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Weld Heat Input Calculator

Find arc energy and heat input in kJ/in or kJ/mm from arc voltage, current and travel speed, with the heat efficiency factor for each welding process.

Level: Grades 7–8

Printable worksheet

Weld Heat Input Calculator

Know the answer you need? Pick an input to work backward.

Inputs

The length of weld you made, divided by the time the arc was on to make it.

Result

Heat input

21.12 kJ/in

Heat input is 21.12 kJ/in (0.8315 kJ/mm) at k = 0.8.

Heat input in kJ/mm
0.8315 kJ/mm
Arc energy (k = 1)
26.4 kJ/in
Arc energy (k = 1) in kJ/mm
1.0394 kJ/mm
Arc power
5.28 kW
Thermal efficiency factor k
0.8
Travel speed
12 in/min
Travel speed in mm/min
304.8 mm/min

See the step-by-step solution ↓

Step-by-step solution

  1. Find the arc's power in watts: volts × amps

    24×220=5,280 W

  2. Find the arc energy per inch: power × 60 gives energy per minute, then divide by the inches welded per minute (and by 1,000 for kJ)

    24×220×601,000×12=26.4 kJ/in

  3. Multiply by the process factor k = 0.8, the share of heat that reaches the work, to get heat input

    0.8×26.4=21.12 kJ/in

  4. For kJ/mm, divide by 25.4, the millimeters in an inch

    21.1225.4=0.831496 kJ/mm

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how much heat your weld puts into each inch or millimeter of the joint. That's called heat input, the energy the arc delivers per unit length of weld. It controls how fast the weld and the metal around it cool down. The cooling speed affects hardness, toughness and how much the part warps (distortion). Welding procedures for many steels set an allowed range for it. Inspectors check a welder's actual volts, amps and travel speed against that range.

The calculator finds the arc energy from the voltage, current and travel speed. Then it multiplies by the process's thermal efficiency factor, k, the share of heat that actually reaches the work. That gives the heat input, in both kJ/in and kJ/mm. Codes handle this differently. The European approach applies k. Several American codes use the term heat input for the arc energy itself, which you get here by choosing no factor. Your welding procedure specification and its code decide which figure and which limits apply.

Where it's used

Why this math matters outside the classroom:

  • Procedure checks. An inspector reads 24 V, 220 A and 12 in/min and works out 26.4 kJ/in of arc energy to compare with the procedure.
  • Metric procedures. A welder at 25 V, 200 A and 300 mm/min has 1.0 kJ/mm of arc energy, or 0.8 kJ/mm with the stick factor.
  • Travel speed changes. Slowing from 12 to 8 in/min at the same settings raises heat input by 50%, from 21.12 to 31.68 kJ/in.
  • Training welders. Welding students time a 6 in bead in 30 seconds, a travel speed of 12 in/min, and calculate their own heat input.

How to use it

  1. Enter the arc voltage and welding current, measured at or near the arc where you can.
  2. Enter the travel speed and choose its unit: inches per minute, millimeters per minute or millimeters per second.
  3. Choose the process to apply its efficiency factor, or choose none for the arc energy alone.
  4. Read the heat input in the unit that matches your speed. The other unit and the arc energy are shown alongside.

What the inputs mean

Arc voltage
Use the voltage across the arc. Readings at the welding machine include losses in the cables. So they can be higher than the true arc voltage, which makes heat input look higher than it is.
Travel speed
Measure the length of weld bead you made, and divide by the time the arc was on for that bead. If you weave, use the speed along the joint, not the side-to-side motion of the torch.
Process (thermal efficiency k)
The factor k is the share of arc energy that ends up in the work, compared with submerged arc welding. These values are the ones listed in EN 1011-1 (also published as ISO/TR 17671-1).

Formula and method

arc energy (kJ/in) = V × A × 60 ÷ (1,000 × travel speed in in/min)
arc energy (kJ/mm) = V × A × 60 ÷ (1,000 × travel speed in mm/min)
heat input = k × arc energy
1 kJ/mm = 25.4 kJ/in

V is the arc voltage in volts, and A is the current in amps. Volts times amps is the arc's power in watts, which means joules per second. Multiply by 60 to get joules per minute. Then divide by the distance traveled per minute, and you're left with joules per unit length. Dividing by 1,000 turns joules into kilojoules (kJ).

The thermal efficiency factor k scales the arc energy down to the heat actually delivered to the joint. It's about 1.0 for submerged arc, and 0.8 for stick, MIG/MAG and flux-cored welding. For TIG and plasma, it's 0.6, since more heat escapes from the open arc. Travel speed is converted exactly, with 1 in = 25.4 mm.

Worked example

A MIG weld is made at 24 V and 220 A with a travel speed of 12 in/min.

  1. Arc power: 24 × 220 = 5,280 W.
  2. Arc energy: 5,280 × 60 ÷ (1,000 × 12) = 26.4 kJ/in.
  3. Heat input with k = 0.8: 0.8 × 26.4 = 21.12 kJ/in.
  4. In metric: 21.12 ÷ 25.4 = 0.8315 kJ/mm.

Result: The heat input is 21.12 kJ/in (0.8315 kJ/mm). The arc energy, before the efficiency factor, is 26.4 kJ/in.

Understanding the result

Higher heat input slows cooling. That tends to lower hardness in the heat-affected zone, the metal next to the weld that heats up without melting. But it can also reduce toughness and increase warping. Lower heat input cools faster. That can raise hardness and the risk of cracking in steels that harden easily. The right range depends on the steel, the thickness and the procedure.

When you compare with a procedure's limits, use the same definition it uses. A figure with k applied and one without can differ by 20 to 40% for the same weld.

Assumptions and limitations

  • Voltage, current and travel speed are steady averages over the bead.
  • The process factor is the typical listed value for that process.
  • Travel speed is measured along the line of the weld.
  • Pulsed and waveform-controlled processes need moment-by-moment power or energy averaging. A single volts-times-amps figure can read too low or too high for them.
  • The factor k is an average. Real efficiency varies with shielding gas, arc length and technique.
  • This calculator doesn't set or check heat input limits; the procedure and code do.

Frequently asked questions

What is the difference between arc energy and heat input?

Arc energy is the electrical energy per unit length, from volts, amps and travel speed. Heat input multiplies it by a process efficiency factor to estimate the energy that actually reaches the joint. Some American codes use the words heat input for what this page calls arc energy. So check how your procedure defines it.

How do I convert kJ/mm to kJ/in?

Multiply by 25.4, since there are 25.4 mm in an inch. 1.0 kJ/mm is 25.4 kJ/in, and 21.12 kJ/in is 21.12 ÷ 25.4 ≈ 0.83 kJ/mm.

Why does travel speed matter so much?

Heat input goes up as travel speed goes down, in direct proportion. Slowing from 12 to 8 in/min at the same volts and amps raises heat input by half. That's why weaving or slow travel can push a weld outside its procedure limits.

Which efficiency factor should I use?

Use the one your welding procedure or code specifies. The common listed values are 1.0 for submerged arc, 0.8 for stick, MIG/MAG and flux-cored, and 0.6 for TIG and plasma. If your code works with arc energy, choose no factor.

Sources

How this calculator is checked

Method: Arc energy = V × I × 60 / (1000 × travel speed); heat input = k × arc energy, with k from the EN 1011-1 table as reproduced by TWI (SAW 1.0; SMAW, GMAW, FCAW 0.8; GTAW and plasma 0.6). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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