SettingsSettings for this calculationUS
Heat through one square foot for each degree Fahrenheit of difference, in BTU per hour, as US labels and code tables give it.
NFRC window and door labels, the U-factor columns of the IECC and ASHRAE 90.1 and US manufacturers' data use this unit. The same element's metric U-value is 5.678 times the number.
U-value
1.703 W/m²K
Multiplied by 5.678263, NIST's factor from BTU/(hr·ft²·°F) to W/(m²·K). Multiply a US U-factor by 5.678 for W/m²K. Invert within one system: 1 ÷ the US figure is the US R-value, 1 ÷ the metric figure is the RSI.
- BTU/hr·ft²·°F entered
- 0.3 BTU/hr·ft²·°F
- US R-value (1 ÷ U-factor)
- 3.33 hr·ft²·°F/BTU
- Thermal resistance, RSI (1 ÷ U)
- 0.59 m²K/W
They open the calculator with your figures already in it
U-Factor Converter (BTU/hr·ft²·°F to W/m²K): 1.7 W/m²K — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- NIST Guide to the SI (SP 811), Appendix B.8, factors listed alphabetically: to convert from British thermal unit (IT) per hour square foot degree Fahrenheit to watt per square meter kelvin, multiply by 5.678 263 E+00
- R = 1 ÷ U within one system: the reciprocal of a US U-factor is a US R-value, the reciprocal of a metric U-value is an RSI, each counting the inside and outside air films
- NFRC 100 window and door ratings, the International Energy Conservation Code and ANSI/ASHRAE/IES Standard 90.1 state their U-factors in BTU/(hr·ft²·°F)
Inputs used
- U-factor (BTU/hr·ft²·°F)
- 0.3
Intermediate steps
- BTU/hr·ft²·°F entered
- 0.3 BTU/hr·ft²·°F
- US R-value (1 ÷ U-factor)
- 3.33 hr·ft²·°F/BTU
- Thermal resistance, RSI (1 ÷ U)
- 0.59 m²K/W
Confidence note: Multiplied by 5.678263, NIST's factor from BTU/(hr·ft²·°F) to W/(m²·K). Multiply a US U-factor by 5.678 for W/m²K. Invert within one system: 1 ÷ the US figure is the US R-value, 1 ÷ the metric figure is the RSI.
Add the equipment this sizes
This result is a specification — 1.703 W/m²K — 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 citations3
- NIST Guide to the SI (SP 811), Appendix B.8, factors listed alphabetically: to convert from British thermal unit (IT) per hour square foot degree Fahrenheit to watt per square meter kelvin, multiply by 5.678 263 E+00
- R = 1 ÷ U within one system: the reciprocal of a US U-factor is a US R-value, the reciprocal of a metric U-value is an RSI, each counting the inside and outside air films
- NFRC 100 window and door ratings, the International Energy Conservation Code and ANSI/ASHRAE/IES Standard 90.1 state their U-factors in BTU/(hr·ft²·°F)
Which documents these citations point at
- International Energy Conservation Code (United States)Building envelope, mechanical, lighting and hot-water efficiency requirements.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
Cite this page
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.
US window labels, energy-code tables and product data give the U-factor on the imperial scale; SAP, the UK's Part L calculations and European declarations work on the metric one. A US figure carried into a metric heat-loss sum unconverted understates that element's loss almost sixfold.