Foot-Pound per Minute per Square Foot [ft·lb/(min·ft²)]

The unit foot-pound per minute per square foot, written as ft·lb/(min·ft²), measures energy flux density or power per unit area in the Imperial system. One foot-pound (ft·lb) is the energy required to move a force of 1 pound-force over a distance of 1 foot, equivalent to 1 ft·lb ≈ 1.35582 joules. Dividing by time (minutes) and area (ft²) gives the rate of energy transfer per unit area over time.

This unit is commonly used in U.S.-based engineering, building physics, and solar energy studies where energy flux through surfaces is measured. Applications include:

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  Heat transfer through walls, floors, or roofs

  
  
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  Solar radiation incident on building surfaces

  
  
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  Mechanical or thermal energy flux in industrial processes

  
  

Mathematically, the flux is expressed as:

where q is flux in ft·lb/(min·ft²), E is energy in foot-pounds, A is area in ft², and t is time in minutes.

Conversion to SI units:

While modern practice favors W/m², ft·lb/(min·ft²) remains relevant in legacy U.S. building and thermal engineering literature, providing an intuitive measure of energy transfer per square foot per minute.

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.