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.

Calorie (th) per Hour per Square Centimeter [cal(th)/(h·cm²)]

The unit calorie (th) per hour per square centimeter, written as cal(th)/(h·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface in 1 hour.

This unit is often used in solar energy studies, calorimetry experiments, and building physics, especially when small surface areas and long durations are considered. Typical applications include:

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  Solar radiation incident on small surfaces

  
  
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  Thermal testing of materials over time

  
  
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  Calorimetry experiments

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in cal(th)/(h·cm²), Q is energy in calories, A is area in cm², and t is time in hours.

Conversion to SI units:

While SI units such as W/m² are standard, cal(th)/(h·cm²) remains convenient in small-scale laboratory and historical studies, providing an intuitive way to quantify energy transfer per hour per unit area.