WeldMaster How to calculate heat input in welding, in kJ/in and kJ/mm
Heat input = volts × amps × 60 ÷ (1000 × travel speed in ipm), in kJ/in. Divide by 25.4 for kJ/mm. Plug in 24 V, 180 A and 8 ipm and you get 32.4 kJ/in, or 1.3 kJ/mm — the number a WPS review usually flags first. WeldMaster's Heat Input calculator (Pro) does the arithmetic and can time a bead to measure travel speed for you.
Why does heat input matter?
Heat input tells you how much energy the arc puts into each inch of weld. Too much and the heat-affected zone grows coarse and can lose toughness; too little and you risk lack of fusion and fast cooling that can crack hardenable steel. Many procedures set a maximum, and some a minimum.
What is the heat input welding formula?
- Heat input (kJ/in) = V × A × 60 ÷ (1000 × S), where S is travel speed in inches per minute.
- The 60 turns minutes into seconds, and the 1000 turns joules into kilojoules.
- Heat input (kJ/mm) = kJ/in ÷ 25.4.
- With travel speed in mm per minute, the same formula gives kJ/mm directly: V × A × 60 ÷ (1000 × mm/min).
WeldMaster shows arc energy with no process efficiency factor. Some codes multiply it by a factor that depends on the process, so check what your WPS asks for before you compare.
What are worked heat input examples?
| Voltage | Amperage | Weld speed | Heat input | App warning |
|---|---|---|---|---|
| 20 V | 200 A | 10 ipm | 24.0 kJ/in (0.9 kJ/mm) | none |
| 24 V | 180 A | 8 ipm | 32.4 kJ/in (1.3 kJ/mm) | none |
| 28 V | 250 A | 6 ipm | 70.0 kJ/in (2.8 kJ/mm) | high heat input |
| 18 V | 130 A | 12 ipm | 11.7 kJ/in (0.5 kJ/mm) | very low heat input |
Step by step for the second row: 24 × 180 = 4,320 W; × 60 = 259,200 J per minute; ÷ 8 in per minute = 32,400 J/in = 32.4 kJ/in; ÷ 25.4 = 1.28, shown as 1.3 kJ/mm.
How do you measure heat input in welding?
Volts and amps come off the machine or a meter — easy. Travel speed is the one people eyeball, and it's the number that moves the result most. Timing it beats guessing. WeldMaster has an optional timer for exactly that: enter the bead length, weld, and it works out the speed.
- Open Heat Input from Calculators (Pro).
- Under Arc Parameters, enter the Voltage and Amperage you are welding at.
- Under Weld Speed Timer (optional), set Weld Bead Length, for example 6 in.
- Tap Start Weld Bead Timer when you strike the arc and Stop & Use Weld Speed when you break it.
- The measured speed fills Weld Speed: 6 in in 45 s is 8.0 ipm. With 24 V and 180 A, Heat Input reads 32.4 kJ/in.
- Or skip the timer and type the Weld Speed directly.
How do you calculate heat input in welding in kJ/mm?
With Metric on in Settings, the speed field takes m/min, the bead length takes mm and the result is shown in kJ/mm. For example, 24 V, 180 A and 0.25 m/min give 1.0 kJ/mm. The app shows kJ/mm with one decimal, so for a tight limit do the last digit by hand with the formula above.
What is the maximum heat input in welding?
There is no single number: the maximum comes from your WPS, which is qualified with a certain heat input and material. WeldMaster flags two general limits: above about 50 kJ/in it warns that a slow travel speed can coarsen the HAZ and to check your WPS limits; below 15 kJ/in it warns about lack of fusion and fast-cooling cracks on hardenable steel.
What are common heat input mistakes?
- Using the machine's preset instead of the actual arc values. Read volts and amps while you weld if you can.
- Timing a weave with the bead length of a stringer. Measure the length of the weld, not the distance your hand travels.
- Comparing arc energy with a code value that includes an efficiency factor, or the other way round.
Questions and answers
Is there a heat input calculator app?
How do I convert kJ/in to kJ/mm?
Does travel speed or amperage matter more?
Does WeldMaster apply a process efficiency factor?
You might also like
- Carbon equivalent formula and preheat temperature for welding A36, 4140 and carbon steel · WeldMaster
More WeldMaster guides
- MIG welding settings chart: voltage, wire speed and gas by metal thickness
- MIG Settings for 1/8-Inch Mild Steel With .035 Wire
- Flux core welding settings and polarity: gasless vs dual shield
- Stick welding amperage chart for 6010, 6011, 6013, 7014, 7018 and 7024 rods
- TIG welding amperage chart, tungsten size and AC balance for steel, stainless and aluminum
- Carbon equivalent formula and preheat temperature for welding A36, 4140 and carbon steel
- MIG porosity, spatter and burn-through: causes and how to fix them
- Which filler rod: stainless to mild steel, 4043 vs 5356, chromoly and more
- Why is my MIG welder popping, and which setting do I change?
- MIG vs flux core welding: which one for which job?
- SS400 welding rod: which electrode, wire and amps?
- All questions about WeldMaster
- Back to WeldMaster