Welding & fabrication
Welding Heat Input Calculator
Calculate conventional arc energy and an efficiency-adjusted heat-input estimate in both kJ/mm and kJ/in. Use representative measured values—not dial settings—for procedure work.
Model reviewed through · freshness policy
Inputs
Result
Adjust the inputs to see your result.
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Welding travel-speed gauge
A repeatable measured travel speed is more useful than guessing from bead length after the fact.
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Clamp meter rated for welding current
Use equipment that is correctly rated for the circuit and process; follow the manufacturer's safety instructions.
What this calculator computes
Welding heat input describes how much energy is delivered along a unit of weld length. It is useful when comparing procedure settings because voltage and current increase the energy rate while travel speed spreads that energy over more or less distance. This tool reports arc energy first, then applies the selected process-efficiency factor to show an estimated heat input.
The distinction matters. TWI explains that arc energy and heat input are related but not interchangeable: heat input applies an efficiency factor to arc energy. Terminology and required calculation methods can vary by standard, contract, and procedure, so treat the number here as a transparent check—not an approval.
The steady-state formula
Arc energy (kJ/mm) = volts × amps × 60 ÷ [travel speed (mm/min) × 1,000]
Estimated heat input (kJ/mm) = arc energy × thermal efficiency
For inches per minute, the calculator first converts travel speed to millimetres per minute. It also reports kJ/in so U.S. shop measurements can be checked without a separate conversion. The process defaults are 0.8 for SMAW, GMAW, and FCAW; 0.6 for GTAW; and 1.0 for SAW, matching the example efficiencies published by TWI.
Worked example: 24 V, 200 A, 12 ipm GMAW
- 12 ipm × 25.4 = 304.8 mm/min.
- 24 V × 200 A = 4.8 kW of arc power.
- 24 × 200 × 60 ÷ (304.8 × 1,000) = 0.945 kJ/mm of arc energy.
- 0.945 × 0.8 = 0.756 kJ/mm estimated heat input.
- The same values are 24.0 kJ/in arc energy and 19.2 kJ/in estimated heat input.
Process-efficiency defaults
| Process | Default | Planning note |
|---|---|---|
| SMAW / stick | 0.80 | Verify the efficiency required by the applicable procedure. |
| GMAW / MIG | 0.80 | Do not apply the simple equation blindly to pulsed waveforms. |
| FCAW | 0.80 | Use the actual process and representative operating values. |
| GTAW / TIG | 0.60 | Lower default reflects the example thermal-efficiency treatment. |
| SAW | 1.00 | A planning default, not proof of procedure compliance. |
Measure the inputs that actually occurred
Dial settings are not always the same as arc values. Measure or use qualified equipment records when the result matters. Travel speed can be checked by dividing a known weld length by the arc time. For example, completing a 12-inch bead during 60 seconds of arc time is 12 ipm. Stop-start time should not be included in that travel-speed measurement; it belongs in the operator factor used by the welding time and cost calculator.
When the traditional equation is not enough
TWI and Lincoln Electric both warn that waveform-controlled power sources can require a different method. Multiplying average voltage by average current can misrepresent pulsed energy because the two signals vary together over time. If the power source reports energy, follow the machine instructions and the governing code or WPS. Never change a qualified procedure solely because this planning result looks favorable.
Sources and methodology
- TWI: difference between heat input and arc energy — equation, terminology, and example process-efficiency factors.
- TWI: measurement of arc-welding parameters — limitations of average measurements and waveform-controlled processes.
- Lincoln Electric: calculating heat input — conventional formula and power-source energy guidance.
Sources and calculator behavior were reviewed on . The implementation is unit-tested against hand-calculated examples. This calculator does not reproduce a welding-code table and does not replace a WPS, PQR, code book, engineer, inspector, or manufacturer instruction.
FAQ