Heat Flux Density Converter: W/m², Btu/(ft²·h) & More
Convert heat flux (power per area)—solar radiation, building envelopes, and electronics cooling—between 28 units.
Convert heat flux density between W/m², Btu/(ft²·h), cal/(cm²·s), hp/ft², and 28 units. For heat transfer rate through surfaces, solar radiation, and electronics cooling. Free, runs in your browser—no sign-up.
What is heat flux density?
Heat flux density q″ is power per unit area (W/m²)—how many watts flow through each square meter of surface. It’s the rate of heat transfer per area, so it shows up in building envelopes (Btu/(h·ft²)), solar radiation (irradiance in W/m²), and electronics cooling (heat sink and CPU flux). Related to the heat transfer coefficient (U-value) by:
So for a given U (W/(m²·K)) and temperature difference ΔT (K), you get q″ in W/m². This converter lets you express that flux in Btu/(ft²·h), cal/(cm²·s), or any of the 28 supported units. Grouped dropdown (Metric, Calorie, Imperial, Scientific) makes it easy to find the unit you need.
Key units explained
W/m²
Factor: 1 (SI base)
Standard unit. Solar constant ~1361 W/m²; typical building envelope flux much lower.
Btu/(ft²·h)
Factor: 1 ≈ 3.155 W/m²
Common in US building and HVAC for heat flow through walls and roofs.
cal/(cm²·s)
Factor: 1 ≈ 41,868 W/m²
CGS unit; high value. Used in some solar and heat transfer literature.
hp/ft²
Factor: 1 ≈ 8027 W/m²
Power per area in mechanical hp per square foot. Relevant for intense cooling.
Common heat flux values
Reference values in W/m² for solar radiation, body heat, and CPU heat sink design. Use the converter above to express these in Btu/(ft²·h), cal/(cm²·s), or other units.
| Application | Typical heat flux (W/m²) |
|---|---|
| Solar radiation (Earth surface) | ≈ 1,000 |
| Human body heat | ≈ 50 |
| CPU heat sink | 100,000+ |
Who uses this converter?
Building and HVAC engineers convert between W/m² and Btu/(ft²·h) for envelope and load calculations. Solar and renewables work in W/m² for irradiance; some datasheets use Btu/(ft²·h). Electronics cooling uses W/m² or W/in² for heat sink and chip flux. Students need to unify units for q″ = U·ΔT and Fourier’s law. All 28 units are grouped (Metric, Calorie, Imperial, Scientific) for quick scanning. Conversions run locally.
Avoiding common mistakes
Don’t mix heat flux (W/m², power per area) with heat density (J/m², energy per area). Flux is rate; density is total. Integrate flux over time to get energy per area. Btu/(ft²·h) is power per area; Btu/ft² alone is energy per area—use our Heat Density converter for the latter. When using q″ = U·ΔT, keep U and ΔT in consistent units (e.g. W/(m²·K) and K).
Heat Flux Density FAQ
? How do I convert Btu/(ft²·h) to W/m²?
Multiply by about 3.155. So 1 Btu (IT)/(ft²·h) ≈ 3.155 W/m². The factor comes from 1 Btu ≈ 1055 J, 1 ft² ≈ 0.0929 m², 1 h = 3600 s. Use the converter above for Btu/(ft²·s), cal/(cm²·s), and 26 other units.
? What is a typical solar heat flux at the Earth’s surface?
Peak solar irradiance on a clear day is around 1000 W/m² (1 kW/m²). That’s the reference used for solar panel ratings. Convert to Btu/(ft²·h) with the tool: 1000 W/m² ≈ 317 Btu/(ft²·h). Our “Common heat flux values” table above lists this and other references.
? How do I convert W/m² to Btu/(ft²·h)?
Divide W/m² by about 3.155. So 1 W/m² ≈ 0.317 Btu (IT)/(ft²·h). Use the converter for cal/(cm²·s), hp/ft², and all 28 supported units. Handy for building envelope and HVAC when specs mix SI and imperial.
? What is heat flux density used for in building and HVAC?
Heat flux (W/m² or Btu/(ft²·h)) describes how much power passes through a wall, roof, or window per unit area. It’s tied to U-value by q″ = U·ΔT. Building codes and load calculations often use Btu/(ft²·h); research and solar use W/m². This tool converts between them and 26 other units.
? What is the difference between heat flux and irradiance?
Irradiance is radiant power per area (W/m²)—often used for solar or optical flux. Heat flux is the same dimension (W/m²) but can include conduction and convection, not only radiation. For solar applications they’re the same; use this converter to express irradiance in Btu/(ft²·h), cal/(cm²·s), or other units.
? How do I calculate heat flux from U-value and temperature difference?
Use q″ = U·ΔT: heat flux (W/m²) = U-value (W/(m²·K)) × temperature difference (K). Keep U and ΔT in consistent units (e.g. both SI). The result is in W/m²; use this converter to express it in Btu/(ft²·h), cal/(cm²·s), or any of the 28 units.