Free tool · no sign-up
Heat input unit converter
One figure, every unit a welding procedure is written in: kJ/mm, kJ/cm, kJ/in and J/mm, converted together rather than one pair at a time. The thermal efficiency factor is a separate, explicit choice, so a value never changes quantity without you saying so.
Travel speed converts alongside it, because a procedure states mm/min or in/min while the arc energy equation takes mm/s. Every factor used is listed with the result, and the calculation runs entirely in your browser.
Every factor on this page, written out
Energy per unit length — 1 kJ/mm = 10 kJ/cm = 25.4 kJ/in = 1000 J/mm
Travel speed — 1 mm/min = 0.1 cm/min = 1/25.4 in/min = 1/60 mm/s
Efficiency — heat input = k × arc energy, so arc energy = heat input / k
1 in = 25.4 mm exactly (international inch), 1 cm = 10 mm, 1 kJ = 1000 J, 1 min = 60 s. k is dimensionless.
The length and time factors are definitions, not measurements, so the unit conversions on this page are exact and add no uncertainty of their own. The factor k is different in kind: it is an engineering allowance for the fraction of the electrical energy that entered the joint, and EN 1011-1 / ISO/TR 17671-1 is the document that names arc energy, heat input and the factor between them. Which value of k belongs to which process is a statement that standard and your welding procedure make, which is why this page asks you to type it instead of choosing one for you.
That separation is the whole point of keeping the two operations apart. Converting kJ/in into kJ/mm changes the unit and leaves the quantity alone. Multiplying by k changes the quantity and leaves the unit alone. A number that has had both done to it without a record of either is the one nobody downstream can check.
How to convert a heat input between units
From a figure written in one procedure to the same figure in the unit the next document expects.
- Find out which quantity you are holding. Arc energy is what the arc delivered per unit length; heat input is the part of it that entered the joint. A number without that label cannot be converted safely, because the conversion depends on whether the efficiency factor has already been applied.
- Enter the value and its unit. kJ/mm, kJ/cm, kJ/in or J/mm. The unit conversion is exact arithmetic and adds nothing to the uncertainty of the original figure.
- Decide what happens to the factor k. Leave it at convert-only when both documents use the same basis. Apply k when you hold arc energy and the receiving document wants heat input; remove k when the reverse is true. Type the factor your procedure or the standard assigns to the process.
- Convert the travel speed as well if the calculation needs it. A procedure states mm/min or in/min while the arc energy equation takes mm/s. The travel speed section converts between them independently of the energy value.
- Record the conversion, not only the result. Write down the unit, the basis and the factor used. A converted number quoted on its own cannot be checked, and a reviewer who cannot repeat the arithmetic has to reject it.
- Convert the number, then say what it is. The unit conversion is arithmetic; the basis is a claim. A figure labelled only "1.2" is ambiguous in two directions at once, and the heat input calculator is the page to start from when you have volts, amperes and a travel speed rather than a converted value.
- The factor of 60 is where speed calculations go wrong. A procedure quoting 250 mm/min and an equation expecting mm/s differ by a factor of sixty, so an unconverted speed produces an energy that is wrong by the same factor. It is a large enough error to be obvious, which is why it survives to the review rather than to the joint.
- A converted limit is still a limit somebody set. No acceptance value is built into this page, in either unit. Where a maximum applies it comes from the specification, and this tool only makes sure both sides of that comparison are expressed the same way.
- Cooling time is what the energy is really about. Energy per unit length matters because it sets how fast the joint cools; the t8/5 cooling time calculator turns a converted heat input into that number.
- Nominal and measured are not the same quantity. A heat input copied from a procedure sheet describes what was planned. What the power source delivered on the day is a measurement, which is the subject of welding parameter monitoring and is recorded pass by pass by WeldTrace.
For why arc energy and heat input are counted separately in the first place, read the heat input calculation guide.
Questions engineers ask about this calculation
How do you convert a heat input from kJ/mm to kJ/in?
Multiply by 25.4, because an inch is 25.4 millimetres exactly and the quantity is an energy divided by a length: a joint that absorbs 1 kJ over every millimetre absorbs 25.4 kJ over every inch. The same identity read the other way means kJ/in divided by 25.4 gives kJ/mm. The two remaining units on this page follow from the metric prefixes rather than from the inch: 1 kJ/mm is 10 kJ/cm because a centimetre is ten millimetres, and 1000 J/mm because a kilojoule is a thousand joules. All four factors are exact by definition, so a conversion introduces no uncertainty of its own — the uncertainty is in the volts, amperes and travel speed the original figure came from.
Does converting the units of a heat input change the thermal efficiency factor?
No. The factor k is dimensionless, so it survives any change of unit untouched. Converting kJ/mm into kJ/in and applying k are two separate operations, and this page keeps them separate for that reason: the unit conversion is arithmetic that is always valid, while applying k is a statement about which quantity you are holding. EN 1011-1 defines arc energy as the electrical energy the arc delivered per unit length and heat input as the fraction of it that entered the joint, with k relating the two. If you multiply by k you have changed quantity, not unit, and the result has to be labelled accordingly.
Which travel speed unit does the arc energy equation expect?
Millimetres per second, when the equation is written in the usual form of volts times amperes divided by travel speed, because that combination returns joules per millimetre directly. Shop-floor documents rarely use it: a WPS states mm/min, a machine readout may show cm/min, and a North American procedure states in/min. Divide mm/min by 60 for mm/s, multiply cm/min by 10 and in/min by 25.4 to reach mm/min first. Mixing the units up by a factor of 60 is the most common arithmetic error in a heat input calculation, and it is large enough that the answer is obviously wrong once you look at it.
Can a heat input limit written in kJ/in be compared with a record in kJ/mm?
Only after converting one of them, and the conversion has to be recorded alongside the comparison so a reviewer can repeat it. Two further things have to match before the comparison means anything. The first is the basis: a limit written for heat input cannot be checked against a figure that is really arc energy, and whether k was applied is not visible in the number itself. The second is the measurement: a nominal figure taken from the WPS and a figure computed from what the power source actually delivered are different quantities that share a unit. A converted value inherits every assumption of the value it came from.
A converted number is only as good as the figure it came from.
WeldTrace records voltage, current and wire feed speed at 25 kHz on every pass, so the heat input you convert is what the arc actually delivered on the joint rather than a nominal value copied off a procedure sheet.
