Btu (IT) per Minute per Square Foot [Btu(IT)/(min·ft²)]

The unit Btu (IT) per minute per square foot, written as Btu(IT)/(min·ft²), measures heat flux density or thermal power per unit area using the International Table British Thermal Unit (1 Btu(IT) = 1055.06 J). It represents the amount of heat energy transferred through 1 square foot of surface every minute.

This unit is commonly used in U.S. building engineering, solar energy studies, and thermal analysis where energy flux per unit area over longer periods is considered. Typical applications include:

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  Heat loss or gain through building walls, roofs, and floors

  
  
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  Solar radiation incident on panels or horizontal surfaces

  
  
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  Industrial heating or cooling rate analysis

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(IT)/(min·ft²), Q is energy in Btu(IT), A is area in ft², and t is time in minutes.

Conversion to SI units:

While SI units such as W/m² are now standard, Btu(IT)/(min·ft²) remains convenient in U.S. engineering and historical thermal literature, providing an intuitive measure of heat transfer per square foot per minute.

Watt per Square Meter [W/m²]

The unit watt per square meter (W/m²) measures heat flux density or radiant flux, representing the amount of energy transferred per unit area per unit time. One watt equals 1 joule per second, so 1 W/m² corresponds to 1 joule of energy passing through 1 square meter every second.

This unit is widely used in physics, engineering, meteorology, and solar energy studies to quantify energy transfer rates across surfaces. Examples of applications include:

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  Solar radiation: measuring sunlight incident on a surface

  
  
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  Heat transfer: quantifying conduction or convection per unit area

  
  
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  Building energy analysis: evaluating heat gain or loss through walls and roofs

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in W/m², Q is energy in joules, A is area in square meters, and t is time in seconds.

For example, solar constant at Earth’s surface is approximately 1361 W/m², indicating the power received per square meter from the Sun outside Earth’s atmosphere.

W/m² is the standard SI unit for energy flux density, replacing older units like cal/cm²·s or Btu/ft²·h, and is essential for thermal engineering, climatology, and renewable energy calculations.