Heat Flux Density Converter
Convert heat flux density (heat flow per unit area) between watts per square metre, kilowatts per square metre, W/cm², Btu per hour per square foot, kilocalories per hour per square metre and many more metric and imperial units.
Select two units to see the conversion factor.
Heat flux density is a rate per area (W/m²). It is not heat density (J/m², an amount) and not the heat transfer coefficient (W/(m²·K)), which also includes a temperature difference.
Heat Flux Density Formula
Heat flux density is heat flow divided by area: q″ = Φ ÷ A, in W/m². Every unit is a fixed multiple of W/m². Result = value × (source factor ÷ target factor). Exact building blocks: 1 W = 1 J/s, 1 cal (IT) = 4.1868 J, 1 cal (th) = 4.184 J, 1 Btu (IT) = 1,055.05585262 J, 1 ft = 0.3048 m, 1 in = 0.0254 m, 1 hp = 745.69987 W, 1 CHU = 1,899.1005 J.
What Is Heat Flux Density?
Heat flux density, or heat flux, is the rate of heat transfer through a surface per unit area. Its SI unit is the watt per square metre (W/m²). Sunshine at midday is about 1,000 W/m². A wall in winter may lose 10 to 30 W/m². Electronic chips can reach 100 W/cm², which is 1,000,000 W/m².
The unit list is large because every system counts energy, time and area differently: calories, Btu, ergs, dynes (dyn/(h·cm) is dimensionally force per time per length, which equals energy per time per area), foot-pounds, horsepower, and time bases of seconds, minutes and hours. Exact definitions make each a fixed multiple of W/m².
Keep it apart from other heat quantities. Heat density (J/m²) is the amount accumulated; flux times time gives density. The heat transfer coefficient (W/(m²·K)) is the flux for each kelvin of temperature difference: q″ = h × ΔT. Thermal conductivity is a material property. This converter changes the units of flux only.
Supported Units
This converter supports the following 26 heat flux density units. The last column gives the value of each unit in the base unit W/m².
| Unit | Symbol | Value in W/m² |
|---|---|---|
| Watt per square meter | W/m² | 1 |
| Kilowatt per square meter | kW/m² | 1,000 |
| Watt per square centimeter | W/cm² | 10,000 |
| Watt per square inch | W/in² | 1,550.0031 |
| Joule per second per square meter | J/(s·m²) | 1 |
| Kilocalorie (IT) per hour per square meter | kcal(IT)/(h·m²) | 1.163 |
| Kilocalorie (IT) per hour per square foot | kcal(IT)/(h·ft²) | 12.51842781 |
| Calorie (IT) per second per square centimeter | cal(IT)/(s·cm²) | 41,868 |
| Calorie (IT) per minute per square centimeter | cal(IT)/(min·cm²) | 697.8 |
| Calorie (IT) per hour per square centimeter | cal(IT)/(h·cm²) | 11.63 |
| Calorie (th) per second per square centimeter | cal(th)/(s·cm²) | 41,840 |
| Calorie (th) per minute per square centimeter | cal(th)/(min·cm²) | 697.3333333 |
| Calorie (th) per hour per square centimeter | cal(th)/(h·cm²) | 11.62222222 |
| Dyne per hour per centimeter | dyn/(h·cm) | 2.777777778e-7 |
| Erg per hour per square millimeter | erg/(h·mm²) | 2.777777778e-5 |
| Foot pound-force per minute per square foot | ft·lbf/(min·ft²) | 0.2432317156 |
| Horsepower per square foot | hp/ft² | 8,026.646615 |
| Horsepower (metric) per square foot | hp(metric)/ft² | 7,916.842657 |
| Btu (IT) per second per square foot | Btu(IT)/(s·ft²) | 11,356.52668 |
| Btu (IT) per minute per square foot | Btu(IT)/(min·ft²) | 189.2754447 |
| Btu (IT) per hour per square foot | Btu(IT)/(h·ft²) | 3.154590745 |
| Btu (th) per second per square inch | Btu(th)/(s·in²) | 1,634,246.178 |
| Btu (th) per second per square foot | Btu(th)/(s·ft²) | 11,348.93179 |
| Btu (th) per minute per square foot | Btu(th)/(min·ft²) | 189.1488632 |
| Btu (th) per hour per square foot | Btu(th)/(h·ft²) | 3.152481054 |
| CHU per hour per square foot | CHU/(h·ft²) | 5.678263341 |
How to Use the Heat Flux Density Converter
- Type the heat flux density in the value box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
- Find the size of each unit in the common base unit (W/m²): 1 Btu(IT)/(h·ft²) = 3.154590745 W/m² and 1 W/m² = 1 W/m².
- Divide the source size by the target size to get the conversion factor: 3.154590745 ÷ 1 = 3.154590745, so 1 Btu(IT)/(h·ft²) = 3.154590745 W/m².
- Multiply by your value: 100 Btu(IT)/(h·ft²) × 3.154590745 = 315.4590745 W/m².
Heat flux density is shown as a non-negative magnitude, so only zero and positive values are accepted. The direction of the flow is a separate matter.
Common Heat Flux Density Conversions
- Imperial to metric: 100 Btu/(h·ft²) equals 315.4590745 W/m².
- Sunshine: 1,000 W/m² equals 316.9983306 Btu/(h·ft²).
- Chip cooling: 50 W/cm² equals 500,000 W/m².
- Kilocalories: 500 kcal/(h·m²) equals 581.5 W/m².
- Per square inch: 10 W/in² equals 15,500.031 W/m².
- Calorie per second: 0.1 cal(IT)/(s·cm²) equals 4,186.8 W/m².
W/m² to Btu/(h·ft²) and kcal/(h·m²)
Quick reference for frequently used values
| W/m² | Btu(IT)/(h·ft²) | kcal(IT)/(h·m²) | W/cm² |
|---|---|---|---|
| 1 | 0.3169983306 | 0.8598452279 | 0.0001 |
| 10 | 3.169983306 | 8.598452279 | 0.001 |
| 100 | 31.69983306 | 85.98452279 | 0.01 |
| 1,000 | 316.9983306 | 859.8452279 | 0.1 |
| 10,000 | 3,169.983306 | 8,598.452279 | 1 |
| 100,000 | 31,699.83306 | 85,984.52279 | 10 |
Where and Why This Conversion Is Used
- Building and HVAC: heat loss and gain through walls, windows and radiant floors.
- Solar energy: irradiance and collector performance.
- Electronics cooling: power per unit chip or heat-sink area.
- Industrial furnaces and boilers: flux on heat-transfer surfaces.
- Fire safety: thermal radiation exposure.
Limitations
- Flux describes the transfer at one surface and moment; it does not give total energy without time.
- Irradiance, absorbed heat and net heat flux can differ; this tool converts units only.
- Horsepower units use mechanical horsepower (745.7 W) unless “metric” is stated.
Important Differences Between Related Heat Quantities
- Specific heat capacity, J/(kg·K): energy needed to warm one kilogram of a material by one kelvin. It is not the total heat capacity of an object (J/K), which also depends on how much material there is, and it is not molar heat capacity (J/(mol·K)).
- Heat density, J/m²: heat or radiant energy delivered per unit area over a period. Heat flux density, W/m² is the rate of that transfer, energy per area per time. They are not interchangeable.
- Heat flux density, W/m² is heat flow per area. The heat transfer coefficient, W/(m²·K) is heat flux per kelvin of temperature difference, so heat flux = coefficient × temperature difference.
- Thermal conductivity, W/(m·K) is a material property. The heat transfer coefficient is a property of a surface or an assembly, and thermal resistance, K/W belongs to one specific object. Resistance is not conductivity or resistivity: R = L ÷ (k × A) for a slab.
- Temperature interval vs temperature: a difference of 10 °C is a difference of 18 °F. A reading of 10 °C is 50 °F. Every unit here that contains “per degree” or “K” uses intervals, so no offset is ever applied.
Each converter changes units within one quantity only. Related tools: Temperature Interval, Thermal Expansion, Thermal Resistance, Thermal Conductivity, Specific Heat Capacity, Heat Density, Heat Flux Density, Heat Transfer Coefficient, Fuel Efficiency - Mass and Fuel Efficiency - Volume.