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Welding Heat Input Formula in kJ/mm and kJ/in

Calculate arc energy and efficiency-adjusted heat input from voltage, current, and travel speed without mixing units or terminology.

Published September 16, 2026Updated September 16, 2026Editorial method

Arc energy and heat input both express energy per unit weld length, but they are not always the same value. Arc energy uses voltage, current, and travel speed. Efficiency-adjusted heat input multiplies arc energy by the process efficiency factor.

Quick answer

Arc energy = V × I × 60 ÷ (travel speed × 1,000)

Heat input = efficiency × arc energy

Use mm/min for kJ/mm or in/min for kJ/in.

In this guideFormulaWorked exampleEfficiencyMeasurement limits

Arc-energy and heat-input formulas

Arc energyAE = (V × I × 60) / (v × 1,000)V = volts · I = amperes · v = travel distance/min · AE = kJ/distance
Efficiency-adjusted heat inputHI = η × AEη is the applicable process-efficiency factor as a decimal

The factor 60 converts seconds to minutes; 1,000 converts joules to kilojoules. If travel speed is recorded per second, do not include the 60 without first converting units.

Metric worked example

At 24 V, 200 A, 300 mm/min, and an entered efficiency of 80%:

  1. AE = 24 × 200 × 60 ÷ (300 × 1,000) = 0.960 kJ/mm.
  2. HI = 0.80 × 0.960 = 0.768 kJ/mm.
  3. Converted heat input: 0.768 × 25.4 = 19.507 kJ/in.
Output Value
Arc power 4.80 kW
Arc energy 0.960 kJ/mm
Heat input at 80% 0.768 kJ/mm

Use the required terminology and factor

TWI distinguishes arc energy before efficiency from heat input after efficiency. Some codes and procedures use different terminology, and process factors differ. Use the value and convention required by the applicable WPS, PQR, code, or engineering procedure rather than silently assuming a factor.

Measurement and process limitations

Voltage should represent the arc as required by the governing procedure; power-source terminal voltage can differ because of cable losses. Current, voltage, and travel speed can vary during the weld, so a single instantaneous reading may not represent the average energy. Pulsed and complex waveforms can require methods beyond multiplying simple mean values.

  • Record current, arc voltage, travel length, and arc time using the required method.
  • Keep arc energy separate from efficiency-adjusted heat input.
  • Document the efficiency factor and its source.
  • Compare the correct result with the WPS/PQR and applicable code.
  • Do not use this planning calculation to replace qualified welding engineering.

Technical references

Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.

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