The formula, and the one term that moves
Arc energy per inch is volts times amps times sixty, divided by travel speed in inches per minute, which comes out in joules per inch. At 26 volts, 220 amps and 12 inches a minute that is 343,200 divided by 12, or 28,600 joules per inch — 28.6 kilojoules. Multiply by an efficiency factor of 80 percent and heat input is 22.88 kJ per inch.
Volts and amps are set on a machine and stay where they are put. Travel speed is set by a person, changes constantly, and is inversely proportional to the answer. Slow from 12 inches a minute to 9.6 and heat input goes up by exactly 25 percent, with the machine untouched. That is the whole reason the page prints a travel speed ladder rather than a single number: the sensitivity is the useful thing.
Arc energy and heat input are two numbers
Arc energy is the raw figure with nothing taken off. Heat input applies a factor for the share of that energy actually entering the plate rather than radiating away or leaving with the spatter. Different processes and different procedures use different factors, and some define heat input as the raw figure with no factor at all. Which is meant on your job is written in the procedure, not here, and that is why the factor is an editable field. Set it to 100 and the page reports plain arc energy.
The wait between passes, from your own stopwatch
Hot steel cools towards the shop temperature at a rate proportional to how far above it the steel is, which means the temperature decays rather than falling in a straight line. Two readings settle the curve. Take a plate at 500 F, read it again 90 seconds later at 300, and with a 70 F shop the time constant works out at 144 seconds. That constant belongs to that part, that thickness, and the bench you left it on.
Once you have it, the wait is arithmetic. A plate at 550 F after a pass, on a job working to 400 F between passes, needs 54 seconds. Over a six pass joint that is five gaps and 4.5 minutes of standing still, against 12 minutes of arc — so 27 percent of the joint is waiting. On heavier sections with a lower interpass figure it inverts completely, and the welder spends most of the joint watching a temp stick.
Nobody publishes this because it cannot be published. It depends on the mass of the part, what it is sitting on, whether there is a draught, and how much of it has already been welded. A stopwatch and a temp stick settle it for your part in an afternoon, and it stays settled.
What is not here
What a heat input does to a steel. That is metallurgy, it is different for every alloy and every thickness, and it is the reason procedures get qualified. The limit field on the form exists only so you can do arithmetic against a number somebody else gave you — the page has no view on whether that number is right, whether your run meets it, or whether the procedure is acceptable. It also assumes the current and voltage are steady, which for short circuit transfer and for pulsed processes is a convenient fiction rather than a fact.
Questions people ask
How do you calculate weld heat input?
Volts times amps times sixty, divided by travel speed in inches per minute, gives joules per inch; divide by a thousand for kilojoules. At 26 volts, 220 amps and 12 inches a minute that is 28.6 kJ per inch of arc energy. Whether a thermal efficiency factor is then applied, and what factor, is defined by the procedure you are working to rather than by a formula.
What is the difference between arc energy and heat input?
Arc energy is the raw volts-amps-travel figure with nothing taken off it. Heat input applies a factor for the share of that energy that actually goes into the plate. Some standards define heat input as the raw number and some apply a process factor, so the two terms are used to mean the same thing in some documents and different things in others. Read the procedure rather than assuming, and set the efficiency field to 100 if you want the raw figure.
Does travel speed change heat input?
It is the only term on the list a welder changes without touching the machine, and heat input is inversely proportional to it. Slowing from 12 inches a minute to 9.6 raises heat input by 25 percent at identical settings. Speeding up to 15 drops it by 20 percent. That is why two welders running the same machine settings on the same joint can be putting very different amounts of energy into the steel.
How long should you wait between weld passes?
Long enough for the plate to fall to whatever interpass temperature the procedure specifies, which depends on your part and not on any published table. Measure it: read the plate twice a known time apart while it cools, and the two readings fix the decay constant. From 550 F down to 400 F in a 70 F shop, with a constant of 144 seconds, the wait is 54 seconds. Change the part or the bench and the constant changes.
What heat input limit should I use?
Whatever the procedure the job is being run to specifies, from whoever is responsible for the weld. This page states no limit and cannot know yours — the limit field is there only so the arithmetic can be done against a number you were given. What a particular heat input does to a particular steel is a metallurgical question, and it is the reason welding procedures are qualified rather than looked up.