 W/(m K) very dry to 1.4 very moist, and saturated sand reaches 2–4 W/(m K) against 0.15–0.25 dry.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) Isotopic purity matters in the best crystals: reducing the ¹³C content of diamond to ppm levels raised room-temperature conductivity to about 3100 W m⁻¹ K⁻¹.[5](https://arxiv.org/abs/2609.24191)
**Anisotropy.** In non-cubic crystals conductivity is a tensor, not a single number. Exfoliated black phosphorus conducts 86 ± 8 W/(m·K) along the zigzag direction but only 34 ± 4 W/(m·K) along armchair, a factor of about 2.5.[12](https://arxiv.org/abs/1510.00051) Single-crystal β-Ga₂O₃, relevant to high-power electronics, measures 22 ± 2.5 W/mK through-plane along [010] against 9.5 ± 1.8 W/mK near [100].[13](https://arxiv.org/abs/1809.04762) Wood is anisotropic too, with parallel-to-grain conduction higher than across the grain.[9](https://research.fs.usda.gov/download/treesearch/37440.pdf)
## Using the numbers
Conductivity converts directly to heat flow through Fourier's law. For a wall of thickness s, area A, and temperature difference dT, \( q = (k/s)\,A\,dT \). The Engineering ToolBox worked example compares 2 mm pot walls with an 80 °C difference: an aluminum wall (k = 215 W/(m K)) passes 8600 kW/m², while a stainless steel wall (k = 17 W/(m K)) passes 680 kW/m², a factor of about 12.6.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) The overall heat transfer through a real surface also depends on convective and radiative coefficients at both sides, not on conduction alone.[1](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) Insulation practice uses the inverse quantity, thermal resistivity, and R-values, which add thickness to the material property.[3](https://energyeducation.ca/encyclopedia/Thermal_conductivity)
## References
1. [The Engineering ToolBox (2003). Thermal Conductivity of Common Materials - Solids, Liquids and Gases.](https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html)
2. [IUPAC, Quantities, Units and Symbols in Physical Chemistry, table of physical quantities (2011).](https://publications.iupac.org/ci/2011/real_pages/July11CI/files/july11.pdf)
3. [Energy Education. Thermal conductivity.](https://energyeducation.ca/encyclopedia/Thermal_conductivity)
4. [Fundamentals of Solid-State Physics: Thermal Properties, Lecture 10, Thermal Conductivity.](https://acangi.github.io/lecture_notes_solids_thermal/lecture_10.html)
5. [Ultrahigh Thermal Conductivity in Isotopically Ultrapure Single-Crystal Diamond, arXiv.](https://arxiv.org/abs/2609.24191)
6. [McGraw-Hill Encyclopedia of Science and Technology, 10th ed., Thermal conduction in solids.](https://staibabussalamsula.ac.id/wp-content/uploads/2024/03/McGraw-Hill-Encyclopedia-of-Science-Technology-10th-Edition-Volume-18-SUR-TYP-staibabussalamsula.ac_.id_.pdf)
7. [FHWA, Computer-Based Guidelines for Concrete Pavements, Volume III, Technical Appendices (2006).](https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/pccp/04127/appb.cfm)
8. [York University PHYS 1012, Table of Specific Heats and Thermal Conductivities.](https://www.yorku.ca/menary/courses/phys1012/2021/exeq.pdf)
9. [USDA Forest Service, Wood Handbook, FPL-GTR-190.](https://research.fs.usda.gov/download/treesearch/37440.pdf)
10. [Marcia L. Huber, NIST. Thermal Conductivity of Gases.](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926438)
11. [TU Graz, Solid-State Physics, Thermal conductivity.](https://lampz.tugraz.at/~hadley/ss1/book/transport/thermal.php)
12. [Anisotropic Thermal Conductivity of Exfoliated Black Phosphorus, arXiv.](https://arxiv.org/abs/1510.00051)
13. [Three-Dimensional Anisotropic Thermal Conductivity Tensor of Single Crystalline β-Ga2O3, arXiv.](https://arxiv.org/abs/1809.04762)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Thermodynamics › Laws, states, and potentials › Equilibrium and state functions › State variables and conjugate pairs › Intensive and extensive variables*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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