Introduction
Thermal conductivity (k) is how well a material conducts heat, measured in W/(m·K) in SI or BTU·in/(hr·ft²·°F) in US customary. Metals are high (copper 401, aluminum 237, silver 429); insulators low (foam 0.025, wood 0.15, glass 1.0); diamond is the highest of any natural solid (~2,200). This calculator swaps between SI and US imperial units. The 17,000× range between copper and polyurethane foam spans the entire engineering universe of 'move heat' versus 'stop heat' — picking the right material is the first design choice in any thermal system.
Why thermal conductivity units exist and how they diverged
Copper is ~13,000× better at conducting heat than polyurethane foam. This range spans most engineering design: we use metals to move heat (heat sinks, cookware, radiators), insulators to stop it (building walls, refrigerators, clothing), and specialty materials (diamond for LED spreaders, silicon nitride for substrates, aerogels for vacuum insulation) where the extremes matter. Values are for pure materials at ~25°C; alloys and composites vary — stainless steel is 16 W/(m·K), two orders of magnitude below pure iron, because alloying elements scatter electrons and phonons. Porosity matters enormously: solid brick is ~0.8 W/(m·K), foamed concrete is 0.15.
In metals, thermal conduction is dominated by free electrons — the same electrons that carry electric current. The Wiedemann-Franz law states that κ/σT is nearly constant across pure metals at room temperature (the Lorenz number, ~2.44 × 10⁻⁸ W·Ω/K²). That's why good electrical conductors are generally good thermal conductors: copper wins on both. In insulators, heat moves by phonons (lattice vibrations) instead — diamond's exceptional thermal conductivity comes from ultra-stiff covalent bonds carrying phonons at high speed with little scattering. Thermal conductivity varies with temperature: for metals it usually decreases with T (above the Debye temperature); for ceramics and amorphous solids it can go either way.
How to convert thermal conductivity
W/(m·K) to BTU·in/(hr·ft²·°F): divide by 0.1442. W/(m·K) to BTU/(hr·ft·°F): divide by 1.731. To estimate heat flow through a wall: Q = k × A × ΔT / thickness (Fourier's law). A 10 cm concrete wall at 20°C/0°C across 10 m² transmits Q = 1.3 × 10 × 20 / 0.1 = 2,600 W. R-value (in SI) = thickness / k; a 10 cm concrete wall has R = 0.077 m²·K/W. Building insulation R-values in the US are (hr·ft²·°F)/BTU; R-30 ≈ 5.3 m²·K/W.
Units supported by this thermal conductivity calculator
- W/(m·K)
- W/(cm·K)
- BTU·in/(hr·ft²·°F)
- BTU/(hr·ft·°F)
- cal/(s·cm·°C)
- kcal/(hr·m·°C)
- kJ/(hr·m·K)
- mW/(m·K)
Common thermal conductivity conversion mistakes
- k vs R-value. R-value (insulation) is thickness / k, with units hr·ft²·°F/BTU (US) or m²·K/W (SI). An R-30 batt is about 5 inches of fiberglass. Low k × thickness = high R.
- k vs U-value. U-value is 1/R — conductance through a wall. A 'U = 0.1 W/(m²·K)' window is the reciprocal of R = 10 m²·K/W.
- Units with hidden inches. BTU·in/(hr·ft²·°F) uses inches for thickness but feet squared for area — multiply by 12 for foot-thickness. Easy mistake.
- Convection and radiation absent. Thermal conductivity handles pure conduction. Real walls also lose heat by convection (air movement) and radiation (IR emission), both of which can dominate at thin boundaries. Building envelope design uses overall U-values that include all three.
- Anisotropic materials. Wood conducts 2× as well along the grain as across it. Composites, graphite, and rolled metals have directional thermal conductivity.
- Temperature-dependent. Room-temperature values are standard references; cryogenic or high-T values can differ 2-10×. Always check k(T) for extreme temperatures.
Real-world thermal conductivity examples
- Aerogel: 0.015 W/(m·K) — one of the best known insulators.
- Air (still): 0.026 W/(m·K).
- Polyurethane rigid foam: 0.024-0.028 W/(m·K).
- Polystyrene (XPS): 0.029-0.033 W/(m·K).
- Fiberglass batt: 0.040 W/(m·K).
- Water: 0.6 W/(m·K).
- Wood (softwood, dry, along grain): 0.12-0.15 W/(m·K).
- Drywall (gypsum): 0.17 W/(m·K).
- Brick (common): 0.7-0.8 W/(m·K).
- Glass: 1.05 W/(m·K).
- Concrete (dense): 1.3-1.7 W/(m·K).
- Ice: 2.2 W/(m·K) (higher than water).
- Stainless steel 304: 16 W/(m·K).
- Lead: 35 W/(m·K).
- Iron: 80 W/(m·K).
- Brass: 109 W/(m·K).
- Aluminum (pure): 237 W/(m·K).
- Gold: 318 W/(m·K).
- Copper: 401 W/(m·K).
- Silver: 429 W/(m·K) (highest metal at room temp).
- Graphite (along basal plane): 1,950 W/(m·K).
- Diamond: 2,200 W/(m·K).
- Isotopically pure C-12 diamond: up to 3,300 W/(m·K).
Tips for accurate thermal conductivity conversion
- For cookware, copper is best for heat control but expensive; aluminum is the everyday compromise; stainless steel is worst (needs a copper or aluminum-clad base for uniform heating).
- For heat sinks, aluminum is standard; copper for high-performance CPUs and GPUs; vapor chambers and heat pipes for ultra-high density.
- For building insulation, air is the active ingredient in most insulators — the foam or fiber just keeps air from convecting. Vacuum-insulated panels push down to 0.004 W/(m·K).
- For electronics TIM (thermal interface material), pads and pastes bridge the gap between chip and heatsink; conductivity typically 1-10 W/(m·K).
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