How this conversion works
Heat flux is power passing through a unit area. The SI unit is the watt per square meter.
US units combine the IT British thermal unit (1,055.05585262 J) per hour with the square foot (0.09290304 m²), so 1 BTU/(h·ft²) ≈ 3.154591 W/m².
Formula: result = value × (base units per source unit) ÷ (base units per target unit)
Where this conversion is used
- Solar irradiance and panel test conditions in W/m².
- Building heat loss and HVAC load data in BTU/(h·ft²).
- Fire testing and heat exposure studies.
Worked example: 1,000 W/m² to BTU/(h·ft²)
- One BTU/(h·ft²) ≈ 3.154591 W/m².
- 1,000 ÷ 3.154591 = 317.00 BTU/(h·ft²).
Unit definitions
| Unit | In watts per square meter | Note |
|---|---|---|
| Watt per square meter (W/m²) | 1 | |
| Kilowatt per square meter (kW/m²) | 1,000 | |
| Watt per square centimeter (W/cm²) | 10,000 | |
| Watt per square inch (W/in²) | 1,550.00310001 | |
| BTU per hour square foot (BTU/(h·ft²)) | 3.1545907451 | |
| Calorie per second square centimeter (cal/(s·cm²)) | 41,840 | Thermochemical calorie (4.184 J). |
| Kilocalorie per hour square meter (kcal/(h·m²)) | 1.1622222222 |
Watts per square meter to BTU per hour square foot
| Watt per square meter (W/m²) | BTU per hour square foot (BTU/(h·ft²)) |
|---|---|
| 1 | 0.317 |
| 10 | 3.17 |
| 100 | 31.6998 |
| 500 | 158.4992 |
| 1,000 | 316.9983 |
| 5,000 | 1,584.9917 |
BTU per hour square foot to watts per square meter
| BTU per hour square foot (BTU/(h·ft²)) | Watt per square meter (W/m²) |
|---|---|
| 1 | 3.1546 |
| 10 | 31.5459 |
| 100 | 315.4591 |
| 317 | 1,000.0053 |
| 1,000 | 3,154.5907 |
Things to watch for
- Peak sunlight at the ground is roughly 1,000 W/m², a figure commonly used to rate solar panels.
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
Is heat flux the same as irradiance?
They use the same units. Irradiance refers specifically to radiant energy (such as sunlight) arriving at a surface; heat flux covers any heat transfer mode.