This heat flux density converter changes heat flux, the rate of heat flow per unit area, between watts per square meter, kilowatts per square meter, watts per square centimeter and BTU per hour square foot. Formula: 1 W/cm² = 10000 W/m²; heat rate = flux × area. The SI unit is W/m². 1 BTU/(h·ft²) is about 3.15459 W/m², and 1 W/cm² is exactly 10,000 W/m².
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How to use the heat flux density converter
- Enter the value, for example 1,000 W/m² for full sunlight.
- In the heat flux density converter, choose its unit and the target unit.
- Read the heat flux density converter result: 1,000 W/m² ≈ 317 BTU/(h·ft²).
For W/m² to BTU/h·ft², divide by 3.15459. Heat flux is power per area, so to get total power multiply by the area.
What is heat flux density?
Heat flux density is the rate of heat flow through a surface per unit area, measured in watts per square meter (W/m²). Building scientists, solar engineers and furnace designers use it.
Conversion formula
The heat flux density converter stores every unit as a multiple of the base unit, the watt per square meter (W/m²). To convert:
value in target unit = value × (factor of source unit ÷ factor of target unit)
where each factor is the size of the unit in watts per square meter.
Common heat flux density conversions
| From | To | 1 unit equals |
|---|---|---|
| BTU per hour square foot (BTU/(h·ft²)) | watt per square meter (W/m²) | 3.154591 W/m² |
| watt per square meter (W/m²) | kilowatt per square meter (kW/m²) | 0.001 kW/m² |
Heat flux density converter worked example
Convert 25 BTUs per hour square foot to watts per square meter:
- Factor of BTU/(h·ft²) = 3.154591 W/m²
- Factor of W/m² = 1 W/m²
- 25 × 3.154591 ÷ 1 = 78.86477 W/m²
Heat flux density unit reference
| Unit | Symbol | Group | Equals (W/m²) |
|---|---|---|---|
| Watt per Square Meter | W/m² | SI | 1 |
| Kilowatt per Square Meter | kW/m² | SI | 1,000 |
| BTU per Hour Square Foot | BTU/(h·ft²) | US | 3.154591 |
| BTU per Second Square Foot | BTU/(s·ft²) | US | 11,356.5267 |
| Calorie per Second Square Centimeter | cal/(s·cm²) | CGS | 41,840 |
| Horsepower per Square Foot | hp/ft² | US | 8,026.646617 |
Reverse factors for each heat flux density unit
The reference table gives each unit in watt per square meters. To go the other way, divide by that factor; the list below does the division for you.
- Kilowatt per Square Meter (kW/m²), SI: 1 kW/m² = 1,000 W/m², so 1 W/m² = 0.001 kW/m².
- BTU per Hour Square Foot (BTU/(h·ft²)), US: 1 BTU/(h·ft²) = 3.154591 W/m², so 1 W/m² = 0.3169983 BTU/(h·ft²).
- BTU per Second Square Foot (BTU/(s·ft²)), US: 1 BTU/(s·ft²) = 11,356.53 W/m², so 1 W/m² = 8.80551 × 10−5 BTU/(s·ft²).
- Calorie per Second Square Centimeter (cal/(s·cm²)), CGS: 1 cal/(s·cm²) = 41,840 W/m², so 1 W/m² = 2.39006 × 10−5 cal/(s·cm²).
- Horsepower per Square Foot (hp/ft²), US: 1 hp/ft² = 8,026.647 W/m², so 1 W/m² = 0.000124585 hp/ft².
Solar and building heat flux examples
Solar irradiance and building heat loss are both heat fluxes. The top-of-atmosphere value is the total solar irradiance of about 1,361 W/m² measured by NASA instruments; 1,000 W/m² is the standard test condition for rating solar panels; the wall row is U × ΔT = 0.3 × 20:
| Situation | W/m² | kW/m² | BTU/(h·ft²) |
|---|---|---|---|
| Solar irradiance at the top of the atmosphere (about) | 1,361 | 1.361 | 431.4 |
| Standard test conditions for solar panels | 1,000 | 1 | 317 |
| Typical winter heat loss through 1 m² of a wall, ΔT 20 K, U 0.3 | 6 | 0.006 | 1.9 |
A wall loses far less heat per square meter than sunlight delivers, but over a whole house and a whole winter it adds up.
Heat flux from U-value and ΔT
Through a building element, heat flux = U × ΔT. With U in W/(m²·K) and ΔT in kelvin, the result is in W/m². For kW/m² to BTU/h·ft², multiply by 317.0. Convert U-factors with the heat transfer coefficient converter.
From flux to total power
Heat flux is per square meter, so multiply by area to get total power. If 150 W/m² passes through a 12 m² glazed wall, the total is 1,800 W. Done in US units, 150 W/m² is about 47.5 BTU/(h·ft²); multiply by 129 ft² and you get about 6,140 BTU/h, which matches 1,800 W.
The solar constant at the top of the atmosphere is about 1,361 W/m². The heat flux density converter gives about 431 BTU/(h·ft²). On the ground, 1,000 W/m² is the standard test value for solar panels.
Related converters and calculators
Related heat transfer tools:
- Heat transfer coefficient converter: U-values
- Thermal resistance converter: K/W and R-value
- Thermal conductivity converter: W/(m·K)
- Power unit converter: W and BTU/h
- Area converter: m² and ft²
Sources
NIST Special Publication 811, Appendix B; NASA Goddard: Solar irradiance science.
Questions about this calculator
What is the SI unit of heat flux?
Watt per square meter (W/m²).
How do I convert W/m² to BTU/h·ft²?
Divide by 3.15459, or multiply by 0.316998. For example, 500 W/m² = 158.5 BTU/(h·ft²).
How do I convert BTU/h·ft² to W/m²?
Multiply by 3.15459. For example, 100 BTU/(h·ft²) = 315.5 W/m².
How do I convert W/cm² to W/m²?
Multiply by 10,000. For example, 0.5 W/cm² = 5,000 W/m².
How strong is sunlight in W/m²?
About 1,361 W/m² above the atmosphere; at the ground on a clear day near noon, roughly 1,000 W/m².
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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