Weld Heat Input Calculator

Arc power and heat input from voltage, current, and travel speed, in metric or imperial units.

What it does

Heat input is the amount of arc energy delivered per unit length of weld. It shapes cooling rate, grain structure, distortion, and the risk of cracking, which is why welding procedures often set a maximum or a range. The calculation takes the electrical power of the arc and divides it by how fast the torch or electrode travels.

Arc power is straightforward. Voltage in volts times current in amps gives watts, and dividing by 1,000 gives kilowatts. A 25 volt arc at 200 amps is 5.00 kW. That power enters the work over the length the arc travels in a minute, so heat input follows as 60 times volts times amps divided by travel speed.

Unit systems change the constant. In imperial units, travel speed is inches per minute and heat input comes out in kilojoules per inch. A 5.00 kW arc moving at 12 inches per minute deposits 25.0 kJ/in. In metric units, the same arc moving at 304.8 mm per minute deposits 0.98 kJ/mm, and the two figures describe the same weld.

The calculator reports both units at once so a procedure written in kJ/in can be checked against a metric specification without a separate conversion. The relationship is 1 kJ/in = 0.03937 kJ/mm, or equivalently 1 kJ/mm = 25.4 kJ/in.

The result is a lower bound on the energy that reaches the metal. Real processes lose energy to spatter, radiation, and the arc atmosphere, so standards sometimes apply an efficiency factor before comparing heat input with a limit. This tool follows the common practice of reporting the nominal arc energy with no efficiency applied.

How to use

1. Pick imperial or metric units.
2. Enter the arc voltage from the welding procedure.
3. Enter the welding current in amps.
4. Enter the travel speed in in/min or mm/min.
5. Read the arc power and heat input in the selected system.
6. Compare the other-unit figure with any specification written in the opposite system.

Formula

V

Arc voltage in volts.

I

Welding current in amps.

Travel

Travel speed in inches per minute or millimeters per minute.

Heat input

Arc energy per unit length of weld, in kJ/in or kJ/mm.

The 60 converts minutes to seconds so the result lands in kilojoules per unit length. The formula uses nominal arc voltage and current and assumes all arc energy enters the weld. Process efficiency, joint geometry, and preheat are not included. For code work, apply the efficiency factor or the measuring method named in the applicable standard.

Worked example

Reference table

Limits

• Assumes all arc energy enters the weld, with no efficiency factor applied.
• Uses nominal voltage and current, not measured waveforms.
• Does not account for preheat, interpass temperature, or joint geometry.
• Applies to a steady travel speed, not to weaving or pulsed techniques with varying speed.

Common mistakes

• Using inches per minute in the metric formula or millimeters per minute in the imperial formula.
• Forgetting the factor of 60 that converts per-minute travel into per-second energy.
• Treating heat input as the total heat in the part rather than energy per unit length.
• Comparing a nominal heat input with a code limit that requires an efficiency factor.
• Changing current without noting that arc power and heat input both move with it.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• AWS: Key concepts in welding heat input
• NASA-STD-5006: General welding requirements for aerospace materials

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