Btu (IT) per Second per Square Foot [Btu(IT)/(s·ft²)]

The unit Btu (IT) per second per square foot, written as Btu(IT)/(s·ft²), measures heat flux density or thermal power per unit area in the Imperial system 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 second.

This unit is commonly used in high-intensity heat transfer applications, solar energy studies, and engineering where energy flux per small surface area is important. Typical applications include:

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  Heat flux from industrial furnaces or heaters

  
  
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  Solar radiation incident on small surfaces

  
  
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  Thermal testing of materials

  
  

Heat flux can be expressed mathematically as:

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

Conversion to SI units:

While W/m² is the SI standard, Btu(IT)/(s·ft²) is convenient in U.S. engineering and historical references, providing an intuitive measure of high-intensity heat flux per square foot per second.

Btu (th)/Hour/Square Foot

The unit Btu (th)/hour/square foot measures the rate of heat transfer through a surface over time. Here, Btu (th) refers to British thermal units of thermal energy, and the measurement quantifies how much heat passes through one square foot of a material in one hour. This unit is widely used in building design, insulation analysis, and HVAC system planning to evaluate thermal performance. A higher value indicates more heat loss or gain, suggesting lower insulation efficiency, while a lower value reflects better thermal resistance. Engineers and architects rely on Btu (th)/hour/square foot to calculate heating or cooling loads, optimize energy consumption, and improve indoor comfort. It is essential for energy audits, material selection, and sustainable building practices. By accurately assessing heat transfer in this unit, professionals can design energy-efficient structures, minimize utility costs, and reduce environmental impact. Understanding Btu (th)/hour/square foot is therefore critical for both practical construction decisions and long-term energy management.