Heat Flux Converter
Convert heat flux between W/m2, kW/m2, W/cm2, BTU/hr/ft2, kcal/hr/m2 and cal/s/cm2, for thermal engineering, fire safety and building insulation.
Type in any field and the others update instantly. Fire engineering works in kW/m², so that one is here too.
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.
Why fire engineering writes it in kW/m². The numbers that matter to a firefighter or a building’s egress design sit between 1 and 20 kW/m², and writing those as 1,000 and 20,000 W/m² buries the useful digits. Two figures are worth remembering: roughly 1 kW/m² is where bare skin starts to hurt, which is also about what full summer sun delivers, and 2.5 kW/m² is the level a firefighter in full structural gear can work in for a sustained period. Above that the clock starts. The kW/m² field above exists so you can move between that world and the W/m² of thermal engineering without moving a decimal point three places in your head and getting it wrong.
There is one more unit worth having, and it belongs to a different crowd entirely. Laser and semiconductor work quotes W/cm², because the areas involved are tiny and the fluxes are enormous: a chip die dissipating 100 W over a square centimetre is running at a million watts per square metre, which is twice the surface flux of a nuclear fuel rod. That is not an exaggeration for effect, it is why processor cooling is a genuinely hard engineering problem rather than a matter of bolting on a bigger fan.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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