U-Value Converter: BTU/(hr·ft²·°F) to W/(m²·K)
Convert imperial and SI U-values for HVAC design, insulation checks, and building-envelope specifications. Use the conversion factor and reference values to compare thermal performance without applying a temperature offset.
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U-Value Imperial to SI formula
W/(m²·K) = BTU/(hr·ft²·°F) × 5.6782633
BTU/(hr·ft²·°F) = W/(m²·K) ÷ 5.6782633
The factor is 5.6782633 W/(m²·K) for each 1 BTU/(hr·ft²·°F). Going the other way, 1 W/(m²·K) is 0.17611018 BTU/(hr·ft²·°F).
U-Value Imperial to SI reference table
| BTU/(hr·ft²·°F) | W/(m²·K) |
|---|---|
| 0.05 | 0.283913 |
| 0.1 | 0.567826 |
| 0.15 | 0.85174 |
| 0.2 | 1.13565 |
| 0.25 | 1.41957 |
| 0.35 | 1.98739 |
| 0.5 | 2.83913 |
| 0.75 | 4.2587 |
| 1 | 5.67826 |
| 1.2 | 6.81392 |
Where this conversion is used
U-values for walls, roofs and glazing go into every heat load calculation. Glass and insulation data sheets and US references may give them in imperial units, while most codes and suppliers in India and the Gulf use W/(m²·K). Use this conversion when the U-value on a data sheet is not in the units your heat load method expects. A temperature difference of 1 K equals a difference of 1 °C, so W/(m²·K) and W/(m²·°C) are the same.
Worked example
A glazing data sheet lists a U-value of 0.28 BTU/(hr·ft²·°F) and the heat load calculation is in SI units.
0.28 BTU/(hr·ft²·°F) × 5.6782633 = 1.58991 W/(m²·K)
Use about 1.59 W/(m²·K) for the glass in the calculation.
Common mistakes
- Confusing U-value with R-value. R is the reciprocal of U, so the factor is applied the other way round for R-values.
- Converting the temperature part as a temperature reading. A U-value uses a temperature difference, so only the size of the degree matters and no offset of 32 applies.
- Using a centre of glass U-value for the whole window. The frame usually makes the overall value worse.
Related HVAC Tools & Resources
Frequently asked questions
How do you convert an imperial U-value to SI units for an HVAC specification?
Multiply the imperial U-value by 5.678263 to convert Btu/(h·ft²·°F) to W/(m²·K). This factor applies to an existing U-value for walls, roofs, and glazing. For example, 0.3 Btu/(h·ft²·°F) equals approximately 1.7 W/(m²·K).
Are W/(m²·K) and W/(m²·°C) equivalent when comparing insulation performance?
Yes, both units express the same numerical U-value when the denominator represents a temperature difference. A temperature change of 1 K equals a change of 1°C. You do not need to recalculate the value when switching between these two unit notations.
How do you convert an imperial R-value to SI units when comparing insulation?
Multiply an imperial R-value by approximately 0.1761 to obtain the corresponding SI thermal resistance, RSI, in m²·K/W. For example, an imperial R-value of 10 converts to approximately RSI 1.76. Check whether the original rating applies to the entire assembly or per unit of thickness.
Why is there no 32°F adjustment when converting U-values between Fahrenheit and SI units?
The 32-degree offset applies when converting absolute temperature readings, not temperature differences. A temperature difference of 1°F equals 5/9 K. The U-value conversion factor already accounts for the temperature, area, and energy units, so no additional offset is required.
What is the difference between a U-value and an R-value for building insulation?
A U-value measures how much heat passes through a building element, while an R-value measures its resistance to heat flow. A lower U-value indicates better insulation performance, whereas a higher R-value indicates greater thermal resistance. Their numerical relationship must use compatible units and refer to the same assembly.
What formula converts U-values between imperial and SI units in both directions?
The U-value conversion formula uses a factor of 5.678263. Multiply Btu/(h·ft²·°F) by this factor to obtain W/(m²·K), or divide the SI value by the same factor to convert back. A thermal transmittance unit converter changes the units of an existing value rather than calculating an assembly's thermal performance.
Can a U-Value Converter help compare glazing specifications in different unit systems?
Yes, a glazing U-value converter puts existing ratings into a common unit system for comparison. Check whether the figures represent the whole window or only the centre of the glass before comparing them. Frame performance, glazing construction, and the rating method also affect how meaningful the comparison is.
When should you use a wall insulation U-value calculator instead of a unit converter?
Use a wall insulation U-value calculator when you need to estimate thermal transmittance from the wall's construction layers and their thermal properties. A converter changes the units of a U-value you already know. Check the calculation method and input data before using an estimated result in a technical specification.