Understanding BTU per Hour-Foot-Degree Fahrenheit to Watts per Metre-Kelvin Conversion
The BTU per hour-foot-degree Fahrenheit, BTU/(h-ft-°F), is the imperial unit of thermal conductivity used throughout North American building science and HVAC engineering to rate how readily a material conducts heat. The watt per metre-kelvin, W/(m-K), is its SI counterpart and the value quoted on international material datasheets for insulation, metals, and glazing. Converting between them is essential when comparing US-published R-values or k-values against European product specifications.
Conversion Formula
To convert BTU per Hour-Foot-Degree Fahrenheit to Watts per Metre-Kelvin, multiply by this factor:
Step-by-Step Example
Convert 25 BTU per Hour-Foot-Degree Fahrenheit to Watts per Metre-Kelvin.
How to Convert BTU per Hour-Foot-Degree Fahrenheit to Watts per Metre-Kelvin
Follow these steps to move an imperial thermal-conductivity value onto the SI scale.
- Note the starting value: Write down the conductivity in BTU/(h-ft-°F), for example 25.
- Apply the factor: Multiply by 1.730735, the exact number of W/(m-K) in one BTU/(h-ft-°F).
- Compute the product: .
- State the result: 25 BTU/(h-ft-°F) equals 43.2684 W/(m-K).
BTU per Hour-Foot-Degree Fahrenheit to Watts per Metre-Kelvin conversion table
| BTU per Hour-Foot-Degree Fahrenheit (BTU/(h-ft-°F)) | Watts per Metre-Kelvin (W/(m-K)) |
|---|---|
| 0 | 0 |
| 1 | 1.730735 |
| 2 | 3.461469 |
| 3 | 5.192204 |
| 4 | 6.922939 |
| 5 | 8.653673 |
| 6 | 10.38441 |
| 7 | 12.11514 |
| 8 | 13.84588 |
| 9 | 15.57661 |
| 10 | 17.30735 |
| 15 | 25.96102 |
| 20 | 34.61469 |
| 25 | 43.26837 |
| 30 | 51.92204 |
| 40 | 69.22939 |
| 50 | 86.53673 |
| 60 | 103.8441 |
| 70 | 121.1514 |
| 80 | 138.4588 |
| 90 | 155.7661 |
| 100 | 173.0735 |
| 150 | 259.6102 |
| 200 | 346.1469 |
| 250 | 432.6837 |
| 300 | 519.2204 |
| 400 | 692.2939 |
| 500 | 865.3673 |
| 600 | 1038.441 |
| 700 | 1211.514 |
| 800 | 1384.588 |
| 900 | 1557.661 |
| 1000 | 1730.735 |
| 2000 | 3461.469 |
| 3000 | 5192.204 |
| 4000 | 6922.939 |
| 5000 | 8653.673 |
| 10000 | 17307.35 |
| 25000 | 43268.37 |
| 50000 | 86536.73 |
| 100000 | 173073.5 |
| 250000 | 432683.7 |
| 500000 | 865367.3 |
| 1000000 | 1730735 |
What is the BTU per Hour-Foot-Degree Fahrenheit?
The BTU per hour-foot-degree Fahrenheit is the customary unit of thermal conductivity in the United States and other imperial-using industries. It measures the heat, in British thermal units per hour, conducted through a material one foot thick for each degree Fahrenheit of temperature difference.
Definition
One BTU per hour-foot-degree Fahrenheit corresponds to a heat flow of one BTU per hour through a one-foot thickness for a one-degree-Fahrenheit temperature difference. In coherent SI units:
The exact value depends on the definition of the British thermal unit used; based on the International Table BTU (1055.05585262 J), 1 BTU/(h·ft·°F) = 1.730734666 W/(m·K).
Origin and History
The unit is built from three imperial and customary quantities: the British thermal unit (the heat needed to raise one pound of water by one degree Fahrenheit), the foot, and the Fahrenheit degree. It became the standard for U.S. thermal-property data long before SI was widely adopted and persists in North American engineering practice.
Law and Notable Facts
While the SI watt per metre-kelvin is preferred internationally, BTU/(h·ft·°F) remains entrenched in U.S. mechanical, HVAC and building codes. A useful benchmark: pure copper conducts at about 231 BTU/(h·ft·°F), roughly 400 times better than the fibreglass batts used to insulate walls.
Real-World Examples and Conversions
- Structural steel is about 26 BTU/(h·ft·°F); concrete roughly 0.8–1.0.
- Water conducts at about 0.35 BTU/(h·ft·°F).
- 1 BTU/(h·ft·°F) ≈ 1.7307 W/(m·K) ≈ 1730.7 mW/(m·K).
- Conversely, 1 W/(m·K) ≈ 0.5778 BTU/(h·ft·°F).
What is the Watt per Metre-Kelvin?
The watt per metre-kelvin is the SI coherent unit of thermal conductivity, quantifying how readily a material conducts heat. It expresses the rate of heat flow through a material per unit thickness for each degree of temperature difference across it.
Definition
A material has a thermal conductivity of one watt per metre-kelvin if a temperature gradient of one kelvin per metre drives a heat flux of one watt per square metre through it:
Because a temperature difference of one kelvin equals one degree Celsius, W/(m·K) and W/(m·°C) are numerically identical.
Origin and History
The unit follows directly from Fourier's law of heat conduction, published by Joseph Fourier in his 1822 Théorie analytique de la chaleur, which states that heat flux is proportional to the negative temperature gradient. The proportionality constant is the thermal conductivity, and it acquired its coherent SI form once the watt, metre and kelvin were established within the International System of Units.
Law and Notable Facts
The watt per metre-kelvin is the standard SI unit for material property tables worldwide. Thermal conductivity spans an enormous range: from aerogels near 0.02 W/(m·K), among the best solid insulators, to diamond at roughly 2000 W/(m·K), the highest of any bulk natural material.
Real-World Examples and Conversions
- Still air conducts heat at about 0.026 W/(m·K); water at about 0.6 W/(m·K).
- Common window glass is near 1.0 W/(m·K); stainless steel about 15 W/(m·K).
- Copper reaches roughly 400 W/(m·K), which is why it is used for heat sinks.
- 1 W/(m·K) = 1000 mW/(m·K) ≈ 0.5778 BTU/(h·ft·°F).
Frequently Asked Questions
What is the factor to convert BTU/(h-ft-°F) to W/(m-K)?
One BTU per hour-foot-degree Fahrenheit equals 1.730735 watts per metre-kelvin. Multiply any value in BTU/(h-ft-°F) by 1.730735 to obtain W/(m-K).
How do I convert W/(m-K) back to BTU/(h-ft-°F)?
Divide by 1.730735, or equivalently multiply by 0.5777893. For example, 10 W/(m-K) equals about 5.778 BTU/(h-ft-°F).
Why does this conversion matter in building insulation?
US insulation and glazing datasheets often quote thermal conductivity in BTU/(h-ft-°F), while international standards (ISO, EN) use W/(m-K). Converting lets engineers compare products on a common basis when specifying wall assemblies or curtain walls.
Is 25 BTU/(h-ft-°F) a high thermal conductivity?
Yes; 25 BTU/(h-ft-°F) is about 43.27 W/(m-K), typical of a moderately conductive metal like cast iron, and far above any insulation material, which sits well below 0.03 W/(m-K).
Does this differ from the U-value or R-value?
Yes. Thermal conductivity (k) is a material property independent of thickness, whereas R-value and U-value describe an assembly of a given thickness. Convert the k-value first, then divide or multiply by thickness for R or U.
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Complete BTU per Hour-Foot-Degree Fahrenheit conversion table
| Unit | Result |
|---|---|
| Watts per Metre-Kelvin (W/(m-K)) | 1.730735 W/(m-K) |
| Milliwatts per Metre-Kelvin (mW/(m-K)) | 1730.735 mW/(m-K) |
| BTU-Inch per Hour-Square Foot-Degree Fahrenheit (BTU-in/(h-ft2-°F)) | 12 BTU-in/(h-ft2-°F) |
| Calories per Second-Centimetre-Degree Celsius (cal/(s-cm-°C)) | 0.004136555 cal/(s-cm-°C) |