What this converter does
This converter turns an imperial U-factor (BTU/hr·ft²·°F) into the metric U-value (W/m²K) and back. US window and assembly labels use the imperial figure; most building codes elsewhere use metric. Type a value and read the result instantly.
A U-factor of 0.30 equals about 1.7 W/m²K. For the reverse, use the W/m²K to BTU converter.
The units it covers
Every unit measures thermal transmittance — heat flow per area per degree — so each converts through W/m²K with a fixed ratio.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Watt per m² kelvin | W/m²K | 1 | SI / metric building codes |
| BTU per hr ft² °F | BTU/hr·ft²·°F | 5.6783 | US / imperial (a.k.a. U-factor) |
| kcal per hr m² °C | kcal/h·m²·°C | 1.163 | Older European practice |
The formula
Each unit has a fixed value in W/m²K, so converting between any two goes through that base once:
result = value × factor_from ÷ factor_toWhere:
- value = the U-value you typed, in the “from” unit
- factor_from = the “from” unit’s value in W/m²K
- factor_to = the “to” unit’s value in W/m²K
Worked example
Convert an Energy-Star window U-factor of 0.28 to metric.
1 BTU/hr·ft²·°F = 5.6783 W/m²K0.28 × 5.6783 = 1.590 W/m²KSo a 0.28 U-factor window is about 1.59 W/m²K.
The units in this example
The US imperial U-factor printed on NFRC window labels. Multiply by 5.6783 for the metric value.
- 1 BTU/hr·ft²·°F = 5.6783 W/m²K
- Energy-Star window ≤ 0.30
- Code wall ≈ 0.06
- U = 1 ÷ R-value
The SI thermal transmittance used in metric building codes. Lower means better insulation.
- 1 W/m²K = 0.176 BTU/hr·ft²·°F
- 1 W/m²K = 0.860 kcal/h·m²·°C
- Passive-house wall ≈ 0.15
- Single glazing ≈ 5.7