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.