Thermal Resistance Converter
Convert thermal resistance, the temperature difference per unit of power.
Written by Rahman · Last reviewed
Quick answer
Thermal resistance measures opposition to heat flow, in kelvin per watt. One degree Celsius per watt is identical because the interval sizes match, and one °F·h/BTU is 1.8956 K/W. A CPU heatsink rated at 0.5 K/W will sit 50 °C above ambient when dissipating 100 watts.
| Kelvin per watt (K/W) | 1 |
| Degree Celsius per watt (°C/W) | 1 |
| Degree Fahrenheit hour per BTU (°F·h/BTU) | 0.5275279263 |
About the Thermal Resistance Converter
Thermal resistance measures how strongly a material or assembly resists the flow of heat. A higher value means better insulation. The SI unit is the kelvin per watt.
It is widely used in building insulation and electronics cooling.
Absolute resistance and R-value are different quantities
This converter handles absolute thermal resistance (kelvin per watt), which describes a specific object: this heatsink, this wall assembly, this component package. It answers how much temperature difference a given heat flow produces across that particular thing.
Insulation R-value is a different quantity. It is area-specific, measured in square metres kelvin per watt or square feet degree Fahrenheit hours per BTU, and describes a material per unit of area rather than a finished object. The two are related by area, so an R-value cannot be converted here without knowing how large the surface is.
- Absolute resistance falls as area rises: doubling the area of the same insulation halves the resistance of the assembly while leaving its R-value unchanged.
- U-value is the reciprocal of area-specific resistance. Lower is better for U, higher is better for R, which is why the two are so often confused.
- Electronics cooling works almost entirely in absolute terms, quoting junction-to-ambient resistance in K/W or °C/W. Those two units are numerically identical, since an interval of one kelvin equals an interval of one degree Celsius.
US R-values and metric RSI are not the same numbers
Insulation is labelled R-19 or R-30 in the US and RSI 3.0 or RSI 5.0 in metric markets, and the numbers are not comparable. The US unit combines square feet, degrees Fahrenheit, hours and BTU; the metric RSI uses square metres, kelvin and watts.
Working the conversion through this site's own factors (a square foot is 0.09290304 m² and a °F·h/BTU is 1.8956342406 K/W) gives a ratio of about 5.678. So RSI 1 is roughly R-5.68, R-19 is about RSI 3.35, and RSI 3.0 is about R-17.
- A product labelled R-3 in a metric market is a very different material from one labelled R-3 in the US.
- Assembly R-value is lower than the sum of its materials, because studs, joists and fixings conduct around the insulation: thermal bridging.
- Rated values assume correct installation. Compressed or gapped insulation performs below its label.
Frequently Asked Questions
What is thermal resistance?
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It is the temperature difference required to drive one unit of heat power through a material, the opposite of thermal conductance.
Does a higher value mean better insulation?
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Yes. Higher thermal resistance means heat flows through more slowly, which is desirable for insulation.
Where is it used?
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It is used in building design, where it relates to R-values, and in electronics for managing heat from components.