Heat converters
Heat Flux Density Converter
Convert W/m², BTU/(h·ft²), kcal/(h·m²) and other heat flux units.
Result
316.998 BTU/(h·ft²)
1000 W/m² = 316.998 BTU/(h·ft²)
All units for this value
| Unit | Symbol | Value |
|---|---|---|
| Watt per square meter | W/m² | 1000 |
| Kilowatt per square meter | kW/m² | 1 |
| Watt per square centimeter | W/cm² | 0.1 |
| Kilocalorie (IT) per hour per m² | kcal/(h·m²) | 859.845 |
| Calorie (IT) per second per cm² | cal/(s·cm²) | 0.0238846 |
| Langley per minute | Ly/min | 1.43308 |
| BTU (IT) per hour per ft² | BTU/(h·ft²) | 316.998 |
| BTU (IT) per second per ft² | BTU/(s·ft²) | 0.0880551 |
| BTU (IT) per minute per ft² | BTU/(min·ft²) | 5.28331 |
About this calculator
Heat flux density describes the rate at which heat energy passes through a surface, expressed per unit area, such as watts per square meter. This converter switches between the SI watt-based unit, the BTU-based units used in US HVAC and insulation work, and calorie-based units still found in some engineering references.
How it works in plain English
Because heat flux density combines a power unit with an area unit, the converter anchors every entry to watts per square meter and rescales through that common base for any pair of units you select. Metric multiples like kilowatts per square meter and watts per square centimetre follow straightforward powers of ten, since a square centimetre is a fixed fraction of a square meter.
The BTU-based units divide the International Table BTU (1055.05585262 joules) by a time unit and by the square foot (0.09290304 m²); the most common form, BTU/(h·ft²), works out to about 3.154591 W/m². The calorie-based units use the International Table calorie of 4.1868 joules per unit time and area, giving figures like kcal/(h·m²), which are common in older European and Asian HVAC documentation.
Select the from and to units, enter your value, and the converted heat flux density appears immediately, with a table showing the same rate across every supported unit at once. A precision control adjusts the number of decimal places shown, and a swap button reverses the two chosen units instantly.
The formula
- value_in_target = value_in_source × (source factor to W/m²) ÷ (target factor to W/m²)
- 1 BTU(IT)/(h·ft²) = 1055.05585262 ÷ 3600 ÷ 0.09290304 = 3.154591 W/m²
- 1 kcal(IT)/(h·m²) = 4186.8 ÷ 3600 = 1.163 W/m²
- 1 W/cm² = 10,000 W/m²
Worked example
Suppose an insulation datasheet lists a wall's heat flux as 25 BTU/(h·ft²) and a building surveyor wants that in watts per square meter for a metric energy-audit comparison. Multiplying by the conversion factor: 25 × 3.154591 = 78.86 W/m².
Entering 25 into the BTU/(h·ft²) field with W/m² selected as the target shows about 78.86 W/m² right away. As a sanity check, that figure sits well below the roughly 1,000 W/m² of noon sunlight and comfortably within the range typical of heat loss through an insulated wall, so the size of the result looks reasonable for the scenario described. The accompanying unit table also converts that same rate to roughly 67.79 kcal/(h·m²) and 0.007886 W/cm², all from that single entered value with no extra calculation needed.
Heat flux density reference values
| Quantity | Value in W/m² |
|---|---|
| 1 BTU(IT)/(h·ft²) | 3.154591 |
| 1 kcal(IT)/(h·m²) | 1.163 |
| 1 W/cm² | 10,000 |
| Solar constant (top of atmosphere) | ≈1,361 |
| Average solar irradiance at Earth's surface (clear noon) | ≈1,000 |
| Typical human body heat loss (resting, indoors) | ≈50-100 |
Standard unit definitions and solar radiation references
Frequently asked questions
What does a heat flux density value actually tell you?
It tells you how quickly heat is crossing a given surface at a given moment, such as through a window, wall, or the ground, expressed as power per unit area. Higher values mean heat is being transferred faster; insulation and window ratings are often compared using this kind of figure alongside overall area and temperature difference.
Why is 1 BTU/(h·ft²) roughly 3.15 W/m²?
That factor comes from combining three separate conversions at once: the International Table BTU into joules (1055.05585262), an hour into seconds (3600), and a square foot into square meters (0.09290304). Multiplying and dividing those together produces the combined factor of about 3.154591 used throughout this converter.
How does heat flux density relate to the solar constant?
The solar constant, about 1361 W/m², is itself a heat flux density: the rate at which solar energy arrives at the top of Earth's atmosphere per unit area facing the sun. It's included here as a familiar reference point for judging whether a given flux value is small, moderate, or very large.
Is heat flux density the same as insulation R-value or U-value?
No. R-value and U-value describe how resistant or conductive a material is to heat flow given a temperature difference, while heat flux density is the actual resulting rate of heat flow through a surface. Multiplying a U-value by a temperature difference gives a heat flux density in matching units.
Can I convert between kcal/(h·m²) and BTU/(h·ft²) directly?
Yes. Choose kcal/(h·m²) as the from unit and BTU/(h·ft²) as the to unit, enter your figure, and the converter routes the calculation through the shared W/m² anchor automatically. There's no need to look up or apply the individual calorie, hour, and area conversion factors yourself.
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