Welding Heat Input Calculator
Heat input in kJ/mm from voltage, current and travel speed, with EN 1011 process efficiency factors — for WPS and CE-marked work.
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How It's Calculated
Heat input follows the EN 1011 formula HI (kJ/mm) = (U × I × 60) ÷ (v × 1000), where U is arc voltage, I is welding current, and v is travel speed in mm/min. The ×60 converts amps and volts (watts) into energy delivered per minute, and ÷1000 converts joules to kilojoules while keeping the length unit in millimeters. Some standards multiply the result by a process efficiency factor k — 1.0 for submerged arc, 0.8 for MMA/stick and MIG/MAG, 0.6 for TIG — to account for how much arc energy actually reaches the plate versus radiating away.
Worked example: a MIG weld run at 24 V and 150 A, traveling at 200 mm/min. Raw heat input is (24 × 150 × 60) ÷ (200 × 1000) = 216,000 ÷ 200,000 = 1.08 kJ/mm. Applying the MIG/MAG efficiency factor of 0.8 gives 0.8 × 1.08 ≈ 0.86 kJ/mm — the number that actually lands in most WPS documents and that this calculator returns.
What You Should Know
Heat input is a direct trade-off, not a simple "more is better" or "less is better" dial. Higher heat input gives deeper penetration and a wider, smoother-looking bead, but it also coarsens the grain structure and widens the heat-affected zone (HAZ) — the region next to the weld that gets hot enough to change its metallurgy without melting. A coarser grain HAZ has lower impact toughness, which matters enormously on fine-grained or quenched-and-tempered structural steels used in cold climates or dynamically loaded structures. Excess heat input on thin material also raises the risk of burn-through and distortion from uneven thermal expansion.
Lower heat input avoids that coarsening and keeps distortion down, which is why thin sheet metal is usually welded with low current and fast travel speed. But push it too low and penetration becomes shallow — the weld may look fine on the surface while lacking fusion at the root, and on hardenable steels a fast cooling rate from low heat input can actually increase the risk of a hard, crack-susceptible zone and hydrogen-assisted cracking, especially without adequate preheat.
- There's an optimum range, not a single target, and it depends on material thickness and grade — a WPS document states the qualified range for a specific joint, and this calculator tells you where a given voltage/current/speed combination actually lands within (or outside) it.
- American practice (AWS) reports heat input in kJ/in without a k factor, since US codes typically define efficiency differently. Multiply a kJ/mm figure by 25.4 to get the equivalent in kJ/in for comparison across standards.
- Travel speed is the easiest number to get wrong in a shop setting — measure it directly (weld length ÷ time) on a real pass rather than reading it off a machine dial, since actual hand or robot speed often drifts from the programmed value.
Frequently Asked Questions
How do I measure travel speed accurately?
Time an actual weld pass over a measured length: 300 mm covered in 90 seconds is 200 mm/min. On mechanized or robotic rigs, read the carriage or program's set speed directly rather than estimating — manual welding speed varies more than people expect pass to pass.
Why does TIG get an efficiency factor of 0.6 while submerged arc gets 1.0?
The factor represents arc efficiency — how much of the arc's energy actually enters the base metal versus radiating into the surrounding air. An open TIG arc loses roughly 40% to radiation and convection; a submerged arc burns hidden under a blanket of flux, so almost none of its energy escapes before reaching the plate.
What is a typical acceptable heat input range?
General structural steel commonly runs 1.0-2.5 kJ/mm. Fine-grained or high-strength low-alloy steels are often capped near 1.5 kJ/mm to protect toughness, and may also carry a minimum heat input tied to preheat requirements that prevent hydrogen cracking. The governing WPS or application standard always has the final word.
Does heat input alone determine weld quality?
No — it's one variable among several, alongside preheat/interpass temperature, filler metal choice, joint preparation and shielding gas. Heat input mainly governs cooling rate and grain size; a weld can sit inside the right heat-input range and still fail for unrelated reasons like poor fit-up or contamination.
How does material thickness change the target heat input?
Thinner material needs lower heat input to avoid burn-through and excessive distortion, while thicker sections often tolerate — or even require — higher heat input to get adequate penetration and fusion through the full joint thickness. That's why a single "correct" kJ/mm number doesn't exist independent of the plate being welded.
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