Heat Flux Converter
Convert heat flux between W/m², BTU/(hr·ft²), kcal/(hr·m²), and cal/(s·cm²) instantly.
Type in any field — the others update instantly.
Heat flux (also called heat flux density or thermal flux) is the rate of heat energy transfer per unit area. It tells you how much heat is flowing through a surface every second.
Formula:
q = Q / (A × t)
Where q is heat flux (W/m²), Q is total heat energy (joules), A is area (m²), and t is time (seconds).
Unit conversions (all relative to W/m²):
- 1 BTU/(hr·ft²) = 3.15459 W/m²
- 1 kcal/(hr·m²) = 1.163 W/m²
- 1 cal/(s·cm²) = 41,840 W/m²
Practical reference values:
| Source / Application | Heat Flux |
|---|---|
| Solar irradiance at Earth’s surface | ~1,000 W/m² |
| Human body (at rest) | ~50–100 W/m² |
| Boiling water on a stovetop | ~10,000–100,000 W/m² |
| Nuclear reactor fuel rods | ~500,000 W/m² |
| Space shuttle re-entry (peak) | ~500,000 W/m² |
| Well-insulated building wall | 5–20 W/m² |
Applications:
Heat flux measurements are critical in many fields:
Thermal engineering — designing heat exchangers, cooling systems, and reactors requires knowing how much heat can be transferred per unit area of surface.
Building science — insulation R-values are inversely related to heat flux. A higher R-value means lower heat flux through a wall for a given temperature difference.
Solar energy — solar panel efficiency depends on incident heat flux and how much is converted to electricity vs. lost as heat.
Meteorology and climate — the sun delivers about 1,361 W/m² above Earth’s atmosphere (the solar constant). At the surface, about 1,000 W/m² reaches on a clear day.
Medical applications — burn injuries are characterized by the heat flux that caused them. Skin begins to feel pain at about 1,000 W/m² and burn damage occurs at sustained exposure above 4,000 W/m².