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Welding Heat Input Calculator (kJ/mm and kJ/in)

Heat input controls cooling rate, and cooling rate controls toughness, hardness and distortion. This calculator turns the three numbers on your welding machine and a travel speed measurement into kJ per millimeter and kJ per inch.

Your numbers

V
A
mm/min

Measure over a known weld length and time. Do not use wire feed speed.

Factors follow EN 1011-1 practice; ASME and AWS often report arc energy without an efficiency factor.

mm

Your results

Net heat input
0.8448
kJ/mm after applying the process efficiency.
Net heat input (kJ/in)
21.4579
Same value in kJ per inch.
Arc power
5.28
Volts times amps, in kW.
Arc energy (no efficiency)
1.056
kJ per mm of weld. Often used in AWS and ASME style procedures.
Arc time for the run
1.7 min
Energy delivered to the run
422.4
Total net kJ over the weld length.

Heat input limits come from your WPS and the governing standard. This calculator uses the common formula HI = (V x I x 60) / (speed x 1000) times efficiency. Always check whether your procedure limit is stated as arc energy or net heat input, and what efficiency factor applies.

Save results:

Net heat input vs travel speed

Sweeps Travel speed from 100 to 600, holding your other inputs fixed.

Check WPS limits automatically

Ask for a short review of how welding parameter data could be checked against procedure limits.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the arc voltage and current that you recorded during welding, not just the machine settings, because the actual values drift. Measure travel speed by timing a known length of weld. Choose the process efficiency your procedure uses. The output shows arc energy, net heat input in both unit systems, arc time and total energy for the run. Compare the net heat input with the minimum and maximum on the WPS. The chart shows how sensitive heat input is to travel speed.

  • Use average values from a pass, not a peak reading
  • Measure speed on the actual joint
  • Convert travel speed units before entering
  • Check the interpass temperature too

The formula and efficiency factors

Arc energy equals volts times amps times 60 divided by travel speed in mm/min and divided by 1000, giving kJ per mm. Net heat input multiplies arc energy by thermal efficiency, since not all arc power ends up in the workpiece. EN 1011-1 lists factors such as 1.0 for submerged arc, 0.8 for MMA and gas shielded metal arc, and 0.6 for TIG. US codes frequently quote heat input without a process factor, so be sure which basis your WPS uses. Pulsed processes require the average or an RMS based calculation per your standard.

Reading the result

With 24 V, 220 A and 300 mm/min, arc energy is about 1.06 kJ/mm, and net heat input with an efficiency of 0.8 is about 0.84 kJ/mm, or about 21 kJ/in. Slowing the travel speed raises heat input proportionally, which can soften fine grained steels and reduce impact toughness. Speeding up lowers it, which raises cooling rate and hardness risk in carbon manganese steels and increases the chance of lack of fusion. Austenitic and duplex stainless steels also have maximum heat input limits.

Common mistakes

Heat input errors are almost always unit or measurement errors.

  • Using wire feed speed instead of travel speed
  • Mixing mm/min with in/min
  • Applying an efficiency factor when the WPS limit is arc energy
  • Using the voltage setting on the machine instead of at the arc
  • Ignoring the effect of weave width on effective travel speed

Frequently Asked Questions

What is the formula for welding heat input?

Heat input in kJ/mm equals volts times amps times 60 divided by travel speed in mm/min times 1000, multiplied by thermal efficiency when the standard requires it. In inch units multiply the kJ/mm result by 25.4 to get kJ/in.

Which efficiency should I use?

Use the one required by your standard or WPS. EN 1011-1 gives typical values such as 1.0 for SAW, 0.8 for MMA, MAG and flux cored, and 0.6 for TIG. If your WPS states arc energy, do not apply a factor.

Why does heat input matter for steel?

It affects cooling rate. Too low can cause hard heat affected zones and cracking in hardenable steels, too high can reduce toughness and strength. Many procedures set both minimum and maximum limits and tie them to preheat.

How do I measure travel speed?

Mark a length of joint, time the welder from start to finish and divide length by time. For mechanized welding read the carriage speed setting. Use the true speed along the joint, not the weave pattern.

If you capture current, voltage and travel speed in an MES or weld monitoring system, we can help check heat input against the WPS automatically.

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