How to calculate heat input

Heat input is a critical parameter for arc welding processes. The Heat input shall 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. 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 a 18 inches long weld. He keeps the voltage at 24 volts and 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 European system;

Heat input = Thermal efficiency X Arc energy


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