Welding Calculator uses HI = (V × A × 60 ÷ speed ÷ 1000) × efficiency to estimate net heat input, gross arc energy, arc power, travel time, and kJ/in or kJ/mm conversions from entered weld data.
This Welding Calculator estimates net heat input from arc voltage, welding current, travel speed, and weld length — applying a process efficiency factor to separate the theoretical gross arc energy from the net thermal energy actually delivered into the base material. Select your welding process and measurement system, enter your parameters, and the calculator returns net heat input per unit length, arc power, arc-on energy, travel time, and BTU equivalent in a single calculation.
What This Welding Calculator Measures
Heat input calculations in welding serve one core purpose: quantifying how much thermal energy is deposited into the weld joint per unit length of travel. The calculator resolves this at two levels — gross and net — and derives several supporting quantities from the same input data.
The theoretical electrical energy delivered per unit weld length, calculated from voltage, current, and travel speed alone. Also called arc energy or gross heat input, it makes no adjustment for how efficiently the arc transfers heat into the workpiece. Standards such as AWS and older BS EN documentation have historically used this value.
Gross arc energy multiplied by the process thermal efficiency factor (η). This is the primary output of the calculator and represents the estimated portion of arc energy that actually enters the base material as heat. Modern standards such as BS EN 1011 and certain AWS documents use net heat input — with η — as the controlled variable in welding procedure specifications.
Arc power (in kW) is the instantaneous electrical power of the welding arc at the entered voltage and current. Arc-on energy (in kJ) is the total electrical energy consumed over the calculated travel time for the entered weld length — the arc power integrated over time. These are total energy quantities, not normalised per unit length.
The theoretical continuous arc-on duration needed to complete the specified weld length at the entered travel speed. Expressed in seconds and minutes, and also as a time-per-unit-length rate. This assumes uninterrupted travel at constant speed — it does not account for start/stop sequences, arc strikes, or repositioning.
Weld Heat Input Formula
The calculation sequence below shows how every output derives from the six inputs. All intermediate results are carried in full floating-point precision before being rounded for display.
P (W) = V × I
Voltage (V) in volts multiplied by current (I) in amps gives arc power in watts. Divide by 1,000 for kilowatts.
AE (kJ/unit) = ( V × I × 60 ) ÷ ( S × 1000 )
S is travel speed in ipm (US) or mm/min (metric). The factor of 60 converts speed from per-minute to per-second to align with the joule (J = W·s). Dividing by 1,000 converts joules to kilojoules. Result is kJ/in for US or kJ/mm for metric.
HI (kJ/unit) = AE × η
η (eta) is the process thermal efficiency factor selected for the welding process. Values used: TIG/GTAW 0.60, MIG/GMAW 0.80, Stick/SMAW 0.80, Flux Core/FCAW 0.85, Sub Arc/SAW 0.95.
Heat Loss (kJ/unit) = AE − HI
The portion of gross arc energy not transferred into the base material — lost to radiation, convection, spatter, and other non-conductive mechanisms. Equals AE × (1 − η).
t (s) = ( L ÷ S ) × 60
L is weld length (in or mm), S is travel speed (ipm or mm/min). Multiplying by 60 converts from minutes to seconds.
E_arc (kJ) = P_kW × t (s)
Arc power in kilowatts multiplied by travel time in seconds. This gives the total electrical energy consumed during the weld run, not normalised per unit length.
kJ/mm = kJ/in ÷ 25.4 | kJ/in = kJ/mm × 25.4
Based on the exact international definition 1 inch = 25.4 mm (NIST SP 811). The Alternate Units result card applies this conversion to both the net heat input and the gross equivalent.
Total Net Energy (BTU) = Total Net kJ × 0.947817
Converts total net heat (net heat input × weld length, in kJ) to BTU using the thermochemical conversion factor 1 kJ = 0.947817 BTU.
How to Use the Inputs
Each field maps to a parameter measured or specified during the welding operation. Use actual recorded values from your welding procedure record or data logger where possible.
US Customary uses inches per minute (ipm) for travel speed, inches for weld length, and outputs kJ/in as the primary unit. Metric uses mm/min for travel speed, mm for weld length, and outputs kJ/mm. Switching systems resets inputs to representative defaults. The Alternate Units card always shows the result in the opposing system.
Selects the process thermal efficiency factor η applied to gross arc energy. The five options and their factors are: TIG/GTAW 60%, MIG/GMAW 80%, Stick/SMAW 80%, Flux Core/FCAW 85%, Sub Arc/SAW 95%. These are standard representative values from welding literature — they are not measured from your specific equipment or setup. See Assumptions for detail.
The arc voltage in volts as measured or specified in the welding procedure. Use the actual arc voltage at the work — not the open circuit voltage or the machine display if it differs from measured arc voltage. For pulsed processes, use the mean arc voltage representative of the process parameters.
Welding current in amps. Use the actual mean current during the arc-on period. For pulsed or variable-current processes, use a mean representative value consistent with how your welding procedure specification defines heat input measurement.
The linear travel speed of the arc along the weld joint — in inches per minute (US) or mm/min (metric). This is the actual welding travel speed, not wire feed speed. Travel speed has a direct inverse relationship with heat input: halving travel speed doubles heat input per unit length, all else being equal.
The total weld length in inches or mm. Used to calculate travel time, arc-on energy, and total net energy in BTU. This does not affect the per-unit-length heat input values (kJ/in or kJ/mm) — those depend only on voltage, current, travel speed, and efficiency.
Understanding the Results
The calculator returns a primary hero result and four output cards. Each is explained below in the same order it appears in the calculator, with the formula used and what the value physically represents.
The hero output. Net heat input is the gross arc energy reduced by the process efficiency factor — the estimated quantity of thermal energy actually transferred into the base material and heat-affected zone per unit length of weld travel. It is expressed in kJ/in (US) or kJ/mm (metric).
HI = ( V × I × 60 ) ÷ ( S × 1000 ) × η
Net heat input is the value most commonly referenced in welding procedure specifications and codes when a heat input limit is specified. If your WPS or applicable code specifies a maximum or minimum heat input, compare the net heat input result from this calculator against that limit — not the gross arc energy value. Always verify that the efficiency factor your specification uses matches the process selection here.
This card shows the gross arc energy — the total electrical energy per unit weld length before any efficiency adjustment — alongside the selected efficiency percentage and the heat loss adjustment that separates gross from net.
AE = ( V × I × 60 ) ÷ ( S × 1000 )
η: GTAW 60%, GMAW 80%, SMAW 80%, FCAW 85%, SAW 95%
Heat Loss = AE − HI = AE × (1 − η)
Some older standards and some AWS documents define heat input without an efficiency factor — effectively using gross arc energy as the controlled value. If you are working to a standard that does not apply an efficiency factor, use the gross arc energy figure shown in this card rather than the net heat input hero value. Always check which definition your applicable standard uses before comparing results to a specification limit.
This card groups the instantaneous power and total energy quantities derived from the entered parameters — distinct from the per-unit-length heat input values on the other cards.
P (kW) = ( V × I ) ÷ 1000
Rate (J/s) = V × I (same as watts)
E_arc (kJ) = P_kW × t (s)
Arc-on energy is the total electrical energy consumed over the entire weld run at the entered parameters. It is not normalised per unit length — it scales with weld length. This value represents total gross electrical energy input and does not have an efficiency factor applied. It is useful for energy consumption estimation, not for comparing against a heat input limit in a WPS.
Travel time is the theoretical continuous arc-on duration to complete the entered weld length at the entered travel speed. The time-per-unit value is its normalised equivalent — seconds per inch or seconds per mm.
t (s) = ( L ÷ S ) × 60
60 ÷ S (s/in or s/mm)
t (min) = t (s) ÷ 60
Travel time assumes uninterrupted arc travel at constant speed across the full weld length. It does not include arc strike and extinguish time, repositioning, inter-pass cooling, or any stop-start sequences. For multi-pass welds, multiply time per pass by pass count.
The Alternate Units card converts the net heat input and gross arc energy into the opposing unit system, and converts total net energy from kilojoules into BTU. All conversions use exact defined factors.
kJ/mm = kJ/in ÷ 25.4
kJ/in = kJ/mm × 25.4
BTU = (HI × L_in or L_mm) × 0.947817
The primary PASS/FAIL comparison — if any — is always made in the selected unit system against the WPS or code limit expressed in the same units. The Alternate Units card is a convenience reference. If your applicable specification states a kJ/mm limit and you are working in US Customary, use the kJ/mm value shown in the Alternate Units card for comparison, not the primary kJ/in result.
Worked Example
Using the calculator's default values — MIG/GMAW at 24 V, 200 A, 12 ipm over 10 inches — the following results are produced step by step.
How to Interpret the Result
This calculator does not compare the calculated heat input against a WPS limit, code requirement, or pass/fail threshold — it has no access to your applicable specification. The following guidance describes the general relationship between heat input level and weld behaviour, not a code-compliant acceptance decision.
More energy per unit length generally means a slower cooling rate, a wider and deeper heat-affected zone (HAZ), and potentially greater grain growth in the HAZ. For some materials — particularly certain low-alloy steels, stainless steels, and nickel alloys — excessive heat input can reduce toughness or cause sensitisation. Some WPS documents specify a maximum heat input for this reason.
Less energy per unit length generally means a faster cooling rate. For hardenable steels, this can increase hardness and reduce ductility in the HAZ, potentially increasing susceptibility to hydrogen-assisted cracking. Some WPS documents specify a minimum heat input — particularly for preheat-sensitive materials — to ensure the cooling rate stays within acceptable limits.
If your welding procedure specification or applicable code specifies a heat input range or limit, compare the net heat input result from this calculator against that limit directly. Confirm that the efficiency factor (η) used here matches the definition in your specification. Some specifications use gross arc energy rather than net heat input — if so, use the Gross Arc Energy value from the Arc Energy card, not the net heat input hero value.
⚠ Assumptions and Limitations
The results produced by this calculator are estimates based on steady-state input values and standardised efficiency factors. Before using any result in an acceptance decision, engineering judgement, or procedure documentation, note the following:
- Steady-state assumption: The calculator assumes constant voltage, current, and travel speed throughout the weld. Real welding conditions vary — voltage and current fluctuate with arc length, technique, and equipment response.
- Efficiency factors are representative, not measured: The η values used (0.60 for GTAW through 0.95 for SAW) are standard literature values drawn from sources such as TWI guidance and published welding engineering references. Your actual process efficiency depends on equipment, polarity, arc length, joint geometry, shielding gas or flux, technique, and position.
- No pulsed process correction: For pulsed GMAW or pulsed GTAW, use mean arc voltage and mean current values representative of the pulse parameters as defined in your procedure or measurement method.
- Weld length and arc-on time: Travel time assumes continuous uninterrupted arc travel. It does not include arc strike time, repositioning, inter-pass cooling, or any stop-start sequences.
- Not a substitute for procedure qualification: This calculator produces an estimated heat input for reference and checking purposes. It does not qualify a welding procedure, verify conformance to a code, or replace the heat input measurement methods defined in your applicable welding standard.
- Compare against your applicable specification: Always verify your heat input result against the WPS, project specification, code, or engineering requirement applicable to your work, using the heat input definition (gross or net, with which η) specified in that document.
References
The formulas, efficiency factors, unit conventions, and terminology used in this calculator are consistent with the sources below. Consult them for authoritative guidance on heat input definitions, efficiency factor selection, and the distinction between arc energy and net heat input used in your applicable welding standard.