What is the welding heat input formula?

Heat input equals voltage times current times 60, divided by 1000 times the travel speed in mm per minute, giving kilojoules per millimetre. Multiply by 25.4 to convert kJ/mm to kJ/in. The 60 converts amp-volts (watts) into energy per minute of arc time.

Why apply a thermal efficiency factor?

Not all arc energy reaches the workpiece; some radiates and convects away. ISO/TR 18491 gives typical efficiencies of about 1.0 for submerged arc, 0.8 for stick, MIG, and flux-cored, and 0.6 for TIG. The effective heat input is the arc energy multiplied by that factor.

Why does heat input matter for procedure qualification?

Heat input controls the cooling rate of the weld and heat-affected zone, which sets the final microstructure, hardness, and toughness. Codes such as AWS D1.1 limit the qualified range, so welders must stay between the minimum and maximum recorded on the procedure qualification record.

What happens if heat input is too high or too low?

Too high and the weld cools slowly, causing grain growth, a soft heat-affected zone, and reduced toughness. Too low and it cools quickly, which can harden hardenable steels and promote hydrogen cracking. The correct window depends on the alloy and thickness.

Does this work for SAW and TIG?

Yes. The formula is identical; only the efficiency factor changes. Submerged arc uses about 1.0 because the flux blanket traps almost all the energy, while TIG uses about 0.6 because more heat is lost from the open arc, so the same parameters give a lower effective heat input.

What is the Welding Heat Input Calculator?

Compute welding heat input from voltage, current, and travel speed for SMAW, GMAW, FCAW, SAW, and GTAW. Applies the correct thermal efficiency factor and reports both arc energy and effective heat input in kJ/mm and kJ/in. It runs free in your browser on Gera Tools, with nothing uploaded.

Welding Heat Input Calculator

Name: Welding Heat Input Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Welding heat input is the amount of energy delivered to the joint per unit length of weld. It is the master variable behind cooling rate, and therefore behind the strength, hardness, and toughness of both the weld metal and the surrounding heat-affected zone. Welding codes qualify a procedure over a tested range of heat input, and this calculator lets you confirm where any set of parameters lands.

How it works

The standard arc-energy equation used in AWS D1.1 and D1.5 is:

HI [kJ/mm] = (V x I x 60) / (1000 x S)      S in mm/min
HI [kJ/in] = HI [kJ/mm] x 25.4

V is arc voltage, I is welding current, and S is travel speed. The factor 60 turns instantaneous power (volt-amps, i.e. watts) into energy per minute, and dividing by 1000 converts joules to kilojoules.

Raw arc energy overstates the heat that actually enters the steel, so it is scaled by a thermal (arc) efficiency η from ISO/TR 18491:

effective HI = arc energy x η
SAW  ~ 1.00   SMAW ~ 0.80   GMAW/FCAW ~ 0.80   GTAW ~ 0.60

Worked example

A GMAW pass at 25 V and 200 A travelling at 300 mm/min:

Arc energy = (25 × 200 × 60) / (1000 × 300) = 300,000 / 300,000 = 1.0 kJ/mm
Effective HI = 1.0 × 0.80 = 0.80 kJ/mm  (= 20.3 kJ/in)

Now compare the same joint done with GTAW at 15 V and 150 A at the same travel speed:

Arc energy = (15 × 150 × 60) / (1000 × 300) = 135,000 / 300,000 = 0.45 kJ/mm
Effective HI = 0.45 × 0.60 = 0.27 kJ/mm

The GTAW pass delivers less than a third of the GMAW effective heat input at the same travel speed, even though the arc energy is already lower. This explains why TIG-welded zones cool rapidly and can be prone to hydrogen cracking on hardenable steels without preheat.

Why heat input is a code requirement

Heat input controls the t8/5 cooling time — the time in seconds for the weld metal and HAZ to cool from 800 °C to 500 °C. This window governs microstructure:

Too high heat input → slow cooling → coarse grain growth in the HAZ → reduced toughness and impact energy, risk of soft zones in high-strength steels.
Too low heat input → rapid cooling → risk of hardened martensite in the HAZ of hardenable steels → hydrogen-induced cracking (HIC), especially with moisture or hydrogen-containing consumables.

AWS D1.1 and ASME IX both qualify procedures within a tested maximum heat input. Exceeding that maximum on production welds means the procedure is no longer qualified, even if the bead looks good.

How to adjust heat input

To raise heat input	To lower heat input
Reduce travel speed	Increase travel speed
Increase voltage	Reduce voltage
Increase current (within electrode limits)	Reduce current
Switch to SAW (η=1.00) from GMAW	Switch to GTAW (η=0.60)

Travel speed is usually the easiest variable to adjust. A 10% reduction in travel speed raises heat input by roughly 11%. For pulsed GMAW or GMAW-P, use average voltage and average current measured during the arc-on time, not peak values.

Notes for procedure qualification

Log voltage, amperage, and travel speed for each pass and each layer during PQR test welds — the qualified range (typically ≤ qualified HI, or within a defined band) must be held on production welds.
Many structural codes also permit controlling heat input by measuring bead cross-section area as an alternative to the energy calculation.
Always confirm which formula and which efficiency factors your controlling code requires; some older codes used η = 1.0 for all processes.