Universal Welding Formula

Welding Heat Input Calculator

Free online tool for welders and fabricators — calculate welding heat input in kJ/in and kJ/mm from voltage, amperage, and travel speed.

Built on AWS D1.1:2025 — standard energy input formula.

Voltage (V) Amperage (A) Travel Speed

Steel (optional — for interpass temp)

Understanding Your Result

What Your Heat Input Means

Heat input measures the energy transferred to the base metal per unit length of weld, expressed in kJ/in or kJ/mm. It is a function of voltage, amperage, and travel speed. Higher heat input means a wider heat-affected zone (HAZ), slower cooling, and greater risk of grain growth and reduced toughness in the base metal.

Per AWS D1.1:2025 Clause 7.7: “When quenched and tempered steels are welded, the heat input shall be restricted in conjunction with the maximum preheat and interpass temperatures required.”

Under D1.1:2025, heat input restrictions apply only to quenched and tempered (Q&T) steels per Clause 7.7. For these steels (A514, A517), the producer's maximum heat input recommendations must be followed to preserve the Q&T mechanical properties. ASTM A913/A913M is explicitly exempt from Clause 7.7. For all other structural steels (A36, A572, A992, A500, Groups I through III), D1.1 imposes no numeric kJ/in ceiling.

Even without a code-mandated limit, controlling heat input is good practice. Excessive heat input on thin sections can cause burn-through and distortion. On thick sections, insufficient heat input leads to fast cooling rates that increase hydrogen cracking risk. Pair your heat input with the preheat calculator and carbon equivalent to get the full thermal picture.

Common Questions

FAQ

How do you calculate heat input in welding?

Heat input is calculated as HI = (Voltage x Amperage x 60) / (Travel Speed x 1000), giving the result in kJ/in (when speed is in inches per minute) or kJ/mm (when speed is in mm/min). This is an industry-standard formula used for process documentation and control.

Does D1.1 have heat input limits?

D1.1 has NO numeric kJ/in limits for non-quenched-and-tempered steels (A36, A572, A992, A500, Groups I-III). For QT steels (A514, A517), Clause 7.7 states heat input must follow producer recommendations. A913 is explicitly exempt from Clause 7.7.

What is the difference between kJ/in and kJ/mm?

Both express heat input per unit length of weld. kJ/in (kilojoules per inch) is used in North American practice with AWS D1.1, while kJ/mm (kilojoules per millimeter) is used with ISO and European standards like EN 1011-2. To convert: kJ/mm = kJ/in divided by 25.4. This calculator displays both simultaneously so you can reference either system.

How does heat input affect the heat-affected zone?

Higher heat input produces a wider heat-affected zone (HAZ) with slower cooling rates, which reduces hardness but can lower toughness in some steels. Lower heat input creates a narrower HAZ with faster cooling, increasing hardness and potential for hydrogen-induced cracking. Controlling heat input is critical for steels with high carbon equivalent where rapid cooling causes brittle microstructure.

Does travel speed affect weld quality more than amperage?

Both variables directly affect heat input, but travel speed has the most immediate impact on weld profile and penetration. Decreasing travel speed increases heat input proportionally, widening the weld bead and HAZ. Very high travel speeds reduce penetration and can cause lack of fusion. The heat input formula treats voltage, amperage, and travel speed as equally weighted variables.

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