What is the difference between heat flux and heat flow?

Heat flow (Q) is total power in watts—how much heat transfers per second. Heat flux (q) is heat flow per unit area in W/m²—how intense the heat transfer is at a surface. Heat flow = heat flux × area. Same distinction as current vs current density.

How do I calculate building heat loss from heat flux?

Total heat loss = Σ(q × A) for all surfaces. If walls have q = 15 W/m² and area = 100 m², that's 1500 W through walls. Add roof, floor, windows. Or use Q = U × A × ΔT directly where U is thermal transmittance.

What is critical heat flux in boiling?

Critical heat flux (CHF) is the maximum heat flux a boiling surface can sustain before transitioning to film boiling, where vapor insulates the surface causing temperature spikes. For water at atmospheric pressure, CHF is about 1 MW/m². Exceeding CHF in boilers or nuclear reactors can cause burnout.

How is heat flux measured?

Heat flux sensors use thermocouples across a thin material of known thermal resistance: q = ΔT/R. Other methods include thermopile sensors, gradient heat flux sensors, and calorimetric techniques. Proper calibration and surface contact are critical for accuracy.

Flux thermique Converter

About Heat Flux Conversion

Heat flux measures the rate of heat energy transfer per unit area—watts per square meter—quantifying how intensely thermal energy flows through or from a surface. Unlike total heat flow (watts), heat flux is independent of surface size, making it ideal for comparing materials, designing heat transfer equipment, and specifying thermal performance requirements. It's a vector quantity with both magnitude and direction.

The SI unit is watts per square meter (W/m²). Heat flux appears in Fourier's law of heat conduction: q = -k(dT/dx), where k is thermal conductivity and dT/dx is the temperature gradient. Understanding heat flux is essential for heat exchanger design, building envelope analysis, electronics cooling, solar energy calculations, fire safety engineering, and industrial furnace design. The sun delivers about 1000 W/m² at Earth's surface on a clear day—a fundamental reference point.

Our converter handles heat flux units used in thermal engineering, building science, and industrial heat transfer applications.

Common Heat Flux Conversions

From	To	Multiply By
W/m²	kW/m²	0.001
kW/m²	W/m²	1,000
W/m²	BTU/h·ft²	0.3170
BTU/h·ft²	W/m²	3.155
W/m²	cal/s·cm²	2.39 × 10⁻⁵
cal/s·cm²	W/m²	41,868
W/m²	W/cm²	10⁻⁴
W/cm²	W/m²	10⁴

Heat Flux Unit Reference

Watt per square meter (W/m²) – The SI unit for heat flux, representing power per unit area flowing through or from a surface. Solar radiation is about 1000 W/m² at peak; human body heat loss is 50-100 W/m²; electronics can reach 50,000+ W/m². Used in scientific calculations, international standards, and engineering analysis worldwide.

BTU per hour per square foot (BTU/h·ft²) – US engineering unit widely used in building heat loss calculations, HVAC design, and American construction specifications. 1 BTU/h·ft² ≈ 3.155 W/m². Building codes, equipment ratings, and thermal analysis software in the US commonly use this unit. Typical residential heat loss through walls ranges from 2-20 BTU/h·ft² depending on insulation.

Kilowatt per square meter (kW/m²) – Convenient for high heat flux applications where W/m² gives large numbers: industrial furnaces (10-100+ kW/m²), concentrated solar power (up to 1000 kW/m² at focal point), fire exposure testing, and nuclear reactor heat transfer. 1 kW/m² = 1000 W/m². Fire safety standards often specify critical heat flux thresholds in kW/m².

Calorie per second per square centimeter (cal/s·cm²) – CGS unit occasionally found in older scientific literature. Very large unit: 1 cal/s·cm² = 41,868 W/m². Sometimes called the langley per second when used for solar radiation measurement. Largely superseded by SI units in modern practice.