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Thermal Conductivity Converter

Converters · Added

Thermal conductivity says how readily a material passes heat: watts through a metre of it for each kelvin of temperature difference. The awkward part is that insulation datasheets in the United States quote it per inch of thickness over a square foot, which is a twelfth of the per-foot figure — and mixing the two up is how a specification ends up out by a factor of twelve.

Result

1 W/(m·K) in btu inch per hour square foot degree fahrenheit

6.9334718 BTU·in/(h·ft²·°F)

1 W/(m·K) = 6.9334718 BTU·in/(h·ft²·°F)

The same value in every unit

Watt per metre kelvin (W/(m·K))
1
Milliwatt per metre kelvin (mW/(m·K))
1,000
Kilowatt per metre kelvin (kW/(m·K))
0.001
Watt per centimetre kelvin (W/(cm·K))
0.01
BTU per hour foot degree Fahrenheit (BTU/(h·ft·°F))
0.577789317
BTU inch per hour square foot degree Fahrenheit (BTU·in/(h·ft²·°F))
6.9334718
BTU per second foot degree Fahrenheit (BTU/(s·ft·°F))
0.000160497032
Kilocalorie per hour metre degree Celsius (kcal/(h·m·°C))
0.859845228
Calorie per second centimetre degree Celsius (cal/(s·cm·°C))
0.00238845897

Every factor is derived from exact definitions: the international-table BTU is 1055.05585262 J, the foot is 0.3048 m, the inch is 0.0254 m, and one Fahrenheit degree is five ninths of a kelvin. The IT calorie of 4.1868 J gives the kcal and cal entries.

How to use the thermal conductivity converter

  1. 1Enter the conductivity figure and pick the unit it is quoted in.
  2. 2Pick the unit you want it in. The full table underneath shows it in all of them at once.
  3. 3Watch the two BTU entries: per foot and per inch differ by twelve, and datasheets use both.
  4. 4Use the swap button to reverse the direction without retyping anything.

Examples

A datasheet in imperial

Input
0.25 BTU·in/(h·ft²·°F)
Result
0.0361 W/(m·K) — a typical rigid foam board

Read as BTU/(h·ft·°F) by mistake it becomes 0.433 W/(m·K), which is plaster rather than insulation.

Copper

Input
401 W/(m·K)
Result
231.7 BTU/(h·ft·°F)

Roughly ten thousand times the conductivity of the foam above, which is the range this quantity spans.

An older European figure

Input
1 kcal/(h·m·°C)
Result
1.163 W/(m·K) exactly

The kilocalorie units are still on plenty of legacy building documentation.

About the thermal conductivity converter

What the quantity actually measures

Conductivity is defined by Fourier's law: the heat flowing through a slab is the conductivity times the area times the temperature difference, divided by the thickness. Rearranged, the conductivity is watts per metre of thickness per kelvin of difference, which is where W/(m·K) comes from. It is a property of the material and not of the object — a thick wall and a thin one of the same brick have the same conductivity and very different heat losses.

The range is enormous. Still air is around 0.026 W/(m·K), the best foams sit near 0.020 because they trap gases that conduct even less, timber is around 0.13, glass about 1, concrete around 1.5, steel around 50 and copper 401. Insulation works by approaching still air as closely as possible while staying a solid you can build with.

Why the imperial units survive

The BTU-inch form persists because it makes insulation datasheets readable in whole numbers, and because R-values in the United States are quoted in units derived from it. Neither is going to change, so anyone reading specifications across markets needs the conversion routinely — a European lambda value in W/(m·K) and an American R-value per inch are describing the same physical property through two entirely different arrangements of units.

The calorie-based units are a different survival. They come from an era when heat was measured in calories rather than joules, and they linger in older building and process engineering documentation, particularly in continental Europe and Japan. One kilocalorie per hour metre degree Celsius is exactly 1.163 W/(m·K), the ratio of the international-table calorie to the hour.

Frequently asked questions

What is the difference between BTU per hour foot and BTU inch per hour square foot?
A factor of twelve, and nothing else. Both express the same quantity; the first uses a foot of thickness, the second an inch. The inch form dominates North American insulation datasheets because insulation is thin and quoting per foot would give inconveniently small numbers. Confusing the two is the single most common error in this conversion, and it turns an insulator into a masonry product or the other way round.
How does conductivity relate to R-value and U-value?
R-value is thickness divided by conductivity, so it is a property of a particular board rather than of the material. U-value is the reciprocal of the total R-value of an assembly, including the surface air films and every layer. This converter handles conductivity only — turning it into an R-value needs a thickness, and turning that into a U-value needs the whole construction, which is a building calculation rather than a unit conversion.
Does conductivity change with temperature?
Yes, for everything, and for some materials sharply. Insulation conducts more when warm, which is why datasheets state the temperature a figure was measured at — usually 10 °C or 25 °C in building work. Metals mostly conduct slightly less as they heat. A conductivity figure without a stated temperature is incomplete, and this converter changes units without knowing anything about temperature.
Why do the factors carry so many digits?
Because they are exact rather than measured. The international-table BTU is defined as 1055.05585262 J, the foot as 0.3048 m and the inch as 0.0254 m, all exactly, and one Fahrenheit degree is exactly five ninths of a kelvin. Every factor here is that arithmetic carried out rather than a decimal copied from a chart, so a value converted from one unit to another and back returns exactly what you started with.
Is this the same as thermal diffusivity?
No. Conductivity is about steady-state heat flow — how much gets through once things have settled. Diffusivity is conductivity divided by density and specific heat, and it describes how fast a temperature change travels through a material rather than how much heat does. They have different units and are not interconvertible without knowing the material's density and heat capacity, so they are kept apart here.

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