Kilocalorie (IT) per Hour per Square Foot [kcal(IT)/(h·ft²)]

The unit kilocalorie (IT) per hour per square foot, written as kcal(IT)/(h·ft²), measures heat flux density or thermal power per unit area using the International Table kilocalorie (1 kcal(IT) = 4186.8 J). It represents the amount of heat energy transferred through 1 square foot of surface in 1 hour.

This unit is commonly used in U.S.-based building construction, insulation, and HVAC studies to quantify energy transfer through walls, floors, or roofs. Typical applications include:

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  Heat loss or gain through building materials

  
  
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  Solar energy incident on rooftops or horizontal surfaces

  
  
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  Design and analysis of heating and cooling systems

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in kcal(IT)/(h·ft²), Q is total heat in kilocalories, A is area in ft², and t is time in hours.

Conversion to SI units:

While modern practice uses W/m², kcal(IT)/(h·ft²) remains prevalent in legacy U.S. building and insulation data, offering a convenient way to evaluate thermal energy transfer over a typical hourly timescale.

Watt per Square Inch [W/in²]

The unit watt per square inch (W/in²) measures heat flux density or power per unit area, representing the amount of energy transferred through 1 square inch of surface every second. One watt equals 1 joule per second, so 1 W/in² corresponds to 1 joule of energy passing through 1 in² every second.

This unit is commonly used in high-intensity applications with small areas, such as:

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  Laser beams or focused light sources

  
  
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  Electronic devices to measure power density on chips

  
  
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  Industrial heating processes with concentrated energy

  
  

Heat flux can be expressed as:

where q is heat flux in W/in², Q is energy in joules, A is area in in², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, W/in² is convenient in U.S. engineering, electronics, and laboratory experiments involving small surface areas, where reporting flux per square inch provides an intuitive measure of concentrated energy.