Thermal Resistance Converter
Convert thermal resistance between kelvin per watt and the imperial units degree Fahrenheit hour per Btu and degree Fahrenheit second per Btu, with International Table and thermochemical Btu versions. Enter a value, pick the units and read the result instantly.
Select two units to see the conversion factor.
This is the thermal resistance of a particular object (K/W). It is not the area-based R-value (m²·K/W, ft²·°F·h/Btu), not thermal resistivity and not thermal conductivity.
Thermal Resistance Formula
Thermal resistance R is the temperature difference needed to drive one unit of heat flow through an object: R = ΔT ÷ Q. The base unit is kelvin per watt. A Fahrenheit degree is 5/9 K, an hour is 3,600 s, and 1 Btu (IT) = 1,055.05585262 J, so 1 °F·h/Btu (IT) = (5/9 × 3,600) ÷ 1,055.05585262 K/W ≈ 1.8955 K/W. Result = value × (source factor ÷ target factor).
What Is Thermal Resistance?
Thermal resistance measures how strongly one specific object or layer opposes heat flow. If a heat sink has a resistance of 2 K/W and carries 10 W, the temperature across it rises by 2 × 10 = 20 K. In general R = ΔT ÷ Q, where ΔT is a temperature difference and Q is the heat flow in watts.
For a flat slab of material, R = L ÷ (k × A), where L is thickness, k is thermal conductivity and A is area. A bigger area lowers R and a thicker slab raises it. For this reason thermal resistance of an object differs from three similar quantities: thermal conductivity (a material property, W/(m·K)), thermal resistivity (the reciprocal of conductivity, m·K/W) and the R-value used in building insulation (resistance per unit area, m²·K/W). Those are not converted by this tool.
The degrees in these units are temperature intervals, so no offsets are used. The Btu appears in two definitions, International Table (IT) and thermochemical (th), differing by about 0.07 %.
Supported Units
This converter supports the following 5 thermal resistance units. The last column gives the value of each unit in the base unit K/W.
| Unit | Symbol | Value in K/W |
|---|---|---|
| Kelvin per watt | K/W | 1 |
| Degree Fahrenheit hour per Btu (IT) | °F·h/Btu (IT) | 1.895634241 |
| Degree Fahrenheit hour per Btu (th) | °F·h/Btu (th) | 1.89690283 |
| Degree Fahrenheit second per Btu (IT) | °F·s/Btu (IT) | 0.0005265650668 |
| Degree Fahrenheit second per Btu (th) | °F·s/Btu (th) | 0.0005269174526 |
How to Use the Thermal Resistance Converter
- Type the thermal resistance in the value box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
- Find the size of each unit in the common base unit (K/W): 1 °F·h/Btu (IT) = 1.895634241 K/W and 1 K/W = 1 K/W.
- Divide the source size by the target size to get the conversion factor: 1.895634241 ÷ 1 = 1.895634241, so 1 °F·h/Btu (IT) = 1.895634241 K/W.
- Multiply by your value: 0.5 °F·h/Btu (IT) × 1.895634241 = 0.9478171203 K/W.
Thermal resistance cannot be negative, so only zero and positive values are accepted.
Common Thermal Resistance Conversions
- Imperial to metric: 0.5 °F·h/Btu equals 0.9478171203 K/W.
- Metric to imperial: 2 K/W equals 1.055055853 °F·h/Btu.
- Per second: 1 K/W equals 1,899.100535 °F·s/Btu.
- IT vs thermochemical Btu: 1 °F·h/Btu (IT) equals 0.9993312315 °F·h/Btu (th).
- Heat sink: a 0.3 K/W heat sink is 0.1582583779 °F·h/Btu.
Kelvin per Watt to °F·h/Btu
Quick reference for frequently used values
| K/W | °F·h/Btu (IT) | °F·h/Btu (th) | °F·s/Btu (IT) |
|---|---|---|---|
| 0.1 | 0.05275279263 | 0.05271751322 | 189.9100535 |
| 0.5 | 0.2637639632 | 0.2635875661 | 949.5502674 |
| 1 | 0.5275279263 | 0.5271751322 | 1,899.100535 |
| 2 | 1.055055853 | 1.054350264 | 3,798.201069 |
| 5 | 2.637639632 | 2.635875661 | 9,495.502674 |
| 10 | 5.275279263 | 5.271751322 | 18,991.00535 |
Where and Why This Conversion Is Used
- Electronics cooling: heat sink and junction-to-case resistance in K/W.
- Heat exchangers and insulation of components: resistance of a particular pipe wall or wall section.
- Data sheets from different regions: metric and US customary values.
- Thermal modelling: series and parallel resistances need one unit.
Limitations
- The values describe a specific object; they do not transfer to another size of the same material.
- No area is involved, so R-values (area-normalised) cannot be converted here.
- Temperature is treated as an interval only.
Important Differences Between Related Heat Quantities
- Specific heat capacity, J/(kg·K): energy needed to warm one kilogram of a material by one kelvin. It is not the total heat capacity of an object (J/K), which also depends on how much material there is, and it is not molar heat capacity (J/(mol·K)).
- Heat density, J/m²: heat or radiant energy delivered per unit area over a period. Heat flux density, W/m² is the rate of that transfer, energy per area per time. They are not interchangeable.
- Heat flux density, W/m² is heat flow per area. The heat transfer coefficient, W/(m²·K) is heat flux per kelvin of temperature difference, so heat flux = coefficient × temperature difference.
- Thermal conductivity, W/(m·K) is a material property. The heat transfer coefficient is a property of a surface or an assembly, and thermal resistance, K/W belongs to one specific object. Resistance is not conductivity or resistivity: R = L ÷ (k × A) for a slab.
- Temperature interval vs temperature: a difference of 10 °C is a difference of 18 °F. A reading of 10 °C is 50 °F. Every unit here that contains “per degree” or “K” uses intervals, so no offset is ever applied.
Each converter changes units within one quantity only. Related tools: Temperature Interval, Thermal Expansion, Thermal Resistance, Thermal Conductivity, Specific Heat Capacity, Heat Density, Heat Flux Density, Heat Transfer Coefficient, Fuel Efficiency - Mass and Fuel Efficiency - Volume.