Welding heat input formula
Electrical power in the arc is volts times amps, in watts, which is joules per second. Divide by how fast the arc moves along the joint and you get the energy put into each unit of weld length. Travel speed in inches per minute needs a factor of 60 to turn minutes into seconds:
- Arc energy (J/in) = V × A × 60 ÷ travel speed (in/min). Divide by 1,000 for kJ/in.
- Metric: arc energy (kJ/mm) = V × A × 60 ÷ (travel speed in mm/min × 1,000). Between units, 1 in = 25.4 mm, so kJ/in = kJ/mm × 25.4.
- Heat input = k × arc energy, where k is the process thermal efficiency factor.
- If you time a pass instead of reading a speed, travel speed = weld length ÷ time. Keep length and time in matching units before you divide.
How the efficiency factor works
Not all the arc power ends up in the workpiece, and processes lose different shares. TWI and the Australian Welding Institute both print typical values of 1.0 for SAW, 0.8 for SMAW, GMAW and FCAW, and 0.6 for GTAW. Treat them as typical, and confirm the factor your WPS or code uses.
Some American codes use the words heat input for the plain V × A × 60 ÷ speed result with no k. ASME Section IX, QW-409.1, the heat input variable in procedure qualification, is written that way. If your document gives no factor, choose None (k = 1). Apply a process factor only when the document calls for it.
Worked example 1: SMAW pass in inches
25 V, 200 A, 10 in/min. Power = 25 × 200 = 5,000 W. Arc energy = 5,000 × 60 ÷ 10 = 30,000 J/in = 30 kJ/in, which is 30 ÷ 25.4 = 1.1811 kJ/mm. With the typical SMAW factor k = 0.8, heat input = 24 kJ/in (0.9449 kJ/mm).
Worked example 2: GMAW pass timed with a stopwatch
24 V, 180 A, and an 8 in bead that took 60 seconds. Speed = 8 in ÷ 60 s × 60 = 8 in/min. Power = 24 × 180 = 4,320 W. Arc energy = 4,320 × 60 ÷ 8 = 32,400 J/in = 32.4 kJ/in, and with k = 0.8 heat input = 25.92 kJ/in (1.0205 kJ/mm). Forgetting that the time was in seconds, and dividing 8 by 60 as if it were minutes, would put the speed off by a factor of 60.
These are original arithmetic examples, not exam questions.
Heat input questions
What is the difference between heat input and arc energy?
Arc energy is the raw V × A × 60 ÷ speed number. Heat input multiplies it by the process efficiency factor k. Some codes and exam questions use the two words for the same no-k figure, so read which one your document means.
Why does my answer come out a thousand times too big?
Because J/in was reported as kJ/in. A normal arc welding pass lands in tens of kJ/in. If you get thousands, divide by 1,000, and check that you used 60 only when the speed is per minute.
Do I use in/min or mm/min?
Use whichever your WPS lists, then read the result in kJ/in or kJ/mm. This calculator shows both, so you can check a hand answer in either unit.
Heat input is on CWI Part A, where you work it by hand with no code book. For the rest of the Part A arithmetic, use the CWI math calculator.