Two passes can use the same nominal current and voltage yet deliver different energy per inch. Travel speed is the missing variable. A slower pass keeps the arc working over each inch for longer.
Calculating arc energy helps explain that relationship and check a welding record. It does not establish an acceptable welding procedure, prove fusion, or authorize a change to the welding procedure specification, or WPS. Use the applicable WPS and the project's approved recording method for production work.
Name the quantity before calculating it
Arc energy is electrical energy per unit weld length before applying a process-efficiency factor. In one common convention, heat input is arc energy multiplied by an efficiency factor. Some American codes use the term heat input for the unadjusted arc-energy calculation.
That terminology difference matters. Confirm which definition, units, measurement method, and limits apply to the job. Do not add an efficiency factor to a record simply because a calculator has a field for one.
Find travel speed from actual weld length
For this teaching example, assume a steady, non-waveform-controlled arc and suitable measured values:
- Current: 150 amperes
- Arc voltage: 12 volts
- Weld length along the joint: 6 inches
- Arc-on time for that length: 90 seconds
Convert seconds to minutes: 90 ÷ 60 = 1.5 minutes.
Travel speed = 6 inches ÷ 1.5 minutes = 4 inches per minute.
Use the weld distance progressed along the joint, not the side-to-side path of a weaving electrode. The time and length must describe the same measured segment. Lunch breaks, slag removal, and other arc-off time do not belong in this simple continuous-pass calculation. Interrupted or multipass records must follow the project's approved method.
Calculate arc energy in kilojoules per inch
For the simplified steady-arc case:
Arc energy in kJ/in = voltage × current × 60 ÷ (1,000 × travel speed in in/min)
Insert the example values:
12 × 150 × 60 ÷ (1,000 × 4) = 27 kJ/in.
The factor 60 converts minutes to seconds. The factor 1,000 converts joules to kilojoules. Writing both factors makes a unit mistake easier to spot.
Check the result a second way. The arc power is 12 × 150 = 1,800 watts, or 1,800 joules per second. Over 90 seconds, the energy is 162,000 joules. Divide by the 6-inch weld length: 27,000 joules per inch, which is 27 kJ/in.
The example settings are invented for arithmetic only. They are not a recommended GTAW procedure or a permitted parameter range.
Change one variable on paper
Keep voltage and current the same but complete the 6-inch segment in 60 seconds. Travel speed becomes 6 in/min and arc energy becomes 18 kJ/in.
Take 120 seconds instead. Travel speed becomes 3 in/min and arc energy becomes 36 kJ/in.
This comparison explains why the same machine settings do not guarantee the same energy per inch. It also shows why changing travel speed just to reach a desired calculator result is not a welding instruction. All required procedure variables still apply.
Keep metric and inch values consistent
One inch equals exactly 25.4 millimeters. Therefore:
27 kJ/in ÷ 25.4 = approximately 1.063 kJ/mm.
The same example travels 152.4 mm in 1.5 minutes, or 101.6 mm/min. Substituting that metric speed into the same formula gives approximately 1.063 kJ/mm.
Do not compare 27 kJ/in directly with a limit written in kJ/mm. Convert the units first, retain useful precision through the calculation, and apply the reporting precision required by the job.
Know where the simple calculation stops
Pulsed and other waveform-controlled processes require particular care. Multiplying average current by average voltage can give a different answer from averaging their instantaneous product. Use the approved measurement and calculation method for that process, power source, and governing requirements.
Measurement location matters too. A power-source display or terminal voltage is not automatically the actual arc voltage; cable losses can affect the difference. Only trained, authorized personnel using suitable equipment should make electrical measurements. Do not improvise meter connections, especially on systems with high-frequency arc starting.
A pass-average value also cannot show every local hesitation or restart. Keep weld identification and segment information detailed enough for the inspection team to interpret the result.
Record enough to reproduce the answer
A useful record identifies the weld, pass or segment, WPS revision, process, measured current and voltage, measured length, arc-on time or travel speed, calculation method, and final units. Where required, record the efficiency factor and its approved basis separately.
For the practical effect of changing travel technique, read why a root pass can suck back. For more trade lessons, visit the Næxon Learning Center.
The calculation should make the welding record easier to check. Acceptance still comes from the applicable procedure, code, specification, and inspection requirements.
Related measurement lessons
The same unit-checking discipline applies to pipe slope and elevation calculations and measured grease-gun output.
