How this conversion works
Thermal conductivity (k or λ) is the heat that flows through a material per unit thickness, area and temperature difference.
Because these units use a temperature difference, 1 °F of difference equals exactly 5/9 K; no 32° offset applies. That gives 1 BTU/(h·ft·°F) ≈ 1.730735 W/(m·K).
Formula: result = value × (base units per source unit) ÷ (base units per target unit)
Where this conversion is used
- Comparing insulation products rated in W/(m·K) or US k-values.
- Material property tables for metals, plastics and building materials.
- Heat-transfer calculations that mix SI and US units.
Worked example: Insulation rated 0.04 W/(m·K) as a US k-value
- One BTU·in/(h·ft²·°F) ≈ 0.1442279 W/(m·K).
- 0.04 ÷ 0.1442279 = 0.2773 BTU·in/(h·ft²·°F).
Unit definitions
| Unit | In watts per meter kelvin | Note |
|---|---|---|
| Watt per meter kelvin (W/(m·K)) | 1 | |
| Watt per centimeter kelvin (W/(cm·K)) | 100 | |
| Milliwatt per meter kelvin (mW/(m·K)) | 0.001 | |
| BTU per hour foot °F (BTU/(h·ft·°F)) | 1.7307346664 | IT BTU; a temperature difference of 1 °F equals 5/9 K. |
| BTU inch per hour square foot °F (BTU·in/(h·ft²·°F)) | 0.1442278889 | The “k-value” used for US insulation. |
| Calorie per second centimeter °C (cal/(s·cm·°C)) | 418.4 | |
| Kilocalorie per hour meter °C (kcal/(h·m·°C)) | 1.1622222222 |
Watts per meter kelvin to BTU per hour foot °F
| Watt per meter kelvin (W/(m·K)) | BTU per hour foot °F (BTU/(h·ft·°F)) |
|---|---|
| 0.025 | 0.0144 |
| 0.04 | 0.0231 |
| 0.1 | 0.0578 |
| 1 | 0.5778 |
| 10 | 5.7779 |
| 50 | 28.8895 |
| 200 | 115.5579 |
Watts per meter kelvin to BTU inch per hour square foot °F
| Watt per meter kelvin (W/(m·K)) | BTU inch per hour square foot °F (BTU·in/(h·ft²·°F)) |
|---|---|
| 0.02 | 0.1387 |
| 0.03 | 0.208 |
| 0.04 | 0.2773 |
| 0.05 | 0.3467 |
| 0.1 | 0.6933 |
Things to watch for
- R-value is a different quantity (thermal resistance of a given thickness). For a uniform layer, R = thickness ÷ k; this converter does not convert to R-values.
Precision and sources
Results are calculated from the definitions above and rounded to 12 significant digits; the converter shows up to eight decimal places and the tables four. Reference: NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.
Frequently asked questions
Why don't I subtract 32 when converting °F units?
The unit refers to a temperature difference, not a temperature reading. A difference of 1 °F is 5/9 of a kelvin.
Is lower thermal conductivity better for insulation?
Yes. A lower k means less heat passes through the same thickness.