Thermal Conductivity Converter (W/m·K, kcal/m·h·°C, BTU/h·ft·°F)

Understanding the thermal conductivity converter — concept and principle

Thermal conductivity (λ or k) tells you how easily heat flows through a material. Datasheets quote it as W/(m·K) (SI), kcal/(m·h·°C) (older Asian and European tables) or BTU/(h·ft·°F) and the "per inch" BTU·in/(h·ft²·°F) (US). This converter puts all four side by side so you can compare products and spec sheets fairly.

Four labels for one insulation value: the material is the same, only the rulers for heat, length and temperature change.

How it is calculated

Every value is first turned into W/(m·K), then divided by the size of the target unit: result = value × factor(from) ÷ factor(to).

1 kcal/(m·h·°C) = 1.163 W/(m·K) exactly (international-table kilocalorie: 4,186.8 J ÷ 3,600 s).
1 BTU/(h·ft·°F) ≈ 1.730734666 W/(m·K).
1 BTU·in/(h·ft²·°F) ≈ 0.1442278889 W/(m·K), which is the previous unit divided by 12.

A temperature step of 1 °C equals 1 K, so °C and K are interchangeable here; only the °F step (5/9 K) needs a factor.

The principle

Fourier's law says heat flow per area q = λ × ΔT ÷ d, where d is the thickness. So λ is "watts that pass through one square metre of a one-metre-thick slab for each kelvin of temperature difference". Other unit systems measure the same quantities with different rulers. The BTU·in form keeps area in square feet but thickness in inches, which is why it is 12 times smaller than BTU/(h·ft·°F).

Worked example

Default inputs: 0.035 W/(m·K), a typical value for mineral-wool or EPS insulation.

To kcal/(m·h·°C): 0.035 ÷ 1.163 = 0.030094583 kcal/(m·h·°C).
Check the ratio: 1 W/(m·K) = 1 ÷ 1.163 = 0.8598452279 kcal/(m·h·°C).
To BTU/(h·ft·°F): 0.035 ÷ 1.730734666 = 0.0202226261.
To BTU·in/(h·ft²·°F): 0.035 ÷ 0.1442278889 = 0.2426715129.

How to read the result

Lower λ means better insulation. Rough room-temperature values: still air about 0.026, high-performance foams (PIR/PUR) roughly 0.022–0.028, mineral wool and EPS about 0.031–0.040, wood about 0.1–0.2, concrete around 1–2, steel around 50 and copper around 400 W/(m·K). Use the manufacturer's declared value for real products.

Common mistakes

Confusing thermal conductivity (a material property) with thermal conductance or U-value (which already include thickness).
Using the thermochemical calorie (4.184 J) instead of the international calorie; this gives 1.1622 instead of 1.163.
Mixing BTU/(h·ft·°F) with the per-inch form and getting a 12-fold error.

Keep in mind: Conversions are exact, but λ itself depends on temperature and moisture. For real building or equipment design, use declared values tested to recognised standards and consult a qualified engineer and local codes.

Frequently asked questions

How do I convert kcal/(m·h·°C) to W/(m·K)?

Multiply by 1.163. For example, 0.03 kcal/(m·h·°C) is 0.0349 W/(m·K).

What is a good thermal conductivity for insulation?

Most building insulation sits below about 0.040 W/(m·K); the lower the number, the thinner the layer needed for the same performance.

How does λ relate to R-value?

For a single uniform layer, R = thickness (m) ÷ λ. 100 mm of a 0.035 material gives R ≈ 2.86 m²·K/W.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Guide for the Use of the SI, Appendix B · ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.