Inputs
Results
Heat input = η × V × A / (S × 1000). η = arc efficiency (0.7 for stick, 0.8 for TIG, etc.).
What this calculator actually solves
Heat input is the energy per unit length delivered to the workpiece during welding. It's the single number that controls HAZ (heat-affected zone) width, grain coarsening, distortion, and the risk of cracking in high-strength or hardenable steels. AWS D1.1 caps preheat and interpass temperatures based on this figure; many repair procedures specify a maximum kJ/in.
We use process-specific arc efficiency, which most calculators skip — but the difference between stick (0.70) and sub-arc (0.90) efficiency is a 30% swing in actual heat input.
Who this is for
Welders and fab shops working to a procedure spec (WPS) that calls out heat input limits.
Restoration and custom builders doing repair welds on thin-wall tubing where heat input determines whether the part survives.
How to read the results
For thin-wall (≤1/8"), keep heat input under 30 kJ/in. For 1/4" plate, 50 kJ/in is reasonable. Above 75 kJ/in you're depositing enough energy to grain-coarsen the HAZ of most carbon steels.
Limitations
Heat input doesn't tell you how heat distributes. Two welds with the same kJ/in can have very different HAZ widths depending on joint geometry and preheat.
We don't model pulse welding. Pulsed MIG/TIG can achieve lower heat input for the same deposition rate.
Frequently Asked Questions
What heat input should I cap for chromoly tube?
For 4130 chromoly in 0.095" wall, most race-welding guides cap at 25 kJ/in. Above that, you risk HAZ softening and a brittle zone in the heat-affected area.
Is TIG really lower heat input than MIG?
For the same V × A and same travel speed, TIG's higher arc efficiency means slightly higher heat input — but TIG also runs cooler voltage and current for the same bead. In practice, TIG welds are made with lower total heat input.
Does preheat reduce heat input?
No — preheat reduces the cooling rate, which is what matters for hydrogen cracking. Heat input is unchanged by preheat.
About this tool
What this tool is for: Welding Heat Input Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Heat input kJ/in or kJ/mm from voltage, current, travel speed, and process efficiency. It is energy delivered to the workpiece; HAZ width and microstructure still need a PQR.
How to use the body of this page
The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
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All content on this page, including formulas and calculator outputs, is provided for general informational and educational purposes only. It is not professional engineering advice, it is not a substitute for manufacturer specifications, and it is not a warranty of fitness for any particular purpose.
Mechanical, hydraulic, and welding work can cause serious injury, death, or property damage if performed incorrectly. Numbers produced by this calculator reflect simplified textbook models; they do not capture manufacturing tolerances, material defects, environmental conditions, wear, fatigue, regulatory codes (such as ASME, DOT, or AWS), or the specific service conditions of your job. Always cross-check critical values against the original equipment manufacturer's published specifications and a qualified engineer's review before committing to the job.
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