Thermal Resistance Converter: K/W, °F·h/Btu & More
Convert thermal resistance and R-value—temperature drop per unit heat flow—between SI and US building units.
Convert thermal resistance between K/W, °F·h/Btu (IT and th), and °F·s/Btu. Used for insulation R-value, heat sinks, and heat flow. Five units; free and instant.
What is thermal resistance?
Thermal resistance R is the temperature drop per unit heat flow. If one watt of heat flow causes a 2 K drop, R = 2 K/W. Higher R means the path resists heat flow more—better insulation. In US building practice, R-value is often given as ft²·°F·h/Btu (area-specific); the same idea per unit area appears here as °F·h/Btu, so you can convert between K/W and the units contractors and architects use.
SI unit is K/W. °F·h/Btu is common in US HVAC and building codes. This converter includes both IT and thermochemical Btu so you can match your source.
How conversion works
All units are converted via K/W. One °F·h/Btu (IT) ≈ 1.896 K/W. Multiply your value by the source factor to get K/W, then divide by the target factor. Example: 10 °F·h/Btu → 10 × 1.896 ≈ 18.96 K/W. The tool above handles all five units and both Btu variants.
Key units and R-value
Kelvin per watt (K/W)
Factor: 1 (SI base)
Standard for thermal resistance of a heat path. Used in heat sinks and one-dimensional heat flow.
°F·h/Btu (R-value style)
Factor: 1 °F·h/Btu ≈ 1.896 K/W
US building R-value is often ft²·°F·h/Btu; per unit area this is °F·h/Btu. Contractors and architects search for R-value—this converter bridges to SI.
Who uses this converter?
HVAC and building engineers convert between K/W and °F·h/Btu for insulation specs and code compliance. Electronics and thermal engineers use K/W for heat sink and junction-to-ambient resistance. Students see K/W in textbooks but may find US datasheets in °F·h/Btu. All conversions run locally.
Thermal Resistance FAQ
? How do I convert R-value (ft²·°F·h/Btu) to K/W?
R-value in ft²·°F·h/Btu is area-specific resistance. For a given area A in m², total resistance in K/W = (R-value in °F·h/Btu) × 1.896 / (A × 0.0929) when converting ft² to m². This converter uses °F·h/Btu (same as R-value per square foot); multiply or divide by area as needed for your geometry.
? What is the difference between Btu (IT) and Btu (th) for thermal resistance?
IT uses the international steam table Btu (≈1055 J); th uses the thermochemical Btu (≈1054.35 J). The resulting °F·h/Btu factors differ slightly (about 1.896 vs 1.897 K/W). Building and HVAC specs usually mean IT; we support both so you can match your source.
? How do I convert R-value to SI (m²·K/W)?
R-value in °F·ft²·h/Btu × 0.176 ≈ R in m²·K/W. So R-20 is about 3.5 m²·K/W. This converter handles K/W (total resistance), °F·h/Btu (per unit area), and other variants so you can align with US insulation labels or SI calculations.
? What is the relationship between R-value and U-value?
U = 1/R when both are per unit area. So R-10 (in °F·ft²·h/Btu) gives U = 0.1 Btu/(ft²·h·°F). Higher R means better insulation and lower U. Use our Heat Transfer Coefficient converter to get U in W/(m²·K) or other units.
? What is a good R-value for attic or wall insulation?
Attics in cold climates often target R-38 to R-60 (°F·ft²·h/Btu); walls R-13 to R-21. Codes vary by region. R-value is resistance per unit area—higher is better. Use this converter to express R in K/W for a given area or to switch between Btu (IT) and Btu (th) definitions.
? How do I convert thermal resistance K/W to °F·h/Btu?
1 K/W ≈ 0.527 °F·h/Btu. So 2 K/W ≈ 1.05 °F·h/Btu. K/W is total resistance (for a given area); R-value is often per ft². Use the converter above for exact factors and for area-specific and absolute resistance units.