How to calculate heat input

Heat input is a critical parameter for arc welding processes and it must be controlled to ensure sound weld quality.

Heat input may be defined as “The amount of electrical energy that is supplied to a weld during the welding process”.

How to calculate Heat input?

There are several ways of calculating the heat input. We shall discuss the two most common methods of calculating the heat input.

1. The American system (given in ASME BPVC Section IX – QW 409.1 (a) and various AWS standards):

Heat input:


  • Heat input is expressed in J/In (Joule/Inch) or J/mm (Joule/mm)
  • Voltage is expressed in Volts
  • Travel speed is expressed in In/min or mm/min

The unit for heat input obtained by this formula shall be either in J/In or J/mm. To get the results in KJ/in or KJ/mm, divide the result by 1000.

Example 1: If a welder takes 2 minutes to complete an 18 inches long weld. He keeps the voltage at 24 volts and the current 120 amps. What is your heat input?


Given data parameters are;

Travel Speed = Length of Weld/Time to weld = 18 inches/2 minutes = 9 in/min

Voltage = 24 volts

Current = 120 amps

Heat Input = (24 X 120 X 60)/ 9 = 19200 J/in

= 19.2 KJ/in (Divided by 1000 to obtain the result in KJ/in.)

2. European system (given in EN ISO 1011-1 and PD ISO/TR 18491)

An additional parameter of Thermal efficiency (process efficiency or arc efficiency) is used while calculating heat input as per European standards.

Heat input:

Value of thermal efficiency is different for different arc welding processes, see table 1:

Table – 1

Why Heat input is so important?

Heat input affects the cooling rates in welds and thereby it affects the microstructure of the weld metal and that of the heat-affected zone (HAZ). A change in microstructure directly affects the mechanical properties of weld metal and heat-affected zone (HAZ). Therefore, it’s very important to control the heat input to achieve a sound microstructure and a good quality weld.

Note: The heat input (as calculated by ASME standards) is referred to as “arc energy” under the European system. Hence the following formula holds true with regard to the European system;

Heat input = Thermal efficiency X Arc energy


Sandeep Anand

I am a Mechanical Engineer with more than ten years of work experience in the field of welding and NDT.

8 thoughts on “How to calculate heat input

  • August 27, 2019 at 10:17 pm

    Hai sir please explain about post weld heat treatment PWHT for pressure vessels.

    • August 19, 2020 at 11:01 pm

      Very nice information regarding heat input. Can you please tell the maximum and minimum values of heat input for different grades of materials based on ASME Standards.

  • September 16, 2019 at 9:38 pm

    Hello, I read your article and got very helpful information about calculation of heat input. Really I badly needed that and finally, I got this post. Thanks for your valuable post.

  • November 7, 2019 at 5:59 pm

    Dear sir please post about line checking in Piping ,mechanical punching and punch closing procedures…

  • March 23, 2020 at 8:35 pm

    Dear sir,
    Today I visited your website. I got very usefull notes. Thanks for your usefull notes.
    Dharam Tyagi

  • August 18, 2020 at 11:06 pm

    Related to pipings

  • September 3, 2020 at 3:31 pm

    Very helpful information about calculation of heat input, And thanks for your valuable post.

  • December 13, 2020 at 11:57 am

    The best part of your article is , ” Simple” and To the point which makes it easy to understand.

  • March 8, 2021 at 11:46 am

    Very Nice publication

  • April 16, 2021 at 10:49 pm

    I have been testing, this is one area I have had issues with. After this post, I get it!!! And you even cleared up K/in and KJ/in… Thank you very very much!!!


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