Finding yourself in a little difficulty with heat flux? Whether you are a mechanical engineer, an HVAC technician, or even a window installer, calculations like this are part of your ongoing work.

    Real life applications often require moving from the U.S. BTU/sec/ft2 to the standard W/m2. Then there's the j/s·m2, BTU/min/in2 and non-standard expressions.

    To make these conversions easier, try our free heat flux calculator. Simply select your units and enter the amount to convert above.

    For those just learning, we have prepared a comprehensive guide for your most sought after questions. We cover the flux equation, how it relates to heat transfer, and more.


    What is heat flux?

    Heat flux is, by definition, the rate of thermal energy being transferred through a surface per unit of time.

    Think of measuring the energy of sunshine through a window at a single point. It would be written in Watts or Joules/sec.

    The heat flux of a unit of exposed surface area is known as the heat transfer.

    Now think of making the measurement to express the energy through a square meter sized window, instead of a single point. It would be expressed using Watts/square meter.

    The heat flux calculator makes easy and accurate work of making conversions from one scale to the other. No more arithmetic errors or accidentally swapping figures around.

    Just input your existing measurement units and convert them instantly to your desired measurement units.

    What is the formula for heat flux?

    Like most concepts in physics, heat flux can be expressed through a realtively simple equation:

    HF = Δ T x HTC


  • HF is Heat Flux, in W/m2
  • Δ T is the Temperature difference, in K
  • HTC is the Heat Transfer Coefficient, in W/(m2 K)

  • What is the SI of heat flux?

    The SI for heat flux is Watts/square meter or W/m2.

    However, some applications require derivates of the BTU/square foot, Kilowatt/square meter, or other units. US construction documents often express heat flux in BTU/second square foot.

    When solving equations regarding heat transfer and other partial differentials, it is important that all expressions are on the same scale.

    It is sometimes helpful to write out all the different abbreviations so that you are clear about what scale your answer is in.

    How do you convert between similar units?

    The easiest way to convert units is by using our heat flux calculator.

    There are also several helpful charts available to help you convert energy units from BTU to Watts. The process to change any unit from one to the other is simple arithmetic.

    The first thing to understand is that a Watt is a unit of power. It represents the energy consumption of 1 joule per second.

    You can convert one expression of power to another. So you could convert Watts to horsepower or British Thermal Units.

    Btu to Watts

    In the case of BTU, the number of watts equals the number of BTU per hour divided by 3.412142.

    What about watts to kilowatts?

    1000 Watts = 1 Kilowatt

    Your conversion will have an element of power, time, and area. You'll have to convert each element as needed.

    Can you measure heat flux?

    The best method of measuring heat flux is through the use of a sensor or heat flux transducer.

    It is mounted on the material to be measured and reads the amount of heat transferred to and from the surface.

    The most common type of heat flux sensor is a differential temperature thermopile. Once the sensor is calibrated it can then be used to directly measure heat flux.

    An indirect way of measuring of heat flux is done by observing the difference in temperature over a conductive piece of material.

    To accurately determine thermal resistance, you'll need precise figures for the thickness of the material and its conductivity.

    Heat flux can then calculated using the thermal resistance, and the temperature measurements on each side of the material.

    The formula is referenced in the section above.

    What are the applications of heat flux?


    Engineers and architects often apply heat flux calculations to their work.

    Thermal sensors measure loss or gain through the component of a building envelope. From there, you can determine the thermal resistance or transmittance value of the material.

    As energy efficiency becomes more critical, measurements of heat gain and loss through walls, windows, and the entire building envelope are tested and measured.


    In agricultural or weather study, soil heat flux is an important parameter.

    The thermal properties of soil indicate the absorption and evaporation of water and the energy stored in the soil.


    Medicine looks at heat exchange in the human body to develop clothing, wetsuits and sleeping bags.

    Hair, fat, and skin are temperature heat sinks and provide challenges for sensors. Test subjects also tend to be in motion.


    In industrial settings, heat flux study is part of smelting, computer chip manufacturing, boilers, blast furnaces and more.

    Specialized sensors measure radiative and convective heat flux.

    What if I still don't understand heat flux?

    Good news! Your calculations are accurate to eight decimal places. However, if you need help understanding heat flux, you have many references available to help. Of course, seek your teacher's guidance first.

    Online references can be very helpful. Many offer free lessons on the subject. To find them, it's best to use your favorite search engine. Try these websites:

    • Khan Academy
    • Thermopedia
    • YouTube

    For more practical examples, look at industry and trade magazines for articles and charts.

    • Principles of Heating and Cooling from the U.S. Department of Energy
    • Science Daily
    • Engineers Edge

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