What this converter does
This converter turns a U-value between metric W/m²K and imperial BTU/hr·ft²·°F. U-value is the rate heat passes through a building element per degree of temperature difference — lower is better insulated. Type a value, pick units, and read the result instantly.
A window rated 1.4 W/m²K equals about 0.25 in imperial U-factor. For the reverse, use the BTU to W/m²K 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 a wall U-value of 0.3 W/m²K to imperial.
1 BTU/hr·ft²·°F = 5.6783 W/m²K0.3 ÷ 5.6783 = 0.0528 BTU/hr·ft²·°FSo 0.3 W/m²K is a U-factor of about 0.053 — a well-insulated wall.
The units in this example
The SI unit of thermal transmittance, used in most building codes outside the US. 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
The US imperial U-factor used on window and assembly labels. Numerically about 0.176× 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