Heat Flux Density Converter
Convert between W/m², W/cm², Btu/(h·ft²), kcal/(h·m²), and more heat flux density units — for thermal engineering, solar power, and insulation design.
Each step along the axis is ten times the one before — the heat-flux-density units span 5 orders of magnitude, so a straight scale would bunch almost all of them at one end.
Instant insight
The "per" part is where these go wrong
Heat flux, heat density and heat transfer coefficients are all rates per unit area, volume or degree. Two figures in different units are only comparable once the per-what matches.
Write the full unit out — W/m²·K, not just W — before comparing two sources.
Check the direction of the conversion
Going from w_m2 to kw_m2, the number should get bigger if the new unit is smaller, and smaller if it is bigger.
If it moved the wrong way, the factor has been applied upside down — the most common conversion error there is.
Worked out in your browser — nothing is sent anywhere. Comparison figures are approximate.
Common conversions
Quick Reference — 1 unit = ? Kilowatt per Square Meter (kW/m²)
| Unit | Value |
|---|---|
| Watt per Square Meter (W/m²) | 0.001 |
| Kilowatt per Square Meter (kW/m²) | 1 |
| Watt per Square Centimeter (W/cm²) | 10 |
| Watt per Square Inch (W/in²) | 1.5500031 |
| Btu/(h·ft²) | 0.00315459 |
| Btu/(s·ft²) | 11.3565 |
| cal/(s·cm²) | 41.84 |
| kcal/(h·m²) | 0.001163 |
Formula
Multiply by (from factor / to factor) | q = Q / A (power per unit area)
Everyday Uses
Solar panel math
Solar irradiance in W/m² versus BTU-based specs — convert to compare panels and size arrays.
Safety thresholds
Fire safety and radiant heat limits quote flux in different units — convert where thresholds matter.
Industrial heating design
Furnace and process specs mix W/cm², W/m², and BTU/hr·ft² — translate accurately.
Heat transfer homework
Convert flux units cleanly across metric and imperial problem sets.
Solar gain
Convert irradiance figures between W/m² and BTU/hr·ft² when sizing panels or modelling summer overheating.
Process engineering
Move between unit systems when reading heat exchanger or furnace specifications.
Frequently Asked Questions
What is heat flux density, and how does it differ from heat transfer coefficient?
Heat flux density (q) is the rate of heat energy flow through a surface per unit area (q = Q/A), measured in W/m² — it tells you "how much power is crossing this surface" without needing to know any temperature difference. The heat transfer coefficient (h) instead relates flux to a specific temperature difference (h = q/ΔT). Heat flux density is the more general quantity, used directly for things like solar irradiance and insulation heat-loss calculations.
How intense is solar radiation in W/m², and how does this converter help?
Solar irradiance at the top of Earth's atmosphere (the solar constant) is about 1,361 W/m². At sea level on a clear day, peak irradiance reaching the ground is typically 800–1,000 W/m², the standard figure (1,000 W/m²) used to rate solar panels under "Standard Test Conditions." This converter lets you translate that into Btu/(h·ft²) (≈317) or kcal/(h·m²) (≈860) for comparing against HVAC or older engineering specifications.
How do you convert Btu/(h·ft²) to W/m²?
1 Btu/(h·ft²) = 3.15459 W/m². To convert Btu/(h·ft²) to W/m², multiply by 3.155. To convert W/m² to Btu/(h·ft²), divide by 3.155 (or multiply by 0.317). This conversion is common when comparing US HVAC heat-loss/heat-gain calculations (often in Btu/h per square foot of wall, roof, or window) against international (SI) building energy standards.
What heat flux levels are dangerous to humans or equipment?
Reference benchmarks: comfortable indoor heating panel: 100–300 W/m². Direct summer sunlight: ~1,000 W/m². Pain threshold on skin (radiant heat): roughly 2,500 W/m² for sustained exposure. Industrial furnace walls: can exceed 10,000–50,000 W/m². Fire safety codes often specify critical heat flux thresholds (e.g. ~12.5 kW/m² as the level that can ignite many common materials) when designing fire-rated separations and evacuation distances.
How is heat flux density used in insulation and energy-loss calculations?
Heat flux through a wall, roof, or window equals the U-value (W/(m²·K)) multiplied by the indoor-outdoor temperature difference: q = U × ΔT. For a window with U = 1.4 W/(m²·K) and a 20°C indoor/outdoor difference, the heat flux is 28 W/m² — multiply by the window's area to get total heat loss in watts. This is the calculation behind home energy audits and building heat-loss reports.