Weld Heat Input Calculator
Calculate arc energy and heat input to AWS D1.1, in kJ per inch and per millimetre, with the process efficiency that separates the two.
How to use this calculator
- 1Time a measured run to get travel speed. Mark a length, weld it, stop the clock, divide - it takes a minute and removes the biggest error in the calculation.
- 2Read the voltage as close to the arc as you can. At the machine, on long leads at high current, the cable drop is several volts.
- 3Pick the process so the efficiency is right. Arc energy and heat input differ by a fifth for MIG and by nearly half for TIG.
- 4If your procedure sets a limit, enter it - and check whether it also sets a minimum, because many do.
How the calculation works
Arc energy (kJ per unit length) = volts x amps x 60 / (travel speed per minute x 1000)
Heat input = arc energy x thermal efficiency
Efficiency: 0.80 stick, MIG and flux-cored; 0.60 TIG; 1.00 submerged arc- Volts
- Arc voltage, measured at the arc rather than at the machine
- Amps
- Welding current
- Travel speed
- Distance the arc advances per minute. Timed over a measured length, not estimated
- Efficiency
- The fraction of arc energy that enters the work. What separates arc energy from heat input
This is AWS D1.1 clause 6.8.5, and it is the same equation ASME Section IX uses. The 60 converts minutes to seconds and the 1000 converts joules to kilojoules.
Heat input is inversely proportional to travel speed and directly proportional to both volts and amps. Of the three, travel speed is the one most often estimated and the one with the largest error.
Codes differ on which quantity they regulate. Some specify arc energy, some heat input, and a procedure that quotes one where the other is required is a routine review finding.
Worked example
MIG on structural steel
- 1.24 V at 220 A is 5.28 kW of arc power.
- 2.Arc energy is 24 x 220 x 60 / (12 x 1000) = 26.4 kJ per inch.
- 3.MIG transfers about 80% into the work, so heat input is 21.1 kJ/in - or 0.83 kJ/mm.
- 4.A 24 in weld at 12 in/min takes two minutes of arc time and puts 634 kJ into the joint.
Result: 21.1 kJ/in of heat input, from 26.4 kJ/in of arc energy
The same settings run slower
- 1.Identical volts and amps - the display looks the same and the welder would say nothing changed.
- 2.But travel speed is 8 in/min rather than 12, and heat input goes inversely with it.
- 3.Arc energy rises from 26.4 to 39.6 kJ/in, and heat input from 21.1 to 31.7.
- 4.Half again the heat into the same joint, from a variable nobody was watching. This is why procedures specify travel speed and why it gets recorded.
Result: 31.7 kJ/in - 50% more heat, same machine settings
TIG, where the efficiency gap is largest
- 1.12 V at 120 A travelling 100 mm/min gives arc energy of 12 x 120 x 60 / (100 x 1000) = 0.864 kJ/mm.
- 2.TIG runs about 60% thermal efficiency - a large share of the arc energy goes to heating the tungsten, the gas and the surroundings.
- 3.So heat input is 0.518 kJ/mm, well inside a 1.5 kJ/mm procedure limit.
- 4.Had arc energy been reported as heat input the figure would have been 0.864 - still compliant here, but on a tighter limit that difference decides it.
Result: 0.518 kJ/mm of heat input against 0.864 kJ/mm of arc energy
Why anybody measures heat input
A weld is a casting made at speed inside a heat sink. The molten pool solidifies, and the metal around it - the heat affected zone - is heated and cooled through a range that changes its structure without ever melting it. What that structure ends up being depends almost entirely on how fast it cooled, and how fast it cooled depends on how much energy went in per unit of length.
Too little and the cooling is fast. In hardenable steels that means martensite, which is hard, brittle and intolerant of the hydrogen that every arc introduces - the combination that produces cracks appearing days after the welding finished. Too much and the cooling is slow, the grain in the heat affected zone grows coarse, and toughness falls away. On quenched-and-tempered steels excessive heat input simply undoes the heat treatment the plate was bought for.
So a qualified procedure for anything demanding carries a range rather than a single value, and heat input is how that range is expressed. It is one of the essential variables in ASME IX and AWS D1.1 - change it beyond the qualified range and the procedure is no longer qualified.
Arc energy against heat input
The equation gives arc energy: the electrical energy the power source delivered per unit of weld length. Not all of it reaches the work. Some radiates from the arc, some heats the electrode or the tungsten, some leaves with the shielding gas, and in stick welding some goes into vaporising the coating.
Thermal efficiency captures that. It is around 0.8 for stick, MIG and flux-cored; about 0.6 for TIG, which loses a great deal to the tungsten and to radiation; and essentially 1.0 for submerged arc, where the flux blanket traps everything. Heat input is arc energy times that factor.
The awkwardness is that codes are not consistent about which they regulate, and the words get used interchangeably in the shop. A procedure that specifies a heat input limit and a welder who calculates arc energy will disagree by a fifth, in the direction of thinking they are compliant when they are not. It is worth being explicit about which quantity a number is, every time.
- Stick (SMAW) — About 80%. Losses to coating vaporisation, spatter and radiation.
- MIG (GMAW) — About 80%. Similar losses without the coating.
- Flux-cored (FCAW) — About 80%.
- TIG (GTAW) — About 60%. The tungsten and the gas take a large share.
- Submerged arc (SAW) — Essentially 100%. The flux keeps the energy in the joint.
Travel speed is the variable that moves
Of the three inputs, volts and amps are displayed on the machine and are usually within a few percent of what the operator believes. Travel speed is not displayed anywhere, and it is estimated by eye far more often than it is measured.
It is also the term heat input is most sensitive to, because it sits in the denominator. Running 25% slower than intended raises heat input by a third. Running at half speed doubles it. Two welders on the same machine at the same settings can differ by a factor of two in heat input and produce welds that look similar and behave differently.
Measuring it is trivial and almost nobody does it: mark a length on the joint, weld it, time it, divide. On a qualified procedure the travel speed range is recorded precisely because it is the variable most likely to drift, and a shop that has never timed a run does not actually know what its heat input is.
What this assumes, and where it stops
Assumptions
- The equation is AWS D1.1 clause 6.8.5, which matches ASME Section IX.
- Thermal efficiencies are the standard values used in ISO/TR 18491 and AWS practice.
- Voltage is taken as the arc voltage. Reading at the machine on long leads over-states it.
- Travel speed is constant along the run, which weaving and stop-start welding both break.
Limitations
- It does not predict cooling rate, hardness or microstructure. Those depend on plate thickness, joint geometry, preheat and the steel itself as well as on heat input.
- Weaving raises heat input per unit of joint length above what a straight-line travel speed calculation shows, because the arc travels further than the joint does.
- Pulsed and waveform-controlled processes need the instantaneous power averaged properly. Using the meter average of volts times amps can be materially wrong for those.
- Multi-pass welds have a heat input per pass, and the interpass temperature is a separate control that this page does not address.
- Codes differ on whether they regulate arc energy or heat input. Read the procedure rather than assuming.
Common questions
What is the difference between arc energy and heat input?
Arc energy is what the power source delivered per unit length; heat input is the part of it that actually entered the work. They differ by the thermal efficiency of the process - about 80% for stick, MIG and flux-cored, 60% for TIG, and essentially 100% for submerged arc. Codes are specific about which one they regulate and the words get used loosely in the shop, which is how procedures fail review.
Why does my procedure limit heat input?
Because heat input controls the cooling rate, and the cooling rate controls what the heat affected zone turns into. Too much heat coarsens the grain and destroys toughness - on quenched-and-tempered steels it simply undoes the heat treatment. Many procedures also set a minimum, because too little heat cools fast enough to form brittle martensite and invites hydrogen cracking.
How do I measure travel speed?
Mark a known length on the joint, weld it, time it with a stopwatch, and divide. It takes a minute. Travel speed is the input heat input is most sensitive to, it is the one not shown on any display, and it is the one most often estimated by eye - a shop that has never timed a run does not know its heat input to better than a factor of two.
Does weaving change heat input?
Yes, upward. The straight-line travel speed along the joint is what goes into the equation, but the arc has travelled much further and burnt for much longer over that length of joint. A wide weave can put in several times the heat of a stringer bead at the same nominal travel speed, which is why procedures with heat input limits usually restrict weave width too.
Is high heat input always bad?
No - it is a range, not a maximum. Higher heat input slows the cooling and reduces the risk of hydrogen cracking, which is why preheat exists to achieve the same thing. What it costs is grain size in the heat affected zone, toughness, and distortion. The right value is whatever the qualified procedure says, and where there is no procedure, moderate is a better guess than either extreme.
Sources
- AWS D1.1 Structural Welding Code - Steel — American Welding Society
- ISO/TR 18491 - determination of heat input and arc energy — International Organization for Standardization
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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