Welding Heat Input Calculator
The energy in every millimetre of weld — volts, amps and travel speed into kJ/mm, corrected by the process efficiency.
Welding Heat Input Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Bead geometry, preheat and restraint not modeled
- Efficiency factors are band approximations
In short: Thirty volts at 250 amps, travelling 300 mm a minute, pours 1.5 kJ into every millimetre of weld as raw arc energy. A stick or MIG process delivers about 80% of that into the joint — 1.2 kJ/mm of real heat input; a TIG arc only six tenths. That number is the WPS's ceiling and floor: too hot and the heat-affected zone grows, cooking toughness out of the steel; too cold and the bead cracks from cooling too fast.
Formula
Q = V × A × 60 ÷ (v × 1,000) × η kJ/mm
Arc power is volts times amps; sixty seconds of it poured into one minute of travel gives energy per millimetre, divided by a thousand for kilojoules. The efficiency factor splits arc energy from heat input — submerged arc keeps everything, TIG radiates two fifths to the sky. ISO/TR 18491 and ASME IX frame the correction; AWS D1.1's annex permits a conservative 100% for every process.
Worked Example
- Enter the arc volts and amps as actually measured.
- Set the arc-on travel speed and the process.
- Read kJ/mm against the WPS window.
Defaults: 30 V, 250 A, 300 mm/min, a stick or MIG 80% — 1.5 kJ/mm of arc energy, 1.2 kJ/mm of heat input, 18 m of weld an hour. Run it TIG and the same arc delivers only 0.9.
Strengths & Limits Of This Model
Where this engine is strong
- Arc energy and heat input shown separately
- The process efficiency is an input, not a guess
Where it stops
- Waveform-controlled machines need integrated energy
Practical Use Cases
WPS windows
the kJ/mm line on the sheet
Quality control
drift caught between shifts
Crack-sensitive steels
cooling rate before the bead
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Welding Heat Input Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is the difference between arc energy and heat input?
Arc energy is the electricity per millimetre; heat input is the share that actually reaches the plate. The efficiency factor bridges them — SAW about 100%, stick and MIG near 80%, TIG nearer 60%. Procedures qualify heat input, not arc energy.
Why does travel speed divide the formula?
Because the arc spends less time over each millimetre when it moves faster. Double the speed and every millimetre gets half the energy — the coolest bead on the machine is the fastest one that still fuses.
Why does heat input have a floor as well as a ceiling?
Too little heat and the bead freezes before it wets, trapping slag or cracking hard steels. Too much and the heat-affected zone softens or embrittles. The WPS window is a band, and this page reads where the arc sits in it.
Where should volts and amps be measured?
At the arc — the machine's display carries cable losses and the operator's stick-out habits. A meter at the torch reads what the joint actually receives.
What counts as travel speed?
Arc-on speed only: the clock stops for fits, starts and electrode changes. Averaging a morning's metres over a morning's hours — stops included — flatters the number and lies to the WPS.
Why do the processes get different efficiencies?
Where the arc's heat goes differs: submerged arc traps everything under flux, TIG radiates to the sky through an open arc. The factors are approximate band-aids over physics — ISO/TR 18491 documents them as such.
Does heat input decide distortion?
It drives it — more joules per millimetre is more thermal expansion to shrink back, and the plate bows. Fasters, smaller beads and back-step sequences all spend the same joules more gently.
What is the connection to the steel around it?
The weld is only as good as the zone it cooked. The steel weight page weighs what the arc joins; the machining page sets the speeds that finish the weldment after it cools.