Welding heat input calculator.

Answer first. Arc energy (kJ/mm) = volts x amps x 60 / (travel speed in mm/min x 1000). Heat input = process efficiency x arc energy. At 25 V, 200 A and 300 mm/min, arc energy is 1.0 kJ/mm (25.4 kJ/in) and MIG heat input at a 0.8 factor is 0.8 kJ/mm.

Worked example

25 V x 200 A = 5,000 W. At 300 mm/min (5 mm/s) each millimetre of weld receives 5,000 / 5 = 1,000 J, which is 1.0 kJ/mm. Multiply by 0.8 for MIG/MAG: heat input 0.8 kJ/mm, or 20.3 kJ/in.

How it works

Two terms. TWI defines arc energy as the energy supplied by the arc before process efficiency is considered, and heat input as the efficiency-weighted value, HI = efficiency x arc energy. TWI also notes that a number of American codes use the word heat input for what TWI calls arc energy. Read your procedure or code to see which one it wants, and use the arc energy line when it does not apply an efficiency factor.

Efficiency factors. TWI gives submerged arc 1.0, manual metal arc (stick) 0.8, flux-cored 0.8, MIG/MAG 0.8 and TIG 0.6.

Use of the number. Heat input affects cooling rate, heat-affected zone properties and distortion, and procedures set limits on it for some steels. This tool only does the arithmetic and does not tell you which limit applies.

Safety. Welding produces arc radiation, fumes, sparks and burns. Use a proper welding helmet, gloves and flame-resistant clothing, ventilate or extract fumes, and remove flammables from the area. This is a planning aid, not a welding procedure, engineering design or professional advice.

Sources

• FAQ: What is the difference between heat input and arc energy? (TWI) (opened 2026-10-15)

The efficiency factors and the definitions are from the TWI page. The arc energy expression follows from its definition (power divided by travel speed); 1 in = 25.4 mm.

FAQ

What is the welding heat input formula?

Arc energy in kJ/mm = volts x amps x 60 / (travel speed in mm/min x 1000). Heat input is that value multiplied by the process efficiency factor.

What is the difference between heat input and arc energy?

Arc energy ignores process efficiency. Heat input multiplies it by the efficiency factor. TWI notes some American codes use the term heat input for arc energy.

What efficiency factor do I use for MIG?

TWI gives 0.8 for MIG/MAG and flux-cored, 0.8 for stick, 0.6 for TIG and 1.0 for submerged arc.

How do I get kJ/in?

Multiply kJ/mm by 25.4. The tool shows both.

Does this check my procedure limits?

No. It only calculates the value. Compare it to the limit in your welding procedure specification or code.