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.

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.