Convert W/m²K U-values and thermal resistance between metric and imperial units, or solve U = 1/R and R = 1/U for whole building assemblies.
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W/(m²·K) To U Value Formula
The following reciprocal relation calculates U-value from total assembly thermal resistance. W/(m²·K) is already a U-value unit; converting U-value units does not take its reciprocal.
Variables:
- U is the U-value (W/m²K)
- R is the total assembly thermal resistance (m²·K/W), including applicable surface films for the same assembly boundaries.
For matching whole-assembly boundaries, calculate U = 1 ÷ total R using consistent units. Lower U means less heat transfer per area and temperature difference. A single insulation layer does not necessarily represent a wall or window assembly: applicable surface films, framing and other heat paths also matter.
| Thermal Resistance (m²K/W) | U-Value (W/m²K) |
|---|---|
| 0.10 | 10.000 |
| 0.20 | 5.000 |
| 0.25 | 4.000 |
| 0.30 | 3.333 |
| 0.50 | 2.000 |
| 0.70 | 1.429 |
| 1.00 | 1.000 |
| 1.50 | 0.667 |
| 2.00 | 0.500 |
| 2.50 | 0.400 |
| 3.00 | 0.333 |
| 3.50 | 0.286 |
| 4.00 | 0.250 |
| 5.00 | 0.200 |
| 6.00 | 0.167 |
| 7.00 | 0.143 |
| 8.00 | 0.125 |
| 9.00 | 0.111 |
| 10.00 | 0.100 |
| 12.00 | 0.083 |
| * Rounded to 3 decimals. Relation: U (W/m²K) = 1 ÷ R (m²K/W). | |
What Does “W/(m²·K) to U Value” Mean?
W/(m²·K), watts per square meter per kelvin, is already the SI unit of U-value (thermal transmittance). It describes heat-transfer rate per area per temperature difference. A value of 0.25 W/(m²·K) is therefore a U-value of 0.25 in SI units. To convert units, 1 Btu_IT/(h·ft²·°F) = 5.678263341113488 W/(m²·K); equivalently, multiply an SI U-value by 0.17611018368230584 for imperial units. The calculator uses International Table Btu. Fahrenheit denotes a temperature difference here, so no +32 offset applies. Conductivity in W/(m·K) is a different quantity and requires thickness and a complete assembly model before deriving U-value.
How to Calculate W/(m²·K) To U Value?
The following steps outline how to calculate the U-value (W/m²K) using the formula U = 1 / R.
- First, determine total thermal resistance (R) in m²·K/W for the same assembly boundaries, including applicable surface films.
- Next, use the formula U = 1 / R to calculate the U-value in W/m²K.
- Finally, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Thermal resistance (R) = 0.05 m²·K/W, so U = 1 ÷ 0.05 = 20 W/(m²·K).
