SettingsSettings for this calculationUS
The thermal transmittance in watts per square metre kelvin.
Approved Document L targets, SAP inputs and European window declarations are all written this way. The same wall's U-factor in US units is 5.678 times smaller, so a figure typed into the wrong unit makes a wall look nearly six times better or worse than it is.
U-factor
0.0317 BTU/hr·ft²·°F
1 BTU/hr·ft²·°F = 5.678263 W/m²K, so divide by 5.678263. Divide W/m²K by 5.678 for the US U-factor; take reciprocals only after converting.
- W/m²K entered
- 0.18 W/m²K
- Thermal resistance, RSI (1 ÷ U)
- 5.56 m²K/W
- US R-value (1 ÷ U-factor)
- 31.55 hr·ft²·°F/BTU
They open the calculator with your figures already in it
U-Value Converter (W/m²K to BTU/hr·ft²·°F): 0.0317 BTU/hr·ft²·°F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- NIST SP 811, Appendix B: 1 Btu(IT)/(h·ft²·°F) = 5.678 263 W/(m²·K), from the International Table BTU (1055.05585262 J), the foot (0.3048 m) and the Fahrenheit degree (5/9 K)
- U = 1 ÷ R in either system, with R the total resistance of the element including its surface resistances
Inputs used
- U-value (W/m²K)
- 0.18
Intermediate steps
- W/m²K entered
- 0.18 W/m²K
- Thermal resistance, RSI (1 ÷ U)
- 5.56 m²K/W
- US R-value (1 ÷ U-factor)
- 31.55 hr·ft²·°F/BTU
Confidence note: 1 BTU/hr·ft²·°F = 5.678263 W/m²K, so divide by 5.678263. Divide W/m²K by 5.678 for the US U-factor; take reciprocals only after converting.
Add the equipment this sizes
This result is a specification — 0.0317 BTU/hr·ft²·°F — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations2
- NIST SP 811, Appendix B: 1 Btu(IT)/(h·ft²·°F) = 5.678 263 W/(m²·K), from the International Table BTU (1055.05585262 J), the foot (0.3048 m) and the Fahrenheit degree (5/9 K)
- U = 1 ÷ R in either system, with R the total resistance of the element including its surface resistances
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Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
A U-value is the same physical property in both systems — heat per unit area per degree — written in watts, square metres and kelvin in Europe and in BTU per hour, square feet and degrees Fahrenheit in North America. Specifications cross the Atlantic in both directions, and the factor between them is large enough that a value in the wrong unit is not a rounding error but a wall five times better or worse.