Thermal Conductivity Calculator

Solve Fourier's law of heat conduction (q = k·A·ΔT/L) in any direction, build series and parallel thermal resistances, convert conductivity units, look up 34 materials, and back out k from guarded hot plate measurements. Everything runs in your browser.

Scope note: this page is about conduction — the material property k (thermal conductivity). For convection at surfaces and whole-wall U-values with air films, use our heat transfer coefficient calculator instead.

Fourier's law: q = k · A · ΔT / L — steady, one-dimensional conduction through a flat slab.

Solve for

Heat rate q

q unit

Conductivity k

k unit

Material preset (fills k)

Area A

A unit

Temperature difference ΔT

ΔT unit

Thickness L

L unit

Steady-state, one-dimensional conduction — ignores edge effects, convection, and radiation. Sign of q follows ΔT.

Series: stacked layers

Same heat flows through every layer. R_total = Σ L_i/(k_i·A).

Wall area A (m²)

ΔT across wall (K)

Parallel: side-by-side paths

Each path sees the full ΔT (e.g. studs vs. insulation bays). 1/R_total = Σ 1/R_i.

ΔT across paths (K)

Thermal conductivity of materials

Representative room-temperature (~20 °C) values in W/(m·K), spot-checked against the CRC Handbook of Chemistry & Physics. Real products vary — check manufacturer datasheets for certified values. Lower k = better insulator.

0.015 aerogel (best insulator)2000 diamond

Filter materials

Material	Category	k, W/(m·K)	k, BTU/(hr·ft·°F)	Note
Diamond	Metals & high conductors	2000	1156	natural, ~2200 max
Silver	Metals & high conductors	429	247.9	highest of the metals
Copper	Metals & high conductors	401	231.7	CRC 401
Gold	Metals & high conductors	317	183.2
Aluminium	Metals & high conductors	237	136.9	CRC 237
Tungsten	Metals & high conductors	174	100.5
Zinc	Metals & high conductors	116	67.02
Brass	Metals & high conductors	109	62.98	alloy, varies
Nickel	Metals & high conductors	91	52.58
Iron (pure)	Metals & high conductors	80	46.22	CRC 80.2
Platinum	Metals & high conductors	72	41.6
Bronze	Metals & high conductors	60	34.67	alloy, varies
Cast iron	Metals & high conductors	55	31.78
Carbon steel	Metals & high conductors	50	28.89
Lead	Metals & high conductors	35	20.22
Titanium	Metals & high conductors	22	12.71
Stainless steel	Metals & high conductors	16	9.245	austenitic ~16
Mercury	Liquids	8.3	4.796	liquid metal
Quartz	Other solids	6.5	3.756	crystalline
Marble	Other solids	2.8	1.618
Ice (0 C)	Other solids	2.2	1.271	at 0 C
Concrete	Other solids	1.7	0.9822	dense, varies
Glass (soda-lime)	Other solids	1	0.5778	window glass
Brick	Other solids	0.72	0.416	common clay
Water (20 C)	Liquids	0.6	0.3467	0.598 at 20 C
Polyethylene (HDPE)	Other solids	0.42	0.2427
PTFE (Teflon)	Other solids	0.25	0.1444
Rubber	Other solids	0.15	0.08667	natural
Wood (pine)	Other solids	0.12	0.06933	across grain
Engine oil	Liquids	0.145	0.08378	at 20 C
Glass wool insulation	Insulation & gases	0.04	0.02311	still air trapped
Air (still, 20 C)	Insulation & gases	0.026	0.01502	0.0262 at 300 K
Polyurethane foam	Insulation & gases	0.025	0.01444	rigid board
Silica aerogel	Insulation & gases	0.015	0.008667	among the lowest solids

Thermal conductivity units

Value

From

Quick converters

Heat rate

From

Thickness

From

Temperature difference

From

Area

From

k from experiment: guarded hot plate

Reference lab method (ASTM C177): clamp a flat specimen of thickness L between a metered hot plate and a cold plate; at steady state, k = q·L/(A·ΔT). Use the metered (guarded) area for A.

Heater power q

q unit

Specimen thickness L

L unit

Metered area A

A unit

Steady ΔT across specimen

ΔT unit

How the math works

Fourier's law

q = k · A · ΔT / L

Heat conducted through a flat slab is proportional to the material's conductivity k, the area A, and the temperature difference ΔT, and inversely proportional to the thickness L. Units must be consistent: with k in W/(m·K), A in m², ΔT in K, and L in m, q comes out in watts.

Worked example: a copper plate (k = 401 W/(m·K)), area 0.01 m², with 50 K across 0.02 m of thickness, conducts q = 401 × 0.01 × 50 / 0.02 = 10,025 W. Swap copper for glass (k = 1.0) and the same slab passes only 25 W.

Thermal resistance

R = L / (k · A) in K/W · r = L / k in m²·K/W (per unit area)

Resistance is the conduction analog of electrical resistance: q = ΔT/R. Stack layers in series by adding (brick 0.1 m/k=0.72 plus insulation 0.05 m/k=0.04 gives r = 0.139 + 1.25 = 1.389 m²·K/W); combine side-by-side paths in parallel with 1/R = Σ 1/R_i (paths of 2 and 3 K/W give 1.2 K/W).

Unit conversions

The exact bridges: 1 W/(m·K) = 0.5778 BTU/(hr·ft·°F); 1 BTU/(hr·ft·°F) = 1.7307 W/(m·K). For temperature differences, 1 °F = 5/9 K, so ΔT converts by simple scaling (no 32 °F offset).

Measuring k: guarded hot plate

The reference method (ASTM C177) measures heater power q through a specimen of known L and metered area A at a steady ΔT, then k = q·L/(A·ΔT). Example: 2.5 W through 0.025 m thickness, 0.04 m² area, 10 K drop → k = 0.15625 W/(m·K).

Frequently asked questions

What is thermal conductivity?

Thermal conductivity (k, also written lambda) is the material property that says how easily heat flows through a solid, liquid, or gas by conduction. It is measured in watts per meter-kelvin, W/(m·K). High-k materials like copper (401 W/(m·K)) move heat quickly; low-k materials like polyurethane foam (0.025 W/(m·K)) resist it and work as insulation.

How do you use Fourier's law to calculate heat transfer?

Fourier's law is q = k·A·ΔT/L: multiply the conductivity k by the area A and the temperature difference ΔT, then divide by the thickness L. Example: a copper plate with k = 401 W/(m·K), area 0.01 m², ΔT = 50 K across 0.02 m of thickness carries q = 401 × 0.01 × 50 / 0.02 = 10,025 W. This calculator solves the equation in all four directions.

How do I find k if I measured the heat rate?

Rearrange Fourier's law to k = q·L/(A·ΔT). Example: a slab passes q = 500 W through A = 0.25 m² with ΔT = 40 K across L = 0.1 m, so k = 500 × 0.1 / (0.25 × 40) = 50 / 10 = 5 W/(m·K), roughly the conductivity of a ceramic tile.

What is thermal resistance and how is it calculated?

Thermal resistance R = L/(k·A) tells you how much temperature difference you need to push one watt through a slab, in K/W. Example: a 0.2 m brick wall (k = 0.72 W/(m·K)) with area 10 m² has R = 0.2 / (0.72 × 10) = 0.0278 K/W, so a 30 K difference drives about 1,080 W through it (q = ΔT/R).

How do you combine resistances in series and in parallel?

In series (layers stacked), add them: R_total = R1 + R2. Example: 0.1 m of brick (k = 0.72) plus 0.05 m of insulation (k = 0.04) gives per-unit-area r = 0.1/0.72 + 0.05/0.04 = 0.139 + 1.25 = 1.389 m²·K/W; over 10 m² that is R = 0.139 K/W, so ΔT = 30 K drives 216 W. In parallel (side-by-side paths), add conductances instead: 1/R_total = 1/R1 + 1/R2, so paths of 2 and 3 K/W combine to 1.2 K/W.

How do you convert W/(m·K) to BTU/(hr·ft·°F)?

Multiply by 0.5778, because 1 W/(m·K) = 0.5778 BTU/(hr·ft·°F). Example: water at 0.6 W/(m·K) equals 0.6 × 0.5778 = 0.347 BTU/(hr·ft·°F). The reverse factor is 1.7307: 1 BTU/(hr·ft·°F) = 1.7307 W/(m·K). The unit converter tab on this page handles these plus BTU·in/(hr·ft²·°F), cal/(s·cm·°C), and kcal/(hr·m·°C).

What is the guarded hot plate method?

The guarded hot plate (ASTM C177) is the reference lab method for measuring k of insulation: a heater of known power q presses a flat specimen of thickness L and metered area A, and once the temperature drop ΔT across it is steady, k = q·L/(A·ΔT). Example: q = 2.5 W, L = 0.025 m, A = 0.04 m², ΔT = 10 K gives k = 2.5 × 0.025 / (0.04 × 10) = 0.15625 W/(m·K). The guard ring around the metered area forces one-dimensional heat flow so Fourier's law applies exactly.

What is the difference between thermal conductivity (k) and the heat transfer coefficient (h)?

They describe different physics. Thermal conductivity k is a material property for conduction inside solids (Fourier's law, q = k·A·ΔT/L, units W/(m·K)), which is what this calculator covers. The convective heat transfer coefficient h describes heat carried away by a fluid at a surface (Newton's law of cooling, q = h·A·ΔT, units W/(m²·K)). For wall U-values and convection coefficients, use our heat transfer coefficient calculator instead.

Does thermal conductivity change with temperature?

Yes. For pure metals k falls slightly as temperature rises (more lattice vibration scatters electrons); for gases k rises with temperature; for insulation the change is small near room temperature. The table values on this page are representative room-temperature (~20 °C) figures, so use manufacturer datasheets for cryogenic or high-temperature design.

What k value makes a good insulator?

Look for k below about 0.05 W/(m·K): polyurethane foam at 0.025, glass wool at 0.04, and silica aerogel at 0.015 are all excellent. Still air itself is 0.026 W/(m·K), which is why insulation works by trapping tiny pockets of air. By contrast, structural materials are far higher: brick 0.72, glass 1.0, concrete 1.7.

Why is copper 401 W/(m·K) but stainless steel only 16?

In metals, heat is carried mainly by free electrons. Copper's electron structure lets them move almost freely, giving 401 W/(m·K) (CRC Handbook). Stainless steel's alloyed, disordered lattice scatters electrons, dropping k to about 16 W/(m·K). That is why copper is used for heat sinks and cookware bases while stainless handles and fasteners stay cooler.

What are typical thermal conductivity values for common materials?

Representative room-temperature values in W/(m·K): diamond ~2000, silver 429, copper 401, aluminium 237, iron 80, carbon steel 50, lead 35, glass 1.0, water 0.6, wood (pine) 0.12, engine oil 0.145, glass wool 0.04, still air 0.026, polyurethane foam 0.025, silica aerogel 0.015. The full 34-material table with BTU/(hr·ft·°F) equivalents is on this page.

Thermal Conductivity Calculator

Series: stacked layers

Parallel: side-by-side paths

Thermal conductivity of materials

Thermal conductivity units

Quick converters

k from experiment: guarded hot plate

How the math works

Fourier's law

Thermal resistance

Unit conversions

Measuring k: guarded hot plate

Frequently asked questions

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