Weld Heat Input Calculator
Calculate welding heat input and arc energy in kJ/mm, kJ/in and J/in to AWS D1.1, ASME IX or EN 1011-1. Use average volts and amps, a timed run, or the instantaneous energy or power method for pulsed and waveform-controlled welding. Checks a WPS window, solves the travel speed to hit it, and estimates the t8/5 cooling time.
How it works
Arc energy (kJ/mm) = V × A × 60 ÷ (travel speed mm/min × 1000) · waveform control: arc energy = instantaneous energy (kJ) ÷ bead length (mm), or instantaneous power (kW) × arc time (s) ÷ bead length (mm) · AWS D1.1 / ASME IX heat input = arc energy · EN 1011-1 heat input Q = k × arc energy (k = 1.0 SAW, 0.8 SMAW/GMAW/FCAW, 0.6 GTAW/PAW) · kJ/in = kJ/mm × 25.4 · t8/5 (EN 1011-2): 3D = (6700 − 5T₀) Q [1/(500 − T₀) − 1/(800 − T₀)] F3; 2D = (4300 − 4.3T₀) 10⁵ Q²/d² [1/(500 − T₀)² − 1/(800 − T₀)²] F2
Heat input is the energy put into each unit length of weld. It sets the size of the heat-affected zone, the cooling rate and the distortion, which is why a WPS gives it a range. The basic figure is arc energy: arc power (volts × amps) divided by travel speed. AWS D1.1 and ASME IX call that the heat input as it stands. EN 1011-1 multiplies it by a thermal efficiency factor k for the share of the arc that actually goes into the plate: 1.0 for submerged arc, 0.8 for stick, MIG/MAG and flux-cored, and 0.6 for TIG and plasma. Pulsed and other waveform-controlled processes change the current and voltage thousands of times a second, so averaged meter readings do not give the true power. For those, ASME IX QW-409.1, AWS D1.1 and ISO/TR 18491 use the instantaneous energy logged for the run divided by the bead length, or the instantaneous power times arc time divided by the length. This calculator handles all four ways of measuring, converts between kJ/mm, kJ/in and J/in, checks the result against WPS limits and gives the travel speed that keeps you inside them. For steel it can also estimate the t8/5 cooling time from 800 to 500 °C using the EN 1011-2 formulas.
Worked example
MIG at 25 V and 200 A, moving at 300 mm/min: arc energy = 25 × 200 × 60 ÷ (300 × 1000) = 1.000 kJ/mm, or 25.4 kJ/in (25,400 J/in). That is the heat input for AWS D1.1 and ASME IX. For EN 1011-1, multiply by k = 0.8 to get 0.800 kJ/mm (20.32 kJ/in). With a pulsed program, read the energy meter instead: 300 kJ logged over a 300 mm bead is also 1.000 kJ/mm. If the WPS allows at most 0.7 kJ/mm (EN basis), travel speed must rise to 0.8 × 25 × 200 × 60 ÷ (0.7 × 1000) = 343 mm/min.
Sources: ASME BPVC Section IX, QW-409.1 — heat input (including waveform-controlled welding) · AWS D1.1 Structural Welding Code — Steel · ISO/TR 18491 — Evaluation of instantaneous energy and power for arc welding · Weld Australia — travel speed and heat input calculator
Frequently asked questions
Should I apply a thermal efficiency factor?
It depends on the code. AWS D1.1 and ASME IX use arc energy as the heat input, with no factor. EN 1011-1 and EN ISO 15614-1 heat input multiplies arc energy by k: 1.0 for submerged arc, 0.8 for stick, MIG/MAG and flux-cored, and 0.6 for TIG and plasma. Always compare a value with a limit worked out the same way.
How do I calculate heat input for pulsed MIG?
Use the instantaneous method. ASME IX QW-409.1 and AWS D1.1 allow heat input = total energy (J) ÷ bead length, or average instantaneous power (W) × arc time (s) ÷ bead length. The meter must sample many times per pulse. Averaged volt and amp readings can misstate pulsed heat input by 10% or more.
How do I convert kJ/mm to kJ/in or J/in?
Multiply kJ/mm by 25.4 for kJ/in, and by 25,400 for J/in. 1.0 kJ/mm is 25.4 kJ/in or 25,400 J/in. To go back, divide kJ/in by 25.4.
What raises or lowers heat input?
Higher voltage or current raises it, and faster travel lowers it. Travel speed is the easiest lever: doubling it halves the heat input. Weaving slows the speed along the joint and so raises heat input, which is why travel speed is measured along the weld axis.
What is t8/5 and why does it matter?
It is the time the weld and heat-affected zone take to cool from 800 to 500 °C, the range where steel transforms. A short t8/5 gives hard, crack-prone microstructures; a long one coarsens the grain and lowers toughness. EN 1011-2 estimates it from heat input, preheat, plate thickness and joint shape.
Why do procedures limit heat input?
Too much heat coarsens the grain and softens the heat-affected zone, while too little can leave hard, crack-prone structures. When toughness is tested, ASME IX treats an increase in heat input over the PQR as a supplementary essential variable, so you must stay at or below the qualified value.
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This calculator is for educational purposes. Double-check important results before acting on them.